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H Y D R O L O G Y A N D F L U V I A L
G E O M O R P H O L O G Y
Geography AS Level
Water enters and continually cycles around the earth through
the global hydrological cycle, it is a closed system with no
inputs or outputs. The hydrological cycle refers to the cycle of
water between atmosphere, lithosphere and biosphere.
The Drainage basin system however is an open local system
as it has inputs, outputs and transfers of energy and matter
into and from the system.
A rivers drainage basin is the area of land drained by a river
and its tributaries - also called the rivers catchment
The drainage basin is surrounded by higher land -
this boundary of the drainage basin is called the watershed-
any precipitation falling beyond the watershed enters a
different basin. The watershed separates different drainage
Drainage Basin Features
Source: The beginning of a river. A river may
have multiple sources. The source of a river is
normally found in upland mountainous areas.
Mouth: The end of a river. A river may end in a
lake, but more normally in the sea.
Tributary: A small river that flows into a larger
Confluence: Where two rivers meet.
Watershed: The border between two drainage
Estuary: The tidal section of a river near the
Channel: The physical confines of the river,
encompassing two banks and a bed.
The Hydrological Cycle
Water evaporates from water bodies such as rivers,
lakes and seas, and from plants and trees. The water
vapour rises, cools and condenses to form clouds.
Rain falls from the clouds . The rain water is
intercepted by plants , seeps into the ground before
reaching surface streams, or runs off the land surface
into streams and rivers. The rivers enter lakes or
seas. The hydrological cycle is then repeated.
Drainage Basin Processes Explained
Precipitation: Any moisture that falls from the atmosphere. The main types of precipitation are rain,
snow, sleet, hail, fog and dew.
Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place
from a body of water e.g. a lake, puddle or the sea.
Transpiration: The evaporation of moisture from vegetation's stomata.
Evapotranspiration: The combined action of evaporation and transpiration
River discharge via channel flow: Water entering the sea and leaving a drainage basin. A very small
amount of water also enters the sea via through flow and groundwater flow (base flow).
Through fall: Precipitation that drips from vegetation to the ground
Stem flow: Precipitation that flows down plant stems to the ground.
Surface store: Precipitation lying on the ground (puddles)
Overland flow: movement of water along ground surface to a river.
Infiltration: Process whereby water enters soil layer.
Through flow: is the horizontal occurs downslope along well-defined lines of (percolines) or above
Percolation: process by which water drains to the water table
Ground water flow: is deeper level of gravitational flow in downslope direction through rock to feed
rivers and springs
Channel flow: Water flowing in a river.
Phreatic Zone: The permanently saturated zone within solid rocks and sediments. The upper layer
of this is known as the water table.
Vadose zone: Zone of temporary saturation
Inputs, outputs, stores and transfers.
Inputs: When water is added to a
Outputs: When water leaves a drainage
• Precipitation • Evaporation
• River runoff
Stores: When water is stationary and
not moving in a drainage basin.
Transfers: When water is moving within
a drainage basin.
• Ground water store
• Soil water store
• Vegetation Store
• Channel Store
• Through fall
• Stem Flow
• Overland Flow
• Through flow
• Ground Water Flow
• Channel Flow
Saturated: Ground where the pores are full and can contain no more water.
Unsaturated: Ground where there is still space between the pores.
Water table: The border between saturated and unsaturated ground. The water table
may go up or down.
Permeable: Surfaces that allow water to pass through them.
Impermeable: Surfaces that do not allow water to pass through them.
Pores: Gaps between soil and gravel that water can fill.
Aquifer: Rock that can hold water.
Aquiclude: Rock that can not hold water.
Porous: Rock with pore spaces and cracks in it.
Non-porous: Rock with no pore spaces or cracks in it.
Condenses: When water vapour turn into water droplets. Water can only condense
around condensation nuclei
Antecedent Moisture: Amount of water in the soil before additional precipitation
Topography: The shape of the land
Aquifer (rocks that can hold water)
Using examples , explain how geology can define if a
rock is an effective aquifer (8m)
The porosity and permeability of the rock under the ground
decides whether it will be an effective aquifer
High porosity and permeability rocks with many pores and
big gaps between the pores allow water to transfer well in
them to make a good aquifer
An example of this type of rock is sandstone
Low porosity rocks do not make good aquifers as water can
not pass through them
An example of this type of rock is glacial till
Some rocks have high porosity but are impermeable and so
do not act as a good aquifer
An example of this type of rock is clay
Movement of water – The Water table
Water is infiltrated at the surface and then is percolated
under gravity through pores, joints and bedding planes
to reach an area of permanent saturation where all pores,
joints, etc are full of water.
This may be seasonal or permanent depending upon the
nature of the rock and the level of input.
The water table will generally follow the surface
topography and hence water will flow under gravity and
by the hydraulic gradient to a point where it will emerge
as a spring or base flow of a river. It may also be
abstracted by wells or boreholes.
The Water Balance
The water balance is worked out from inputs and outputs and
affects how much water is stored in the basin. The general
water balance in the UK shows seasonal patterns:
in wet seasons, precipitation exceeds
evapotranspiration creating water surplus. the ground stores
fill with water so there's more surface runoff and higher
discharge- river levels rise
in drier seasons, precipitation is lower than
evapotranspiration. ground stores are depleted as some ware
is used and flows into the river channel but isn't replaced by
at the end of a dry season there's a deficit of water in the
ground. the ground stores are recharged in the next wet
River Discharge = is defined as the volume of water
passing a measuring point or gauging station in a river in
a given time. It is measured in cubic metres per second
precipitation- more precipitation, higher the discharge
hot weather- higher temperature, lower the discharge
because evaporation is higher
removal of water from the river- reduces discharge
Discharge can be illustrated using hydrographs. These can
show annual patterns of flow ( the river regime) in
response to climate. Short-term variations in discharge are
shown using a flood of storm hydrograph.
Flood storm hydrograph
The starting and finishing level show the base flow of a
river. The base flow is the water that reaches the channel
through slow through flow and permeable rock below the
water table. As storm water enters the drainage basin the
discharge rates increase. This is shown in the rising limb.
The highest flow in the channel is known as the peak
discharge. The fall in discharge back to base level is
shown in the receding limb. The lag time is the delay
between the maximum rainfall amount and the peak
The shape of a hydrograph varies in each river basin and
each individual storm event.
Definitions and Explanation of Terms involved in
lag time- is the delay between peak rainfall and peak discharge- delay happens
because it takes time for the rainwater to flow into the river
rising limb- part of the graph up to the peak discharge- increases as rainwater
flows into the river increase in discharge after start of precipitation event.
falling limb- part of the graph after the peak- discharge is decreasing because less
water is flowing into the river – decline in discharge after the precipitation event.
Hydrograph – a graph that shows river discharge and rainfall over time.
Base flow – represents the normal day to day discharge of the river and is the
consequence of groundwater seeping into the river channel.
Storm flow – Water that reaches the steam via overland flow and through flow
Bankfull discharge – the maximum discharge that a particular river channel is
capable of carrying without flooding.
Peak discharge – the point on a flood hydrograph when river discharge is at its
Peak rainfall – the point on a flood hydrograph when rainfall is at its greatest.
Factors affecting flood storm hydrographs
larger drainage basins can catch more precipitation so have
a larger peak discharge, smaller basins generally have shorter lag
steep-sided drainage basins have shorter lag times because water
flows more quickly downhill
circular basins- more likely to have flashy hydrograph because all
points on the watershed are roughly the same distance from the
point of measurement means a lot of water will reach the
measuring point at the same time
basins with lots of streams drain quickly so have shorter lag
the amount of water already present in the drainage basin affects
lag time: if ground is already waterlogged then infiltration is
reduced and surface runoff increases, surface runoff is
much faster than throughflow or baseflow so rainwater reaches
the river more quickly-reducing lag time
Factors affecting storm hydrograph
Rock type- affects lag time and peak discharge:
impermeable rock types don't store water or let water
flow through them reduces infiltration and increases
surface runoff, reducing lag time
Soil type- affects lag time and peak discharge. Sandy
soils allow a lot of infiltration but clay soils allow
little. low infiltration rates increases surface
runoff, reducing lag time, increasing peak discharge
Vegetation- affects lag time and peak discharge.
Intercepts precipitation and slows its movement-
increasing lag time. The more vegetation there is, the
more water is lost before it reaches the river channel,
reducing peak discharge
Factors affecting flood storm hydrograph
Precipitation- affects peak discharge. Intense storms will
generate more precipitation and so greater peak
discharges than light rain showers
the type of precipitation affects lag time e.g. snow thats fallen
in a winter storm can melt and flow into the river in spring,
giving a very long lag time
temperature- affects lag time and peak discharge
hot, dry conditions and cold, freezing conditions both result
in hard ground- reduces infiltration and increases surface
runoff- reducing lag time and increasing peak discharge
high temperatures can increase evapotranspiration, so less
water reaches the river channel, reducing peak discharge
Factors affecting storm hydrographs (human)
in urban areas- much of the soil is covered with man-
made impermeable materials like concrete
water can't infiltrate into the soil, which increases surface runoff, so
water flows more quickly into the river making lag time short and
increases peak discharge
man-made drainage systems affect the hydrograph in a
similar way. water flows down drains into the river
before it can evaporate or infiltrate into the soil,
causing a shorter lag time and increased peak
Deforestation means less interception, so rain reaches
the ground faster. The ground is likely to become
saturated and surface run-off will increase
River Channel Processes and Landforms
Rivers - Source to Mouth Having understood the
basics of a Drainage Basin we now need to consider the
journey that a river within a Drainage Basin takes from
its beginning to its end.
The path the river follows from its source to mouth is
known as the river's course.
When studying rivers we often divide it into 3 main
sections, the upper course; middle course and
Each part of the river has distinctive features which form
and the characteristics of the river and its surrounding
valley change downstream.
Processes in different stages of river profile:
Processes in the Upper Course
In the upper course, the river has a lot of gravitational potential energy so it has a lot of energy to
erode vertically. The bed of the river is eroded greatly while the banks aren’t eroded as much. The
river mainly transports large pieces of angular rock and does so by traction because it doesn’t have
enough kinetic energy to move the load in any other way. This increases erosion of the bed by
abrasion as a result of the load being dragged along the bed of the river. Vertical erosion is further
increased by the rough nature of the channel in the upper course which increases the water’s
turbulence and its ability to erode. Erosion and transportation only takes place in large quantities
in the upper course when the river’s discharge is high after periods of heavy precipitation. When
the river’s discharge falls the river stops transporting the large boulders its transporting and
Processes in the Middle Course
In the middle course, the river has less gravitational potential energy and more kinetic energy so
erosion shifts from vertical to lateral erosion. Abrasion is still the main erosive process as large
particles are transported by saltation. The average load size has decreased in the middle course, so
more load is being transported in suspension. In the middle course, the river can flood and in
doing so, it deposits gravel and sand sized particles onto its flood plain.
Processes in the Lower Course
In the lower course, the river has next to no gravitational potential energy so erosion is almost
exclusively lateral. There isn’t much erosion though because the channel is smoother resulting in
less turbulent flow. The main place where erosion takes place is where the river meanders. The
average particle size is very small now, another reason for the reduction in erosion. The river’s load
is mainly composed of silts and clays and it is transported in suspension or even solution. Like in
the middle course, when the river floods it deposits its load but deposition now also takes place at
the mouth of the river where the river meets the sea or a stationary body of water.
River Channel Processes
As a river flows along its course it undertakes 3 main
processes which together help to shape the river
channel and the surrounding valley. These are
At any one time the dominant process operating
within the river depends on the amount of energy
River erosion is the wearing away of the land as the water flows past the bed
and banks. There are four main types of river erosion. These are:
1. Abrasion (corrosion): This is the scraping and rubbing action of material carried
along by a river (bed load). Rivers carry rack fragments in the flow of water or drag
them along the bed, and in doing so wear away the banks and bed of the river channel.
Abrasion is most effective in short turbulent periods when the river is at Bankfull or in
flood. During times when river levels are low, the load consists of small particles such
as sand grains, and these tend to smooth the surface of the river.
2. Hydraulic Action: this is where the water in the river compresses air in cracks in the
bed and banks (Cavitation – force of air exploding) This results in increased pressure
caused by the compression of air, mini 'explosions' are caused as the pressure is then
released gradually forcing apart parts of the bed and banks
3. Solution: Is most active on rocks that contain carbonates such as limestone and
chalk. The minerals in rocks are dissolved by weak acids in the river water and are
carried away in solution.
4. Attrition: Is the reduction in the size of fragments and particles within a river due to
the collision of boulders with one another as they move down the river. The fragments
strike one another as well as the river bed, therefore, becoming smoother, smaller and
rounder. Consequently larger, more angular fragments tend to be found upstream,
which smaller, more rounded fragments are found downstream.
Rivers erode because they have energy. Their total energy
depends on 3 main factors…
1. The weight of the water: The greater the mass of the
water, the more energy it will posses due to the
influence of gravity on its movement.
2. The height of the river above its base level: this gives it
a source of potential energy and the higher the source
of the river the more gravitational potential energy it
3. The steepness of the channel: this controls the speed of
the river which determines how much kinetic energy it
The type of transport taking place depends on...
(i) the size of the sediment and
(ii) the amount of energy that is available to undertake the transport.
• There are four types of transportation:
1. Traction: Large rocks and boulders are rolled along the river bed by water
moving downstream. This process operates only at times of high discharge.
2. Solution: When dissolved minerals invisible to the naked eye are
transported within the mass of moving water.
3. Saltation: Small stones bounce along the channel bed in a skipping motion.
This process is associated with relatively high energy conditions. Small
particles land then dislodge other particles upwards causing more
bouncing movements to take place.
4. Suspension: Very small particles of sand and silt are carried along by the
flow of a river.
When a river loses energy and therefore velocity, deposition occurs. This is
because the river doesn’t have enough energy to carry the material it is
This could happen in an estuary when the river meets the sea and slow
down, depositing its load and creating a delta.
The main factors leading to deposition are:
1. Low rainfall reducing precipitation
2. A river entering the sea or a lake – reducing velocity
3. Water becoming shallower
4. Increase in load
5. River overflows its bank, depositing material on a flood plain.
May result in the formation of features such as slip off slopes (on the inner
bends of meanders); levees (raised banks) alluvial fans; meanders; braided
streams and the floodplain.
Remember - it is the largest material that will be dropped first as it requires
the most energy to be transported. Eroded material carried in suspension
and solution will be dropped last.
=the total amount of material it can carry
capacity is the total volume of the load
the load of a river can be divided into different
categories according to particle size- which varies
from fine silt and clay to big boulders
the competence describes the maximum particle size
that a river is capable of transporting at a given point
The efficiency of a channel can be quantified as the
channel’s hydraulic radius. The hydraulic radius shows you how
efficient a channel is. The larger the hydraulic radius, the higher the
channel’s efficiency. The hydraulic radius can be calculated using
the following formula:
Where Rh is the hydraulic radius, A is the cross-sectional area of the
channel and P is the wetted perimeter of the channel. The wetted
perimeter is the length of the river’s bed and banks that is in contact
with the water.
A large hydraulic radius is more efficient because it means that a
smaller proportion of the river’s water is in contact with the bed &
banks so there is less friction. The ideal channel shape for a large
hydraulic radius would be a narrow and deep channel. Wide and
shallow channels are less efficient and have a smaller hydraulic
-The capacity of a stream refers to the
largest amount of debris that a stream
-The competence refers to the
diameter of the largest particle that
can be carried
-The critical erosion velocity is the
lowest velocity at which grains of a
given size can be moved
-The relationship between these
variables is shown by means of
the Hjulstrom Curve.
• Most of the time, larger particles such as boulders, need a higher velocity for them to be picked
up because of their large size
• However, the exception to this rule is clay and silt, as even though the particles are very small ,
the particles tend to stick together, making them hard to pick up.
• Higher velocities are needed for picking up (entrainment) than just for transporting.
• When velocity falls below a certain level (settling velocity), particles are deposited
Patterns of Flow in a river:
Turbulent Flow: provides upward motion in the flow that
allows the lifting and support of fine particles which will
contribute to depositional landforms further down the river.
The flow is a series of erratic eddies, both vertical and
horizontal in the downstream direction.
conditions necessary for turbulent flow to occur are:
1. complex channel shapes such as meandering channels and
alternating pools and riffles
2. high velocities
3. cavitation in which pockets of air explode under high
• Laminar flow
it is common in groundwater and in glaciers but not in rivers although it can occur in the bed in
the lower course of a river. This is the horizontal movement of water where the water moves at
uniform velocity with one layer of water molecular sliding over the next without mixing. It
cannot support solid particles in suspension.
best condition are:
• shallow channels
• smooth straight channels
• low velocities
A corkscrew movement in a meander. It is
responsible for moving material from the outside of
one meander bend and depositing it on the inside of
the next bend.
Braided stream: Occurs when the river is forced to split into
several channels separated by islands/eyots.
It is a feature of rivers that are supplied with large loads of
sand and gravel and they are most likely to occur when a river
has variable discharge. The banks formed from sand and
gravel and unstable and easily eroded and as a consequence,
the channel becomes very wide in relation to its depth.
Streams with high sediment loads that encounter a sudden
reduction in flow velocity generally have a braided channel. In
a braided stream, the main channel divides into a number of
smaller, interlocking or braided channels. Braided channels
tend to be wide and shallow because bedload materials are
often coarse (sands and gravels) and non-cohesive.
Landforms formed by fluvial erosion:
Riffles: Areas of shallow
water, due to deposition
of coarse material.
Pools: Areas of deeper
water between rifles
V-Shaped valleys are found in the upper course of the river and are a result of both erosion by the
river and weathering. V-Shaped valleys are deep river valleys with steep sides that look like a letter
V when a cross section of them is taken, hence the name. They’re found in the upper course
because this is where the river has the greatest gravitational potential energy and so the greatest
potential to erode vertically. It does so during periods of high discharge. When the river’s discharge
is high, it is able to transport its large bedload by traction eroding the river’s bed and valley by
corrasion, deepening it. Not much lateral erosion takes place so the channel and valley remains
As the channel and valley deepens the sides of the valley are exposed and become susceptible to
weathering. The valley’s sides also undergo mass movements resulting in large volumes of material
falling into the river’s channel, adding to its erosive power and causing the valley sides to take up a
V shape. The steepness of the valley sides and whether the valley actually looks like a V is
dependent on the climate, vegetation and rock structure among things. In cold, wet climates, freeze
thaw weathering is abundant and rainwater can act as a lubricant, aiding mass movements.
Vegetation can impede mass movements because it will help bind the soil. If the valley is composed
of hard rock the valley sides will be very steep because they won’t be weathered easily.
The steepness of Valley sides depends on factors such as:
1. Climate: valleys are steeper where there is sufficient rainfall for mass movement.
2. Rock Structure Resistant, permeable rocks such as limestone produce vertical sides
3. Vegetation: helps to bind soil together and keep hill slope more stable.
How does a V-Shape Valley Form
1. Vertical erosion (in the form of abrasion, hydraulic action and solution)
in the river channel results in the formation of a steep sided valley
2. Over time the sides of this valley are weakened by weathering processes
and continued vertical erosion at the base of the valley
3. Gradually mass movement of materials occurs down the valley sides,
gradually creating the distinctive v-shape.
4. The material is gradually transported away by the river when there is
enough energy to do so.
As the river flows through the valley it is forced to swing from side to side
around more resistant rock outcrops (spurs). As there is little energy for
lateral erosion, the river continues to cut down vertically flowing between
spurs of creating interlocking spurs.
Interlocking Spur - spurs are ridges of more resistant rock around which
a river is forced to wind as it passes downstream in the upper course.
Interlocking spurs form where the river is forced to swing from side to side around more
Waterfalls develop when a change of lithology (rock type) takes place along the
river’s course resulting in differential erosion. When the rock type of the river’s
channel changes from a resistant rock to a less resistant one (e.g. granite to
limestone), the river erodes the less resistant rock faster producing a sudden drop
in the gradient of the river with the resistant rock being higher up than the less
resistant rock. As the river flows over the resistant rock, it falls onto the less
resistant rock, eroding it and creating a greater height difference between the two
rock types, producing the waterfall.
When water flows over the waterfall it
creates a plunge pool at its base and
the splashback from the falling water
undercuts the resistant rock. The
unsupported rock is known as the cap
rock and it eventually collapses into
the plunge pool causing the waterfall
to retreat upstream. Over thousands of
years, the repeated collapse of the cap
rock and retreat of the waterfall
produces a gorge of recession
Rapids and Potholes
Potholes: Potholes are cylindrical holes drilled into the bed of a river that
vary in depth & diameter from a few centimetres to several metres. They’re
found in the upper course of a river where it has enough potential energy to
erode vertically and its flow is turbulent. In the upper course of a river, its
load is large and mainly transported by traction along the river bed. When
flowing water encounters bed load, it is forced over it and down cuts behind
the bed load in swirling eddie currents. These currents erode the river’s bed
and create small depressions in it.
• Rapids: Rapids are sections of a river
where the gradient of the river bed is
relatively steep resulting in an increase in
the river’s turbulence, velocity and
therefore erosive power. They form
where the gradient of the river is steep
and the bed is composed mainly of hard
Floodplains and Levee’s
Floodplains are large, flat expanses of land that form on either side of a river. The
floodplain is the area that a river floods onto when it’s experiencing high discharge.
When a river floods, its efficiency decreases rapidly because of an increase in
friction, reducing the river’s velocity and forcing it to deposit its load. The load is
deposited across the floodplain as alluvium. The alluvium is very fertile so
floodplains are often used as farmland.
The width of a floodplain is determined by the sinuosity of the river and how much
meander migration takes place. If there’s a lot of meander migration, the area that
the river floods on will change and the floodplain will become wider.
Levees are natural embankments produced when a river floods. When a
river floods, it deposits its load over the flood plain due to a dramatic
drop in the river’s velocity as friction increases greatly. The largest &
heaviest load is deposited first and closest to the river bank, often on the
very edge, forming raised mounds. The finer material is deposited
further away from the banks causing the mounds to appear to taper off.
Repeated floods cause the mounds to build up and form levees.
Levees aren’t permanent structures. Once the river’s discharge exceeds
its bankfull discharge1, the levees can be burst by the high pressure of
the water. Levees increase the height of the river’s channel though, so
the bankfull discharge is increased and it becomes more difficult for the
river to flood
Exempler Answer (floodplain)
River transportation is an essential process in the formation of a floodplain. At
this stage, the river will carry a large load, by solution and suspension and also
by saltation and traction. When the river floods over the surrounding land it
loses energy and deposition of its suspended load occurs. The shallower depth
of water flowing over the surface results in frictional drag and a reduction in
velocity (speed) of flow. As the floodwater loses energy, the capacity and
competence of the flood-water is reduced, leading to deposition. The heaviest
materials (bedload) are deposited first nearest the channel, as these require
the most energy to be transported and therefore build up around the sides of
the river forming raised banks known as levees. Finer material such as silt and
fine clays continue to flow further over the floodplain before they are deposited
(alluvium). Regular flooding results in the building up of layers of nutrient rich
alluvium which forms a flat and fertile floodplain. The slopes of the river valley
border the edge of the floodplain. These slopes are known as the “bluff line”.
Deltas are depositional landforms found at the mouth of a river where the river
meets a body of water with a lower velocity than the river (e.g. a lake or the sea). For
a delta to develop, the body of water needs to be relatively quiet with a low tidal
range so that deposited sediment isn’t washed away and has time to accumulate.
When a river meets a stationary body of water, its velocity falls causing any material
being transported by the river to be deposited. Deltas are made up of three sediment
beds that have been sorted by the size of the sediment. The bottom most bed,
the bottomset bed, is composed primarily of clay and some other fine grained
sediments. Clay is the main constituent because when clay meets salt water a
process called flocculation takes place where clay & salt particles clump together
(flocculate) due to an electrostatic charge developing between the particles. This
makes the clay particles sink due to their increased weight producing the bottomset
bed. The bottomset bed stretches a fair distance from the mouth of the river as the
fine sediments can be transported a reasonable distance from the river’s mouth.
The foreset bed lies on top of the bottomset bed. The foreset bed is composed of
coarser sediments that are deposited due to a fall in the river’s velocity and aren’t
transported very far into the stationary body of water that the river flows into. The
foreset bed makes up the majority of the delta and is dipped towards deep water in
the direction that the river is flowing in.
The topset bed is, as the name suggests, the topmost bed of the delta. It too is
composed of coarse sediment but, unlike the foreset bed, the topset bed doesn’t dip,
it’s horizontally bedded
There are three types of Deltas
1. Arcuate: Have rounded, convex
outer margins e.g. Nile River.
2. Cuspate: Where material brought
down by a river is spread out
evenly on either side of its
channel due to waves hitting it
head on, spreading the deposited
sediment out. E.g. Tiber River
3. Birds Foot: They extend
reasonably far into a body of
water and form when the river’s
current is stronger than the sea’s
waves. Bird’s foot deltas are
uncommon because there are
very few areas where a sea’s
waves are weaker than a river’s
current. As the same suggests it
looks like a birds foot. E.g.
Meandering channels are produced
when the thalweg follows a sinuous path
through pool and riffles to cause erosion
on the outer bank and deposition on the
inside bank. This imparts a secondary
flow called helical (Helicoidal) flow
which is a spiral flow elevating the water
on the outside of the meander with a
return current at the inside of the
meander. This produced the river cliff
and point bar.
A cross section of a meander would show that on the outside bend, the channel is very
deep and concave. This is because the outside bend is where the river flows fastest and
is most energetic, so lots of erosion by hydraulic action and abrasion takes place. River
cliffs form on the outside bend as the river erodes laterally. The inside bend is
shallower with a gentle slip-off slope made of sand or shingle that is brought across
from the outside bend by the helicoidal flow and centripetal force of the river. The
river flows much slower on the inside bend so some deposition takes place,
contribution to the slip-off slope.
River cliff and Slip off slope formation
Water flows fastest on the outer bend of the river where the channel is deeper
and there is less friction. This is due to water being flung towards the outer bend as
it flows around the meander, this causes greater erosion which deepens the
channel, in turn the reduction in friction and increase in energy results in greater
erosion. This lateral erosion results in undercutting of the river bank and the
formation of a steep sided river cliff.
Slip off slope
In contrast, on the inner bend water is slow flowing, due to it being a low energy
zone, deposition occurs resulting in a shallower channel. This increased friction
further reduces the velocity (thus further reducing energy), encouraging further
deposition. Over time a small beach of material builds up on the inner bend; this is
called a slip-off slope
As the outer banks of a meander continue to be eroded
through processes such as hydraulic action the neck of the meander
becomes narrow and
Eventually due to the narrowing of the neck, the two outer
bends meet and the river cuts through the neck of the meander
usually during a flood event when the energy in the river is at its
highest. The water now takes its
shortest route rather than flowing around the bend.
Deposition gradually seals off the old meander bend forming a new
straighter river channel.
Due to deposition the old meander bend is left isolated from
the main channel as an ox-bow lake.
Over time this feature may fill up with sediment and may gradually
dry up (except for periods of heavy rain). When the water dries up,
the feature left behind is known as a meander scar.
This is the line of fastest flow in a stream and is usually exaggerated
variation of the stream channel shape that crosses to the outside of each
meander at the point of inflection. Because erosion is greatest where the
stream flow is fastest, the thalweg is also the deepest channel in the
stream. It is found in the top middle of a straight channel because this is
where the water is the deepest and is where there is the least friction.
Alluvium (material in a
river) is dropped by the river
when it loses momentum as
it enters a wide, flat valley
known as a piedmont, after
leaving a narrow mountain
channel. This happens as
water velocity, gradient and
speed reduces as the water
enters a wide unconfined
channel, so it is deposited at
the junction. It is the
terrestrial (land) equivalent
of a delta
Meander - a bend in a river
River Cliff - a small cliff formed on the outside of a meander bend due
to erosion in this high energy zone.
Slip off Slope - a small beach found on the inside of a meander bend
where deposition has occurred in the low energy zone.
Ox-bow lake - a lake formed when the continued narrowing of a
meander neck results in the eventual cut through of the neck as two
outer bends join. This result in the straightening of the river channel
and the old meander bend becomes cut off forming an ox-bow lake.
Meander scar - feature left behind when the water in an ox-bow lake
The Human Impact
The influence of human activity on the
iodide, solid carbon
dioxide (dry ice) or
into the air to
droplets to form.
-Mixed success but
in Australia and the
USA it has increased
precipitation by 10-
-In Urban areas
precipitation can be
increased by 10%
due to extra
pollutants in the air
The influence of human activity on the
The human impact on evaporation and
evapotranspiration is relatively small in relation to the
rest of the hydrological cycle but nevertheless important.
There are a number of impacts:
Dams: the construction of large dams have increased
evaporation. For example: Lake Nasser behind the
Aswan Dam loses up to a third of water due to
evaporation. Water loss can be reduced by using
chemical sprays or covering the dam in a form of plastic.
Urbanisation: Leads to a huge reduction in
evapotranspiration due to the lack of vegetation. There
may also be a slight increase in evaporation because of
higher temperatures and increased surface storage.
The influence of human activity on the
If a river’s drainage basin or floodplain has been heavily urbanised,
a river becomes much more prone to flooding. Urbanisation
(generally) involves the laying down of tarmac and concrete,
impermeable substances that will increase surface runoff into the
river and therefore increase the river’s discharge.
Urbanisation often involves deforestation. This (obviously) reduces
vegetation cover, reducing infiltration and increasing surface runoff
into a river.
To stop roads and streets from flooding, humans will often build
storm drains that collect rainwater and channel it into a river or
stream. Humans will often send this water to the local river or
stream so, although roads and streets won’t be flooded by rainwater
the entire town will be as the rainwater enters the river much faster
than it would without the storm drains.
Flooding: (Physical Factors)
Flooding occurs when a river’s discharge exceeds its channel’s volume causing the river to
overflow onto the area surrounding the channel known as the floodplain. The increase in
discharge can be triggered by several events. The most common cause of flooding is prolonged
rainfall. If it rains for a long time, the ground will become saturated and the soil will no longer
be able to store water leading to increased surface runoff. Rainwater will enter the river much
faster than it would if the ground wasn’t saturated leading to higher discharge levels and floods.
As well as prolonged rainfall, brief periods of heavy rain can also lead to floods. If there’s a
sudden “burst” of heavy rain, the rainwater won’t be able to infiltrate fast enough and the water
will instead enter the river via surface runoff. This leads to a sudden and large increase in the
river’s discharge which can result in a flash flood.
Although many floods are triggered directly by precipitation just a few hours after it falls some
floods can be triggered by precipitation that fell many months ago. Precipitation that falls as
snow can remain as snow on the ground until it melts. This mightn’t be until the end of winter,
so potentially several months. When the snow does melt, large volumes of meltwater will enter
the river increasing its discharge and triggering floods. These floods are often annual, occurring
every year when snow melts in the spring. In Bangladesh, for example, melting snow in the
Himalayas triggers annual floods in the summer.
Flash floods can also be triggered by slightly more catastrophic events. Erupting volcanoes can
trigger very large flash floods called jökulhlaups when glaciers are partially or even fully melted
by an erupting volcano or some other form of geothermal activity. The meltwater can enter
rivers and greatly increase the river’s discharge leading to a flood. The eruption of
Eyjafjallajökull1 in 2010 triggered jökulhlaups as the volcano had been capped by a glacier that
melted when it erupted. Similarly earthquakes can bring about landslides – loosened soil may
be deposited in rivers causing overflowing.
Effects of flooding
Flooding can have numerous social, economic and environmental effects that can vary
depending on the demographics of a population and the economic development of an area.
The biggest, most obvious effect is death. Floods, especially flash floods, will kill people. Flood
water can travel surprisingly quickly and weighs3 a lot, so people can easily get swept away by
floods. Large chunks of debris and objects like cars can easily get picked up by floodwater and
can easily kill a person should they get hit by the debris. In a LEDC, you’re generally going to get
much more deaths than you would in a MEDC. In a MEDC, people and governments are better
prepared for floods. Rescue services can be dispatched to a flood quickly in a MEDC whereas in a
LEDC, rescue teams mightn’t arrive until several hours after the flood started.
During a flood, sewage pipes are often broken and raw sewage leaks into the floodwater. This
has two effects. First, it contaminates not just floodwater but drinking water too which leads to a
spread of waterborne diseases such as cholera especially in LEDCs where emergency drinking
water mightn’t be available. Second, the sewage gets into people’s homes.
In LEDCs, famines can follow floods which can lead to even more deaths. Floods will commonly
inundate farmland because farmland normally develops on floodplains. If the floodwater is
polluted by sewage, it will contaminate the farmland and make any food grown on it dangerous
to eat. Furthermore, cattle are often killed by floods which can lead to people starving because
they either don’t have a source of food or don’t have a source of income to buy food with.
Effects of Flooding
The big economic effect of a flood is property damage. Water can cause a lot of damage to
property and when it picks up large chunks of debris such as cars, it can act like a wrecking ball,
taking out chunks of buildings when cars crash into them. Very large and powerful floods can
even dislodge buildings from their foundations and move them. In a MEDC, property damage is
often extensive as people have lots of expensive possessions. This isn’t the case in LEDCs but
that’s only because people don’t have a lot to lose in the first place. This means that the overall
cost of a flood is generally substantially higher in a MEDC than in a LEDC.
Floods can cause extensive damage to infrastructure such as power lines, roads, water pipes etc.
Bridges frequently collapse during a flood as they aren’t designed to withstand the high
discharge of the river. The Northside Bridge in Workington, Cumbria collapsed when there were
large floods in 2009. Repairing bridges and other types of infrastructure is very costly. Not only
this, it can lead to a decline in the local economy as businesses are unable to operate without
power or road connections. Unemployment can even increase if businesses are unable to fully
recover from a flood. The economic impact of infrastructure damage and unemployment is
larger in MEDCs since these countries have modern and expensive infrastructure in place. In
LEDCs, this infrastructure is lacking, so there isn’t much economic damage. In fact, in a LEDC,
floods can lead to positive economic effects in the long term. An influx of funding to a less
developed area from charities and NGOs after a flood can result in new infrastructure being
constructed that is substantially better than the previously existing infrastructure. This, in turn,
creates new economic opportunities in an area by, for example, creating new trade routes.
Another economic benefit comes from when a river floods and deposits sediment across the
floodplain. This improves the fertility of the floodplain and can improve agricultural yield in an
area (assuming the floodwater wasn’t polluted).
Effects of flooding
Floodwater that is contaminated with sewage will pollute
rivers and land when it drains back into the river.
Similarly, if the river floods onto farmland, the water can
be polluted by pesticides and other chemicals sprayed
onto the farmland that, when drained back into the river,
can pollute it and kill off wildlife that inhabits the river. If
the floodwater isn’t polluted though, flooding can create
wetlands that can help introduce new habitats for many
species of animals.
Vegetation may be destroyed, along with natural habitats
and animal species.
Reducing impacts of floods
Using weather satellites to predict high rainfall amounts
Estimating rainfall and snow pack amounts
Using river gauges to study river levels over time and map flood recurrence
Create computer flooding models including information on human infrastructure and what
would be most at risk.
2) Preparing people for floods
Loss sharing adjustments (e.g. disaster aid and insurance)
Removal of settlements from flood plains
Education on what to do in a flood.
3) Prevention and amelioration of floods
There are two types of flood protection methods which act to prevent or ameliorate flooding.
Hard engineering= Defence schemes that halt a rivers natural processes.
Soft engineering = involves the use of the natural environment surrounding a river, and the
schemes often work with the river’s natural processes.
Examples on next slides…
Flood and drought control
Irrigation – 60% water from
Aswan Dam is used for
irrigation and up to 4000km
of the desert is irrigated.
Hydro-electricity – accounts
of 7000million kW hours each
Recreation and tourism –
Aswan Dam contributes
500million to the Egyptian
economy each year.
Water losses – provide less than half
the amount of water expected due to
Displacement of population – up to
100000 Nubian people have been
removed from their ancestral homes
Seismic stress – the earthquake of
November 1981 is believed to have
been caused by the Aswan Dam; as
water levels in the dam decrease so
too does seismic activity increase.
Loss of nutrients – it is estimated
that it costs 100million to buy
commercial fertilisers to make up for
the lack of nutrients each year.
Diseases have spread – such as
Is an extended period of dry weather leading to conditions of extreme
Absolute drought is a period of at least 15 days with less than
0.2mm of rainfall
Natural causes of droughts: Insufficient rainfall can be caused by
1. Global atmospheric circulation leads to descending air over sub-
tropical areas and therefore a lack of rain ( no clouds are formed)
2. An area’s distance from the sea can limit the amount of water
carried by the wind
3. Some places are affected by rain shadow effects. This is where air
passes over mountains, and rain is released, but the air has
therefore lost all its moisture as it reaches the far side of the
Human causes of droughts
Deforestation: Reduced vegetation cover results in lower
rates of transpiration. Less water vapour in the
atmosphere leads to fewer clouds formed. Soil exposed to
direct sunlight dries up quickly.
Enhanced greenhouse effect: Global warming can cause
droughts in places with drier climates. High
temperatures increase the rate of evaporation, drying up
land, rivers and lakes.
Over use of water: places with rapid population growth
require more water for hoes, industry and agriculture.
Water sources such as rivers and ground water may not
be able to sustain an increase in water usuage.
Impacts of droughts
Economic impacts range from direct losses in the broad agricultural and
agriculturally related sectors (including forestry and fishing), to losses in recreation,
transportation, banking, and energy sectors. Other economic impacts would include
added unemployment and loss of revenue to local, state, and federal government.
Environmental losses include damages to plant and animal species, wildlife habitat,
and air and water quality; forest and range fires; degradation of landscape quality;
and soil erosion. These losses are difficult to quantify, but growing public awareness
and concern for environmental quality has forced public officials to focus greater
attention on them.
Social impacts mainly involve public safety, health, conflicts between water users,
and inequities in the distribution of impacts and disaster relief programs. As with all
natural hazards, the economic impacts of drought are highly variable within and
between economic sectors and geographic regions, producing a complex assortment
of winners and losers with the occurrence of each disaster.
How to reduce impacts of droughts:
Management of watershed and agricultural
practices: reistance crops prevent desertification.
Using proper irrigation techniques – helps conserve
Cloud seeding – enables water droplets to form
Hard engineering techniques
Dams – these are built across the river channel to stop the flow of the water. A lake or reservoir will form
behind the dam and water can be let out in a controlled manner to prevent flooding. One of the
disadvantages of a dam is that a large area of land has to be flooded and this can destroy natural habitat or
even mean humans have to be relocated. Dams are expensive to build and maintain although they can be
used to produce HEP. Dams do provide a high level of control to reduce the chances of flooding and are very
effective in reducing the risk in this way however, they stop sediment from flowing downstream and this can
lead to greater erosion in a similar way that holding back material on the coast by using groynes reduces
protection further along the cliff. A good example of a dual purpose dam is the Karibaon
the Zambezi in Mozambique .
Levees (reinforcing or man-made) – a levee is a naturally occurring feature on the bank of a river in
the middle and sometimes lower stages of a river, on the flood plain. When the river floods, any load it is
carrying in suspension is dropped and the heavier material is dropped first, just on the river bank. Over time
this will build up and has the effect of increasing the capacity of the river as the banks are higher. These
levees may be enforced in some way by humans. Planting vegetation on them helps to protect them. Adding
even more height to them is also effective. An artificial levee can be built from scratch and this has the same
effect as a natural one. Levees are very common in Holland where much of the country (more than 25%) is
below sea level and is at risk from flooding. The Dutch have a complex network ofwing dykes (see below) and
levees to protect the land.
Channel straightening – getting the water out of an area at risk of
flooding as quickly as possible is a way to reduce likelihood of problems
during times of peak discharge. Meanders may be cut through and the
channel is literally straightened so that water can move very quickly.
This takes the water away from built up area for example where water
can cause havoc to houses and businesses. Straightening has been one
of many management techniques used along the Mississippi in
the USA. This of course means that the water reaches further
downstream more quickly too. Straightening often just diverts the
problem elsewhere rather than providing a solution.
Wing Dyke – these work in a similar way to groynes on a beach in that
they trap sediment moving through the river channel. They are usually
placed in pairs either side of the channel and once sediment has built
up behind them water is forced between them more quickly. Some good
examples are on theMissouri river in the USA. As with channel
straightening, they mean that water reaches downstream more quickly
so careful planning is needed when they are installed to lessen the
impact of increased discharge further along the river’s path.
Afforestation – This may be part
of wetland and river restoration when vegetation
may be planted to return an area to its original form.
Large scale afforestation can not only lower flood
risk by intercepting and storing water but it can
reduce the erosion of soil which ends up in the river
channel. Material in the river channel effectively
decreases its depth and the river level is higher,
increasing flood risk during times of high discharge.
Afforestation is widely used in Australia to achieve
several things which includes helping to manage
water flow in a catchment area (drainage basin).
Meteorology= The study of the Atmosphere
Weather= Short term atmospheric conditions of a particular place.
Climate= Long term atmospheric conditions of a particular place.
Atmosphere=is an area of transparent gases surrounding the earth
The gases stretch to 500-1000km above the earth’s surface
There are several layers to the atmosphere
The area between layers is called a pause
Weather occurs only in the lowest part of the earth’s atmosphere
called the troposphere.
Troposphere - layer characteristics:
Decrease of temperature with height (6.4 degrees per 1000m).
Increase in wind speeds with height.
Fall in pressure with height.
An unstable layer due to the presence of cloud, pollution water vapour
The tropopause marks the outer edge of the troposphere and the limit to
the earth's weather and climate.
Stratosphere - layer characteristics:
Temperatures increase with height in this layer, and it is here that ozone
is concentrated, which absorbs UV radiation from the sun.
Winds increase with height but pressure falls.
The boundary is marked by the stratopause.
Mesosphere - layer characteristics:
A rapid fall in temperature with height, caused by a lack of water
vapour, cloud and dust).
Temperatures are extremely low and winds high.
Its boundary is marked by the mesopause.
Thermosphere - layer characteristics:
The outer layer of the atmosphere.
A rapid increase in temperature with height, exceeding 1000 degrees
Is the amount of energy entering, leaving and transferring within
Some parts of the earth receive lots of solar energy (surplus),
whilst others receive little (deficit).
In order to transfer this energy around, to create some sort of
balance, the Earth uses pressure belts, winds and ocean currents.
The energy budget has a huge effect on weather and climate .
Energy budgets are usually considered at a global scale (macro
scale) and can be at a local scale (micro scale).
Incoming radiation (short wave radiation)
Incoming solar radiation (insolation) is short wave radiation that comes
directly from the sun (100%)
19% is reflected off clouds. And 6 % is lost to scattering ( radiation
diverted by gas molecules)
17% of this is absorbed by the gases in the atmosphere such as carbon
dioxide and ozone.
4% of this is absorbed by clouds
7% is reflected by the earth’s surface (called albedo)
So only 47% actually reaches the earth’s surface to be absorbed
Outgoing Radiation (Long wave radiation)
Energy received by the earth is converted into heat energy when it reaches
the surface. As the ground warms, some is re-radiated as long wave
8% of this re-radiated energy is lost to space
Evaporation and condensation account for a loss of 25 % of the heat energy
from the earth as heat energy is used up when liquid is turned into vapour
(this is called latent heat transfer)
7% of this re-radiated energy is absorbed by clouds, water vapour and CO2
Global Energy Budget
The daytime energy
budget consists of Six
I = Insolation
R = Reflected Solar
S = Surface Absorption
L = Latent Heat
S = Sensible Heat Transfer
L = Long wave radiation
Daytime energy budget
Atmosphere’s main energy input which is strongly influenced by cloud cover and latitude. At the equator, the
sun’s rays are more concentrated than at the poles. (75% of insolation reaches equator, 5% reaches Poles)
Reflected Solar Radiation-
The proportion of reflected solar radiation varies greatly with the nature of the surface.
The reflectivity of a surface is known as the albedo.
Fresh snow & ice have the highest albedos, reflecting up to 95% of sunlight.
Ocean surfaces absorb most sunlight, and so have low albedos.
Energy arriving at the surface has the potential to heat that surface, as heat is absorbed by it.
The nature of the surface has an effect, e.g. If the surface can conduct heat rapidly into the lower layers of the
soil its temperature will be low. If the heat is not carried away quickly it will be concentrated at the surface &
result in high temperatures there.
Latent Heat (evaporation)
The turning of liquid water into vapour (evaporation) it consumes a considerable amount of energy.
When water is present at the surface, a proportion of the incoming solar radiation will be used to evaporate it.
Consequently, that energy will not be available to raise local energy levels and temperatures.
Sensible Heat Transfer
This term is used to describe the transfer of parcels of air to or from the point at which the energy budget is
being assessed. If relatively cold air moves in, energy may be taken from the surface, creating an energy loss. If
warm air rises from the surface to be replaced by cooler air, a loss will also occur. This process is best described
as convective transfer, and during the day it is responsible for removing energy from the surface and passing it
to the air.
Long wave Radiation
This is emitted by the surface, and passes into the atmosphere, and eventually into space.
There is also a downward-directed stream of long-wave radiation from particles in the atmosphere
The difference between the 2 streams is known as the net radiation balance.
During the day, since the outgoing stream is greater than the incoming one, there is a net loss of energy from
Daytime energy budget
The night time energy budget consists of Four processes
L= Long wave radiation
L= Latent Heat (condensation)
S= Sensible Heat Transfer
Night time Energy Budget
Long Wave Radiation: During a cloudless night, little long wave
radiation arrives back at the surface of the ground from the
atmosphere and consequently the outgoing stream is greater than
incoming stream leading to a net loss of energy. Under cloudy
conditions this loss is reduced because long wave radiation can reflect
off clouds back to the surface; they act like a blanket around the earth.
Latent Heat (Condensation): At night water vapour in the air, close to
the ground can condense and form dew as the air is cooled by the
cold surface. This releases stored energy, resulting in a net gain of
Subsurface supply: Heat transferred by the sun to the surface during
the day, may be released back to the surface at night which can off set
the night time cooling at the surface
Sensible heat transfer still occurs and cold air moving into an area
may reduce temperatures whereas warm air moving in will raise
Night Time Energy Budget
Absolute Humidity: The amount of water in the atmosphere
Relative Humidity: Ratio of the amount of water vapour currently
in the air compared to how much the air can hold at that
temperature (usually expressed as a percentage).
Saturated Air: Air with a relative humidity of 100%
Dew point: The temperature at which condensation occurs,
allowing the formation of dew, mist or fog.
WARMER AIR CAN TYPICALLY HOLD MORE WATER VAPOUR
THAN COOLER AIR CAN
Mist and Fog: are cloud at ground level. A cloud is a collection of water droplets.
Mist occurs when visibility is between 1000m and 5000m. Whereas Fog occurs
where visibility is below 1000m. So Fog is thicker cloud cover than mist.
These clouds form at ground level because, air can only hold a certain amount of
moisture. Colder air can hold less moisture than warmer air. Once this
maximum amount of moisture is reached, air is saturated and the water vapour in
the air turns to liquid (dew point). This is when clouds form as condensation of
water vapour to water droplets occur.
For these clouds of fog/mist to form close to the ground level , one of two things
must have occurred:
1. Air must have been cooled close to the ground
e.g. Advection Fog: As warm, moist air passes horizontally over a cold surface, it is chilled,
and condensation takes place as the temperature of the air is reduced and therefore it
reaches dew (saturation) point
e.g. Radiation Fog: Occurs in low lying areas when the ground surface loses heat at night
by long wave radiation and therefore the air immediately above it is cooled causing
condensation and fog.
2. More water vapour must have been added to the atmosphere close to the ground.
This can occur over warm, wet surfaces like large lakes, where water is evaporated from
the warm surface of the lake and condenses in the cold air above to form fog)
For mist or fog to form, condensation nuclei are needed (e.g. dust or salt particles in the
air). These are more common in urban or coastal areas, so mist of fog are more common
Mist and Fog
During the day the ground is heated by the sun’s short wave radiation, and then
after a short time, it heats the air above it when it emits long wave radiation.
At night the ground surface and the air, lose the heat energy they have
absorbed during the day. However, the ground loses heat energy faster than the
air as it is a more efficient conductor of heat. By the end of the night the ground
surface is therefore very cold, and the air directly above it will be cooled too
due to close proximity to the surface. However, the air layer above this, will be
warmer as it has cooled at a slower rate than the ground surface, causing a
temperature inversion. Temperature inversions usually occur during anti-
cyclones when there is little air turbulence to allow these layers to mix.
Temperature inversions act like a lid, causing pollutants to remain in the lowest
A relative increase in temperature with height in the lower part of the atmosphere
The Global Energy Budget
• The atmosphere is an open system
• 47% of insolation reaches the earths surface
• The atmosphere receives 39% of heat back from the earths surface.
• Most incoming short-wave radiation is let through, but some outgoing long
wave radiation is trapped by green house gases, known as the green house
• There are variations in the amount of solar radiation due to two factors. Latitude
and Season, resulting in an unbalance
• (+) positive budget in the Tropics (more energy received)
• (-) negative budget in the Poles (more energy lost)
• At the equator there is little seasonal different
• At higher latitudes, there are large seasonal differences due to decreased
insolation and changes in day length.
Temperature decreases with height above sea level:
The atmosphere is heated from ground level upwards via long-
The higher up a mountain, the smaller the ground surface area
available to heat the atmosphere above.
This, in combination with a decrease in the ability of the air to
retain heat results in lower temperatures.
The Coriolis Effect is the
deflection of moving
objects caused by the
easterly rotation of the
Earth. In the northern
hemisphere, air moving
from high to low is
deflected to the right of
its path and to the left in
Wind: The horizontal movement of air on the Earth’s surface. It is a
result from the difference in air pressure and always moves an
area of high to low pressure. When the air temperature of an area
increase the air expands and rises reducing the air pressure. When
the temperature of the air decrease the air contracts and becomes
denser and sinks increasing the air pressure. The temperature of
the wind is influenced by the origin of where the wind has come
Planetary surface winds
Pressure is measure in Malabar's (mb) and is represented by isobars (lines
of equal pressure).
Poor weather = low pressure
Fine Weather = high pressure
The North Hemisphere has much more land so there is a lot of seasonal
change, whereas the South Hemisphere has a lot of water, so little seasonal
Air pressure: The gases in the atmosphere press down on the Earth’s
surface, exerting a force called air pressure.
It is differences in air pressure that cause different weather in our
atmosphere. You don’t feel it because you have equal pressure pushing out
from inside your body
Winds and air pressure: Changes in air pressure make winds blow. They
are due to seasonal differences in the overhead sun.
Air moves from areas of high pressure to areas of low pressure, and this
“Winds blow from high to low !“
• Doldrums (ITCZ): Areas of
pressure in which sailing ships
have a hard time moving due
to lack of wind.
• Trade winds (30>equator)
Where lots of ships travel die
to strong easterlies.
• Coriolis effect: Due to the tilt of
the earth and its movement on
its axis, winds and ocean
currents curve instead of
traveling straight. This curving
is known as Coriolis effect.
• Hoarse Latitude: (30-60) little
wind, so in the past, horses
were thrown over board to
remove some weight.
• Summer in Southern Hemisphere, means winter in Northern Hemisphere…
this increases differences in polar and equatorial air.
• High level of Westerly's are stronger in NH in Winter
Angle of the overhead sun, latitude and thickness of atmosphere: Lower
latitudes (equatorial regions) have higher temperatures than higher latitudes
(Poles) this is as a result of the amount of heating that each area receives. Places
near the equator receive direct heat on a small surface area, and experience little
energy loss via absorption, scattering and reflection, as there is a relatively small
amount of atmosphere to pass through. Towards the Poles, the surface area to be
heated increases, as does the amount of atmosphere to pass through, increasing
losses via, absorption, scattering, and reflection.
Height above sea level: It is important to remember that the atmosphere is heated
from ground level upwards via long-wave radiation. The higher up a mountain
you go, the smaller the surface area available to heat the atmosphere above. This,
in combination with a decrease in the ability of the air to retain heat results in
Distance from land and sea: Land and sea have vastly different specific heat
capacities (the amount of energy needed to raise 1kg of a substance by 1 degree).
They have different abilities to absorb, transfer and radiate heat energy. Generally,
land surfaces respond to heating on a daily basis (diurnal) meaning that
differences between day and night temperatures can be into double figures, but
sea surfaces respond over a period of months and retain heat for longer. The sea
heats up and cools down more slowly than the land, acting to moderate
temperatures for coastal locations.
Exploring variations in
Temperature and winds:
Surface pressure belts There are many pressure belts existing on Earth, due to the rotation and tilt of the Earth on its axis,
In the equator region, warm air rises causing a low pressure belt. Whereas at the polar regions cold air
sinks, thus creating a high pressure belt. The sub polar regions, around latitudes 60-65 degrees North
and South of the equator, the rotation of the Earth flings the bulk of the air towards the equator,,
creating a low pressure belt. These four main pressure belts however are not continuous because the
surface of the earth is composed of both land and water, which are heated in different ways.
1. The first main pressure belt is the equatorial low pressure belt, which extends 5 degrees north and
south. Being at the equator is receives direct sunlight and thus the air here is warm, this air expands
and rises, creating low pressure in the process. This is a region of calm air known as the Doldrums,
due to its very little winds.
2. The second pressure belt, is the subtropical high pressure belt, that coincides with latitudes of 30-
33degrees north and south. The air that rises eventually meets the tropopause where it can rise no
further, it cools while rising and spreads outwards towards the poles, gradually cooling back down
to the surface of 30degrees, which causes an increase in air pressure. The air flings off the polar
region due to the rotation of the earth and also descends in this region thus adding to the already
high pressure existing in this region. The subtropical high pressure belt is an area of low winds and
so it is also known as the horse latitudes (an area where ship crew would throw horses overboard to
lighten the load and spare food after being caught in these areas).
3. The third major belt is the sub polar low pressure belt at latitudes 60-65degres. It is created mainly to
the rotation of the Earth, which swings the bulk of air towards the equator, these are areas of
storminess, especially in winter.
4. The forth and final pressure belt is the polar high pressure belt, located in the polar region. This belt
is created because in this region the air is extremely cold and heavy, leading to a high pressure.
5. Pressure belts are caused mainly due to the temperature differences on the Earths surface and
therefore move in response to the migration of the sun. The sun shines vertically oer the Tropical of
Cancer on June 21st. At this time all the pressure belts move about 5degrees North. On the 21st
September, the sunshine's vertically over the equator and on December 22nd the sun shines vertically
over the tropic of Capricorn, thus all belts move 5degrees south (winter). The shifting of pressure
belts affects the direction of wind flow, causing wind belts during the year and as these wind belts
shift with the season, belts of precipitation change also.
Ocean currents can be either warm or cold and they act to either raise or
lower temperatures of the coastal areas. Warm currents transfer heat away
from the Equator and towards the poles whilst cold currents carry water
towards the Equator. Major ocean currents circulate clockwise in the NH
and anticlockwise in the SH. They are caused by the influence of prevailing
winds blowing across the oceans, and mean in the same motion.
The dominant pattern is roughly circular and known as a GYRE
Gyres move clockwise in NH and Anti clockwise in SH, due to
Like atmospheric circulation, ocean currents help to redistribute energy
across the earth. Because they cover 67% of the earth's surface, the
oceans receive 67% of the sun's energy that reaches earth. The ocean
holds on to this heat for longer than the land does and the ocean
currents move this heat around, from the tropics to higher latitudes. In
total, ocean currents transfer about 25% of the global heat budget.
Ocean currents flowing away from the equator are called warm
currents. The water in these currents is not necessarily warm, but it's
warm compared to what you would expect for that latitude. The Gulf
Stream is a good example of a warm current. If a current
flows towards the equator it is a cold current, for example the Canaries
Water always flows down toward the lowest point.
Water’s density is determined by the water’s temperature
and salinity (amount of salt).
Cold water is denser than warm water.
Water with high salinity is denser than water with low salinity.
Ocean water always moves toward an equilibrium, or balance. For
example, if surface water cools and becomes denser, it will sink. The
warmer water below will rise to balance out the missing surface water
1. Cold, salty water from polar regions sink into the depths and move
towards the equator
2. The densest water is found around Antarctica, due to the amount of
salt left in the water after ice is formed.
3. Surface currents bring warm water to North Atlantic from the
Indian and Pacific Oceans, loosing heat in the northern areas. Water
sinks, reversing convection current.
4. North Atlantic Is warmer than North Pacific – this leads to more
evaporation in the Atlantic. This evaporation leaves more salt
behind, making water denser, causing it to sink. Water will then
travel back to the North Pacific, picking up more water which
reduces its density.
Ocean Conveyor belt
The ocean is not a still body of water. There is constant motion in the
ocean in the form of a global ocean conveyor belt. This motion is
caused by a combination of thermohaline currents (thermo =
temperature; haline = salinity) in the deep ocean and wind-driven
currents on the surface. Cold, salty water is dense and sinks to the
bottom of the ocean while warm water is less dense and remains on
The ocean conveyor gets its “start” in the Norwegian Sea, where
warm water from the Gulf Stream heats the atmosphere in the cold
northern latitudes. This loss of heat to the atmosphere makes the
water cooler and denser, causing it to sink to the bottom of the ocean.
As more warm water is transported north, the cooler water sinks and
moves south to make room for the incoming warm water. This cold
bottom water flows south of the equator all the way down to
Antarctica. Eventually, the cold bottom waters return to the surface
through mixing and wind-driven upwelling, continuing the conveyor
belt that encircles the globe.
Ocean conveyor belt
These are Meso (small) scale / local winds caused a pressure gradient
between land and sea.
Created on a daily basis, as a result of the differences in heating and
cooling of the land and sea (i.e. specific heat capacities)
During the day, onshore winds are created, as land temperatures are
higher than sea temperatures; thus low pressure is formed over the
land, air rises and cools
Cool air then drifts out over the sea, increasing in density and starts to
sink. Thus creating high pressure over the sea.
The sea breeze is caused by air flowing from high to low pressure (sea
The situation is reversed at night, leading to high pressure over the land
and thus an off shore breeze. Pressure is lower over the sea as it is
warmer than the land, and air above it rises
Local pressure gradients
The two winds that exist in mountain
and valley locations are uphill, anabatic
winds and downhill, katabatic winds.
Anabatic: An uphill wind develops
under sunny morning conditions
when slopes receive sunlight, become
warm and then heat the atmosphere
above them. Air above these slopes
expands and rises. A pressure
gradient results accompanied by a
strong uphill wind
Katabatic: Downhill winds form, as
heat is lost from a valley during the
evening. Colder, denser air from
higher areas drains into the valley
Mountain and Valley winds
General circulation model
• Warm air is transferred pole
wards and is replaced by cold
air moving towards the
• Air that rises is associated with
low pressure, whereas air that
sinks is associated with high
• Low pressure produces rain,
while high pressure produces
• There are three major cells
present: Hadley, Ferrell and
• They shift northwards and
southwards throughout the
year due the shift in location of
the sun’s rays focusing most
intensely on the Earth’s surface.
At the equator, trade winds meet and
form the Inter-tropical Convergence
Zone (ITCZ). Winds are light and
known as the doldrums. Air is warm
and unstable, having crossed warm
oceans, and rises due to convection
currents. As the air rises it cools and
large cumulonimbus clouds develop.
The pressure at the equator is low.
Eventually, the rising air diverges 30
degrees north and south of the
equator,where it cools via radiation
and therefore falls. As the air
contracts, more air can move in,
increasing the air pressure at the
subtropical high pressure zone. The
dense air will then sink, causing
stability. Air is then either returned to
the equator at ground level, or travels
to the Poles as warm south-westerly
The Ferrel cell circulation is not as easily explained as the Hadley and
Polar cells. Unlike the other two cells, where the upper and low-level
flows are reversed, a generally westerly flow dominates the Ferrell cell
at the surface and aloft. It is believed the cell is a forced phenomena,
induced by interaction between the other two cells whereby it acts like
a gear. The stronger downward vertical motion and surface
convergence at 30°N coupled with surface convergence and net
upward vertical motion at 60°N induces the circulation of the Ferrel
cell. This net circulation pattern is greatly upset by the exchange of
polar air moving southward and tropical air moving northward. This
best explains why the mid-latitudes experience the widest range of
This is the northernmost cell of circulation and its mean position is
between 60°N and the North Pole. At the pole, cold, dense air
descends, causing an area of subsidence and high pressure. As the
air sinks, it begins spreading southward. Since the coriolis force is
strongest at the poles, the southward moving air deflects sharply to
the right. This wind regime is called the surface polar easterlies,
although the upper winds are still predominantly from the
southwest. Near 60ºN, the southeasterly moving air moving along
the surface collides with the weak, northwesterly surface flow that
resulted from spreading air at 30°N. This colliding air rises, creating
a belt of low pressure near 60°N.
At the Equator, the sun warms the Earth and this transfers heat to the air above,
which causes it to rise. The rising air creates an area of low pressure with clouds
and rain - this is called the Inter Tropical Convergence Zone (ITCZ) and is where
the trade winds meet in the equatorial zone.
As the rising air cools, it begins to move away from the Equator and then further
cooling, increasing density and diversion by the Coriolis force cause it to slow
down and descend, forming the descending limb of the Hadley Cell.
The cool air sinks at 30o north and south of the Equator, creating an area of high
pressure with clear skies and stable conditions - this is where sub-tropical jet
streams are found
The cool air reaches the ground surface - some is returned to the Equator as
surface winds (trade winds) whilst the remaining air is diverted polewards.
60o north and south of the Equator, warm south-westerlies/ north-
westerlies which have collected moisture from the sea meet the cold air from the
Poles - the warmer air is less dense and this causes it to rise, creating an area
of low pressure
Some of the air joins the Ferrell Cell and moves back towards the Equator and the
rest joins the polar cell and moves towards the Poles
At the poles, the cool air sinks to create a high-pressure zone - the high-
pressure is then drawn back to the Equator as part of the surface winds
Combined effect of the three
Between the different atmospheric cells high up in the tropopause at a
height of about 10km are the jet streams, named the polar jet stream
(40-60°N+S) and the subtropical jet stream (25-30°N+S). These jet
streams move air at a high speed (up to 300km/h) around the Earth
horizontally and give rise to Rossby waves. The jet streams were first
discovered when Zeppelins were blown off course in WW1. The
greater the temperature difference – the greater the jetstream.
Rossby waves were discovered by Carl-Gustaf Rossby, a Swedish
meteorologist, in the 1930’s. They are waves or zigzags in the jet
streams as the travel around the Earth. The number of waves varies
throughout the year but usually in summer it’s between four and six
while in winter it’s three. Rossby waves are formed by major releif
barriers (like mountains), thermal differences and uneven land-sea
Jet streams have also been known to influence flights, for example it’s
quicker to travel by aeroplane from London to New York then it is the
other way around because the altitude planes travel at is similar to
these high speed winds.
Jet Streams and Rossby Waves
Jet stream and Rossby Wave
The wave like
meandering of air is
described as a
rossby wave, which
are affected by
barriers. As the
more exaggerated it
leads to blocking
A weather front is a term used in meteorology to describe the boundary
where two air masses converge, sparking weather events. There is a cold
front and a warm front.
• Cold fronts often come with thunderstorms or
other types of extreme weather. They usually
move from west to east. Cold fronts move faster
than warm fronts because cold air is denser,
meaning there are more molecules of material in
cold air than in warm air.
• Strong, powerful cold fronts often take over
warm air that might be nearly motionless in the
atmosphere. Cold, dense air squeezes its way
through the warmer, less-dense air, and lifts the
warm air. Because air is lifted instead of being
pressed down, the movement of a cold front
through a warm front is usually called a low-
pressure system. Low-pressure systems often
cause severe rainfall or thunderstorms.
• Warm fronts usually show up on the tail end
of precipitation and fog. As they overtake cold
air masses, warm fronts move slowly, usually
from north to south. Because warm fronts
aren't as dense or powerful as cold fronts,
they bring more moderate and long-lasting
weather patterns. Warm fronts are often
associated with high-pressure systems, where
warm air is pressed close to the ground. High-
pressure systems usually indicate calm,
Atmospheric moisture exists in all three states – vapour, liquid and
Energy is used in the change from one phase to another.
When evaporation occurs, it takes 600 calories of heat to change 1
gramme of water from liquid to vapour, thus a heat loss occurs.
Condensation however released locked latent heat, causing a rise in
Changes between vapour and ice releases heat when vapour is
converted to ice.
By contrast, heat is absorbed in the process of sublimation (snow
patches that disappear without melting.
Moisture in the atmosphere
Initial humidity of the
air- if air is very dry
evaporation occurs; if
it is saturated then
very little occurs.
Supply of heat – the
hotter the air, the
more evaporation that
Wind Strength –
under calm conditions
air becomes saturated
rapidly and therefore
when either enough
water vapour is
evaporated into an air
mass for it to become
saturated or when the
temperature drops so
that dew point is
reached. The Cooling
occurs in three main
1. Radiation cooling of
2. Contact cooling of the
air when It rests over a
3. Adiabatic cooling of
air when it rises.
Precipitation refers to all forms of deposition of moisture from the atmosphere
– including rain, hail , snow and dew. Because rain is the most common form
of precipitation in many areas, the term is sometimes applied for rainfall
alone. For any type of precipitation to form, clouds must first be produced.
When minute droplets of water are condensed from water vapour, they float
in the atmosphere as clouds. If droplets coalesce they form large droplets
which, when heavy enough to overcome gravity, fall as rain.
The BERGERON THEORY suggests that for rain to form, water and ice must
exist in clouds at temperatures below 0 degrees C. At such temperatures water
droplets and ice droplets form and grow by condensation until big enough to
overcome turbulence and cloud updrafts, so they fall. As they fall, crystals
coalesce to form larger snowflakes, which generally melt and become rain as
they pass into the warm air layers near the ground. Thus according to
Bergeron, rain comes from clouds that are well below freezing at high
altitudes, where the coexistence of water and ice is possible.
Other mechanisms must also exist as rain as rain also comes from clouds that
are not so cold. These include….
Condensation on extra-large hydroscopic nuclei
Coalescence by sweeping, whereby a falling droplet sweets up others in its
The growth of droplets by electrical attraction.
Humidity and Precipitation
Relates to the rising and sinking of air. This means that the temperature of the
air is changed internally, without any other influence. It is the rising
(expanding and cooling) and sinking (contracting and warming) of air that
causes its temperature change.
Air moves for four reasons…
1. Convection: The most powerful lifting mechanism initiated by the heat of
the Sun warming the ground, causing air to warm, expand and rise.
2. Orographic barriers: When air is forced to rise over a hill, mountain etc.
3. Turbulence: in air flow
4. Frontal Systems
When air rises from one elevation to another, the temperature changes. The
decrease of pressure with height allows the rising parcel of air to expand. As
it expands it uses up energy from within the parcel. Likewise when air is
sinking it gains heat from contraction. Therefore adibiatic heating is an
internal mechanism without any heat exchange.
Adiabatic processes (lapse
The Environmental lapse rate (ELR) is the actual temperature decline
with height – on average this is 6degrees per 1000 metres.
Adiabatic cooling and warming in dry air occurs at a rate of 10
degrees/ 1000m. This is known as the dry adiabatic lapse rate (DALR)
Air in which condensation is occurring cools at the lower saturated
adiabatic lapse rates (SALR) between 4-9 degrees/1000m. This is
because latent heat released in the condensation process partly offsets
the temperature loss from cooling. The rate varies according to the
amount of latent heat released. 4degrees being in warm saturated air,
9/1000 being in cold saturated air.
Lapse rates can be shown on a temperature height diagram.
Elementary Plate Tectonics
• The theory of plate tectonics states that the Earth is made of a number of layers
1. The Crust: Thin outer layer which holds tectonic plates.
2. The Mantle: Thickest layer making up 82% of Earths volume. Made up of
magma, with diameter of approximately 2900KM
3. The Outer Core: Hot layer surrounding inner core. Liquid layer made up of iron
and nickel, with temperatures of approximately 5000degrees.
4. The Inner Core: Dense, solid core made of iron and nickel with temperatures
exceeding 5500degrees Celsius.
• There are two types of crust:
1. Oceanic: Dense, thinner plate made up of Basaltic rock, approximately 16km
2. Continental: Much thicker plate made up of granite, silica and aluminium, less
dense than oceanic plate.
• The upper mantle and crust make up a layer called the Lithosphere which is broken
into a number of plates. These move over the Asthenosphere, which is a plastic
layer in the mantle, which drives plate movement.
Alfred Wegener's theory for
• Wegener in 1912 proposed his hypothesis on continental drift, using
several lines of evidence to support his ideas that the continents were
once joined together in one super continent called Pangaea (which
means “entire earth” in Greek) These included…
1. The apparent fit of the continents like a jigsaw puzzle
2. The correlation of multiple fossils such as the mesosaurus, found in
only South America and Africa. As a creation who could only live in
shallow water, couldn’t swim well or fly, how could it have travelled
over an entire ocean? Solution: The continents were once connected.
3. Matching rock formations and mountain chains found in South
America and Africa, consisting of the same rock and same age.
4. Glacial striations found in tropical rainforests suggests that countries
were always in their current climatic regions.
• However whilst Wegener had some very valid points and a good
argument, he had no driving mechanism to make this happen. He
believes that somehow continents were pushing ocean plates along –
however critics commented that continental plates lacked momentum
to achieve this and thus his theory fell through.
Harry Hess’s hypothesis of Sea
floor spreading 1960s
• Harry Hess in the 1960s suggested that convection currents within the
mantle could be forcing magma to rise and crack the crust above it
forcing it apart.
• He believes that as it welled up and cooled on the ocean floor at
divergent zones, new oceanic crust was forming at mid ocean ridges,
pushing older, colder and more dense crust towards deep sea trenches,
where it is subducted, recycled back into the mantle or creates
• However when there is no trench for old crust to subject under (such as
the coast of Africa), then in pushes the continent along with it as crust
• As there are few trenches in the Atlantic Ocean it is expanding
• As there are many trenches in the Pacific Ocean it is shrinking
• How did Hess support his theory? Rock Magnetism. Hess looked at the
polarity on either sides of the ridge, a correlation of identical bonds
between the two sides supported his theory.
• Magnetic grains in the rock align with the Earth’s magnetic field at the
time of cooling (known as paleomagnetism)
J Wilson 1965
• In 1965 J Wilson linked together ideas of continental drift and
sea floor spreading, developing the concept of plate tectonics.
• Wilson said that Earth’s crust, or lithosphere, was divided into
large, rigid pieces called plates. These plates “float” atop an
underlying rock layer called the asthenosphere. In the
asthenosphere, rocks are under such tremendous heat and
pressure that they behave like a viscous liquid (like very thick
honey). The term “continental drift” was no longer fully
accurate, because the plates are made up of continental and
oceanic crust, which both “drift” over Earth’s face.
Tuzo Wilson predicted three types of boundaries between
plates: mid-ocean ridges (where ocean crust is created),
trenches (where the ocean plates are subducted) and large
fractures in the seafloor called transform faults, where the
plates slip by each other.
Types of Plate boundaries
• Divergent/Constructive: These plates are moving away from each other.
They are usually found in the middle of the oceans and mid ocean
ridges are found here.
• Convergent/Destructive: These plates are moving towards each other
causing earthquakes, volcanoes, deep ocean trenches and fold
• Transform/conservative: These plates are sliding past each other. At
these zones land is not being created nor destroyed , however frequent
earthquakes are common. An example is San Andrea Fault in California
How do the plates move?
• There are three main theories explaining plate movement…
1. Convection currents: This states that huge convection currents
occur in the earths interior causing hot magma to rise to the
surface and then spread out at mid ocean ridges, whilst the
cooler magma gets denser and sinks back deep into the mantle
where it is reheated.
2. Dragging Theory: Plates are dragged or subducted by their
oldest edge when they become cold and dense. Plates are hot
at mid ocean ridges, but cool as they are pushed further away.
As the cold plates descend at the trenches, pressure causes the
rocks to become heavier and therefore they are subducted.
3. Hotspot: Hotspots are plumes of molten rock which rise
underneath a plate penetrating weaknesses in the crust and
resulting in volcanic activity. As plates are moving and hotspots
stay still, these therefore led to chains of land creation such as
Hot Spots Dragging Theory
• Subduction occurs when an oceanic lithospheric plate collides
with another plate. As the density of the ocean plate Is similar
to that of the asthenosphere it can easily be subducted.
Subduction zones dip mainly at 30-70 degree angles.
• If a continental and oceanic plate meet, the oceanic plate will
be subducted beneath the continental as it is more dense.
• Evidence of subduction:
• The existence of certain landforms such as deep sea trenches and
folded sediments (usually arc shaped and containing volcanoes)
• Benioff zone – a deep active seismic area dipping away from the
deep sea trench.
• Earthquake focal mechanisms (ring of fire)
• Island arcs are features of
convergence. They are
chains of volcanoes which
are aligned in an arc shape
and sit close to the boundary
where two plates meet.
During subduction hot re-
melted material from the
subducting slab rises and
leaks into the crust forming a
series of volcanoes. These
volcanoes can make a chain
of islands called Island arcs.
Many are found in the Pacific
and Western Atlantic.
• Plate tectonics are associated with mountain building. Where
oceanic plates meet continental, the light less dense plate may
be bulked and folded up creating fold mountains, such as the
• Where two continental plates meet, both may be folded and
buckled, forming mountains such as the Himalayas formed by
the collision of the Eurasian and Indian plates.
• The Indian subcontinent moved rapidly north during the last
70 million years, eventually colliding with the main body of
Asia. The huge ocean Tethys has been entirely lost between
these masses in the collision zone and the crust has thickened
because Asia overrides India, resulting in crust thickening
causing the uplift of the Himalayas.
• Describes the processes that break up rocks. There are three
types of weathering…
1. Chemical Weathering (Decomposition): Processes that break
down rocks atom by atom through chemical reactions.
Water plays a key role here.
2. Mechanical Weathering (Disintegration): The tearing apart
and breaking of rocks through physically destroying them
3. Biological Weathering: When animals and vegetation (root
wedging) break up rocks.
• Hot wet climates enhance chemical weathering
• Cold wet climates enhance mechanical weathering.