Lecture 7

The Atmosphere
The Blue Planet: Chapter 11

Outline
• The Habitable Planet
• Composition and Structure of our
Atmosphere
• Moisture in the Atmosphere
• The Atmosphere in the Earth System

The Habitable Planet
• The atmosphere is the gaseous envelope
that surrounds a celestial body
• Air is the invisible, odorless mixture of
gases and suspended particles that
surrounds Earth
• In addition to supporting life chemically, it
protects it, stores moisture and solar
energy and moves Earth materials

• After accretion, Earth had a primordial or
primary atmosphere, which was stripped
away by solar winds early in solar system
history
• Little by little, through volcanic outgassing,
abundant volatiles were released to surface,
forming Earth’s secondary atmosphere
– Unlike today’s atmosphere, this would have
been composed of water vapor, methane,
hydrogen, nitrogen, carbon dioxide and argon

• Earth had virtually no oxygen in its
atmosphere more than 4 billion years ago,
and now it makes up 21%
• Traces of oxygen were probably generated
through breakdown of water molecules by
ultraviolet light
• Almost all the free oxygen originated through
photosynthesis
– The oxygen combined with other elements to
make compounds, eventually oxygenating the
atmosphere around 2.5 to 1.8 billion years ago

• The transition from an oxygen-poor to
an oxygen-rich atmosphere is recorded
in the alternating black (reduced) and
red (oxidized) banded iron formations
• With the buildup of molecular oxygen
came and eventual increase in ozone
– By absorbing harmful UV radiation, ozone
made it possible for life to flourish in
shallow water and finally on land

• The critical stage in the evolution of the
atmosphere was reached between 1100 and
542 Ma
– Explosive diversification of life occurred 542 million
years ago
• Oxygen levels have fluctuated over the past 200
million years from 10% to 25%
• The other major chemical change in the
atmosphere was the removal of carbon dioxide
by sinking it into limestone and organic sediment

Composition and Structure of
our Atmosphere

Composition and Structure of our
Atmosphere
• The air of our present day atmosphere
varies in composition from region to
region due to the presence of
– Aerosols: tiny suspended liquid or solid
particles
– Water vapor: humidity from evaporation
• The remaining “dry” gases in the
atmosphere are consistent
– Nitrogen, oxygen and argon
– Carbon dioxide, methane, ozone and NOx

Atmosphere
• Two things energize the atmosphere
– The Sun’s energy
• Warms the atmosphere
• Energy source for clouds, rain, snowstorms,
wind, and weather
– The Earth’s rotation
• Axial tilt is responsible for seasons
• Large scale flow patterns in the atmosphere
– Jet stream, global wind patterns

Atmosphere
• There are 4 thermal layers of the
atmosphere, separated by pauses
– Troposphere: temperature decreases with
altitude, most of our weather occurs here
– Stratosphere: temperature increases with
altitude because of the presence of ozone
– Mesosphere: temperature decreases with
altitude, the coldest layer of the atmosphere
– Thermosphere: temperature increases with
altitude, reaches the highest temperatures,
hosts the ionosphere, where auroras occur

Atmosphere
• Air pressure is the force exerted by the
weight of the overlying air
– Air pressure decreases smoothly with
altitude
– Because air is highly compressible, 50% of
the of the atmosphere lies below 5.5 km,
99% lies below 32 km, the remaining 1%
extends from 32 km to 500 km
– It is measured with a barometer, and
changes can indicate imminent changes in
weather

Moisture in the Atmosphere
• Relative humidity
– When the number of molecules that evaporate
equals the number that condense, the vapor is
saturated, this is the dew point temperature
– The amount of water vapor in under-saturated air
is the relative humidity
– This is the ratio of the actual vapor pressure to
the saturation vapor pressure
– Temperature exerts a strong control on water
vapor capacity of air

• Relative humidity can be changed
either by the addition of water vapor or
by a change in temperature

• Adiabatic lapse rate
– When air is compressed, it warms
– When compressed air expands, it cools
– Warm air rises, but since air pressure
decreases, the air expands and cools
– Cool air descends, but with increased air
pressure it compresses and warms
– The rate of temperature change over these
processes is called
• Dry adiabatic lapse rate (unsaturated air)
• Moist adiabatic lapse rate (saturated air)

• Clouds are visible aggregates of minute
water droplets, ice crystals, or both
• They form when air rises and becomes
saturated with moisture in response to
adiabatic cooling and condensation
• There are four principal reasons for the
upward movement of air, which in turn
leads to the formation of clouds

1. Density lifting
• Warm, low-density air rises convectively
1. Frontal lifting
• Two flowing air masses of different density
meet, one forcing the other up
1. Orographic lifting
• Flowing air is forced upward due to terrain
1. Convergence lifting
• Flowing air masses converge and are both
forced upward

• When the dew point is reached one of
two things happens
– Water condenses
– Ice crystals form
• These nucleation processes require
energy to form a new surface
– Nucleation sites may be
• The ground, aerosols

• Clouds are classified on the basis of
shape, appearance, and height
– Cumulus: puffy individual clouds, where
the flat base marks the condensation level
– Stratus: sheets of cloud cover spread
laterally rather than vertically
– Cirrus: highest of the clouds, wispy
feathers composed of ice crystals
– Nimbus: rain, as in cumulonimbus

The Atmosphere in the Earth
System
• The atmosphere and the life zone
– Most organisms live in conditions of fairly
stable temperatures
– Humans need moist air for proper lung
function, and our brains, hearts, lungs,
livers, and digestive systems cannot
function within a +/- 2˚C temperature
variation
– Over 90% of the human population lives
where the annual mean temperature is
between 6˚C and 27˚C

System
• Considering our total dependence on
air and the atmospheric blanket
maintaining conditions supportive of life,
one might expect us to know and care a
lot about the atmosphere
– A wide variety of anthropogenic
contaminants cause pollution at low levels
in the atmosphere
– Many common air pollutants are known to
have negative impacts on human and
ecosystem health

System
• Today there is growing concern about
anthropogenic pollutants causing
changes in the concentration of
radiatively active gases in the
atmosphere, which, it is thought, will
lead to changes in the greenhouse
effect, and ultimately, global climatic
change

Wind and Weather Systems
The Blue Planet: Chapter 12

Outline
• Why Air Moves
• Global Air Circulation
• Regional Wind and Weather Systems
• Local Wind and Weather Systems
• Severe Weather
• Weather and the Earth System

Why Air Moves
• Weather is the state of the atmosphere at a
given time and place, determined by five
variables
1. Temperature
2. Air pressure
3. Humidity
4. Cloudiness
5. Wind speed and direction
• Weather is short-term, climate is long-term;
the average weather condition of a place

Why Air Moves
• Wind is air movement that arises from
differences in air pressure
– Flowing from an area of high pressure to an area
of low pressure
• Most places have wind speeds that average
between 10 and 30 km/hr
• In places where temperatures drop below
freezing, the windchill factor is reported
– This is a measure of the heat loss from exposed
skin due to low temperature and wind

Why Air Moves
• Wind speed and direction are affected
by three factors
1. Pressure-gradient force: drop in air
pressure per unit of distance
2. Coriolis force: the deviation from a
straight line of the path of a moving body
as a result of Earth’s rotation
3. Friction: the resistance to movement that
results when two bodies are in contact

Why Air Moves
• Pressure-gradient force
– Air always moves from an area of high
pressure to an area of low pressure, the
stronger the pressure gradient, the stronger
the resulting flow of air
– Places of equal air pressure are shown on
weather maps by lines called isobars
• Analogous to contour lines on topographic maps
• Isobars close together show a steep gradient
• Isobars far apart show a low gradient

Why Air Moves
• Coriolis force
– Includes all freely moving objects on the
surface of a rotating planet - such as wind
– The directions of all winds, like ocean
currents, are subject to this force
• Friction
– From topography, trees, and other solid
objects, friction slows the speed of wind

Why Air Moves
• Geostrophic winds
– Winds are always subject to more than one factor,
even the least complicated example
– A high-altitude wind, not in contact with the
ground, starts to flow due to air pressure gradient,
the Coriolis effect deflects the wind so that it is no
longer perpendicular to the isobars
– Eventually the pressure-gradient force and the
Coriolis effect are in balance, and the wind flows
parallel to the isobars
– This is a geostrophic wind

Why Air Moves
• The three effects interact such that winds
around a low-pressure center develop an
inward spiral motion (convergence, cyclone)
• By the same process, winds around a high-
pressure center develop an outward spiral
motion (divergence, anticyclone)
• In the northern hemisphere, convergent
centers rotate counterclockwise and
divergent centers rotate clockwise - the
reverse is true for the southern hemisphere

Global Air Circulation
• Air masses, enormous volumes of air
driven by the pressure-gradient force
and the Coriolis force, are responsible
for our wind and weather systems
• Earth’s axial tilt, the Coriolis force, and
the solar heat imbalance, also means
that warm equatorial air must flow to the
poles and cold air to the equator in
several huge convection cells

• Hadley cells and the ITCZ
– Major circulatory cell that stretches from
the equator to 30˚ N and S latitude
– Warm air rises in the tropics and creates a
low-pressure zone of convergence (ITCZ)
– Air piles up here, creating two belts of high-
pressure air sinking towards the surface,
and creating a zone of divergence and
completing the convection cells
– Contains high-level winds: the westerlies
and low-level winds: the trade winds

• Ferrel cells
– On the poleward side of the Hadley cells
are midlatitude convection cells
– Surface winds are westerlies
• Polar fronts and Jet streams
– On the poleward side of the Ferrel cells,
meeting along a zone called a polar front
– Dry, high-altitude air descends near the
pole, creating a zone of divergence

• Polar fronts and Jet streams
– Surface air then moves toward the equator,
forming the polar easterlies
– High-level winds in the polar cells are
westerly
– Upper atmosphere westerlies associated
with steep pressure gradient are called jet
streams

Regional Wind and Weather
Systems
• Monsoons
– A seasonally reversing wind system
– Most distinct in Asia and Africa
– The controlling factor is the intertropical
convergence zone
• In India, ITCZ lies on the equator in winter and
moves north in summer, bringing with it
southwesterlies and warm, moisture-laden air
that result in humidity and torrential rains

Systems

Systems
• El Niño and the Southern Oscillation
– Off the coast of Peru, cold upwelling sustains
fishing grounds with nutrients
– Periodically a mass of unusually warm water
appears off the coast, when this happens, the
trade winds slacken, upwelling is reduced, fish
population declines and coastal birds die off
– Dry parts of Peru receive heavy rains, Australia
experiences drought conditions, and cyclones
appear in Hawaii and French Polynesia

Systems
• El Niño and the Southern Oscillation
– There is a link between El Niño and changing
atmospheric pressure anomalies over the equator,
the Southern Oscillation
• A periodic variation in air pressure differential across the
tropical Pacific
– Which disrupts the Walker circulation wind pattern
– During an El Niño event, the pressure differential
weakens, weakening the Walker circulation and the
trade winds, which allows anomalously warm
surface water to accumulate and stop cold
upwelling

Local Wind and Weather Systems
• Coupled local wind systems
– Near coasts, the land heats up more
rapidly than the sea during the day, and
causes air to heat up and expand
– This develops a pressure gradient, and air
flows onto the land, creating a sea breeze
– An upper level reverse flow sets in, forming
a convection cell
– During the night, heat is radiated more
rapidly from the land and the situation
reverses, creating a land breeze

• Kabatic wind: high-speed, cold wind
– The flow of cold, dense air under the
influence of gravity
– Occur in places where a mass of cold air
accumulates over a high plateau or valley
• Chinook wind: warm, dry wind
– Strong regional winds rise and compress
higher-level air masses as they pass over
a mountain range, forcing the air
downslope, it warms as it compresses

Severe Weather
• Cyclones
– Refers to any cyclonic circulating wind
system around a low-pressure center
• Wave cyclones
– Extratropical or midlatitude cyclones
– Form between 30˚ and 60˚ N and S
– Up to 2000 km across
– Last many days, responsible for most
everyday weather events in the midlatitude
regions of the world

Severe Weather
• Tropical cyclones
– A hurricane or tropical storm that develops at
5˚N or S or higher when sea-surface
temperature is 26.5˚C or higher
– Draws energy from the water, so wind speeds
quickly diminish when it moves on shore
– Most hurricane damage occurs within 250 km
of the coast
• Wind damage, storm surge, and torrential rain

Severe Weather
• Thunderstorms
– Develop when an updraft of warm, humid
air releases a lot of latent heat very quickly
and becomes unstable
– Most form along cold fronts, pulling in
warm, moist air, releasing more heat and
intensifying updrafts
– Cumulonimbus clouds form and heavy
rainfall, hail, thunder and lightening result

Severe Weather
• Tornadoes
– Violent windstorms produced by a spiraling
column of air that extends downward from a
cumulonimbus cloud
– Develop from large thunderstorms that have
multiple updrafts (supercell thunderstorms)
– Approximately funnel shaped
– Tornado activity from April to August
– Small relative to the thunderstorms they are
associated with, but incredibly violent and
destructive

Severe Weather
• Drought
– A region experiencing below-average
rainfall for an extended period
• Often with emphasis on affected water supply
or harvests
– Semi-arid areas adjacent to deserts are
highly susceptible to drought
– The expansion of desert conditions into
adjacent areas is called desertification
• Due to natural causes (drought)
• Overgrazing and poor land-use practices

Severe Weather
• Dust storms
– Once in the air, dust constitutes the wind’s
suspended load
– Grains of dust are tossed around in eddies
– Strong winds associated with large dust
storms can carry very fine dust into the
upper troposphere
– Most frequent in arid and semi-arid areas
• Related to cycles of drought

Weather and the Earth System
• Weather and climate are sensitive
indicators of changes in the Earth
system as a whole
• Effects from small differences can
propagate quickly due to feedbacks
• Earth’s weather system is full of
feedbacks, both positive and negative
– Prolonged drought -> vegetation dies ->
dust storm

Weather and the Earth System
• In a positive feedback situation, a
prolonged drought might reach a point
where it becomes impossible to recover to
its former state
• A new drier “normal” would be established
• This is a threshold situation, meaning a
system can handle and respond to changes
by returning to its starting point, but only up
to a certain point

Lecture 7

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