Extreme weather events ; astronomical predictions: lunar cycle, sunspot cycle, solar-lunar tides, Chandlers Compensation, blocking highs

1. CCS HARYANA AGRICULTURAL UNIVERSITY
COLLEGE OF AGRICULTURE
Hisar– 125004
ASSIGNMENT ON EXTREME WEATHER EVENTS;
ASTRONOMICAL PREDICTIONS: LUNAR CYCLE, SUNSPOT
CYCLE, SOLAR-LUNAR TIDES, CHANDLERS COMPENSATION,
BLOCKING HIGHS
SUBMITTED TO SUBMITTED BY
Dr. CHANDER SHEKHER DAGAR ABHILASH
DEPARTMENT OF AGRICULTURALMETEOROLOGY 2016A55D
COA, CCS HAU, HISAR
COLLEGE OF AGRICULTURE
CCS HARYANA AGRICULTURAL UNIVERSITY
HISAR -125004

2. What is Extreme Weather Event?
 An extreme weathereventis the occurrence of a value of a weather variable above (or below) a threshold
value near the upper (or lower) endsof the range of observedvaluesof the variable.These eventsare not
a signof climate change by itself,as they always existedbutthe occurrence and severityofat leastsome of
these eventshave increased.Extreme weather includesunexpected,unusual,unpredictable severe or
unseasonal weatheri.e.weatheratthe extremesof the historical distribution.
 An extreme weathereventis an eventthat is rare withinits statistical reference distributionata particular
place.Definitionof“rare” vary, but an extreme weathereventwouldnormally be as rare as or rarer than
the 10th
or 90th
percentile.Bydefinition,the characteristicsof “what is calledextreme weather” may vary
from place to place.
(IPCC)
 It includescold wave,Heat wave,Cyclones,Tidal Waves,Snowstorm, Hail storm, Drought, Fog, Frost,
Thunder storm, Dust storm, Heavy & Unseasonal Rainfall,Cloudburstand Flood.

3. 1. Land summer température distribution has shifted
From 1951 to 1961, only 1% of the land area in the Northern Hemisphere was exposed to
temperatures higher than 3 standard deviations (SD) from the mean for 1951–1980. But from
2001-2011, 11% of land area was exposed to temperatures higher than 3 SD away from the
average. About 1% of land area, an area twice the size of France, experienced heat extremes of
5 SD from the mean. The same trend is seen in the Southern Hemisphere.
2. Extreme high temperatures become more frequent
The annual frequency of warm nights (90th percentile) and warm days (90th percentile) for
the period 1950-2010 is increasing, relative to the period 1961 to 1990, in many regions of the
World. The same trend is seen in decadal values (right panel). Occurrence of warm nights is
more widespread than that of warm days. From WG1 IPCC AR5 Figure 2.32.

4. What is the significance of expressing Climatic values as percentile....?
Changes in the (very high or very low) percentiles with climate change tell you how the tails of the
distributions are moving. We are often less concerned with mean or median changes than with changes
in the tails because extreme events have more serious consequences for society.
So if some day had a Tmin temperature less than the 10th percentile, it means the coldest temperature
on record for the day was in the coldest 10 percent within the record. In other words, it was the among
the coldest of the cold. Similarly we look at the 90th percentile for Tmax because we are worried
about these unusually/extremely hot days (among the hottest 10 percent in the record).
Changes in these extremes are what could really cause damage in terms of human health / food
security effects since societies tend to be poorly adapted to them because they BY DEFINITION do
not happen regularly in the baseline climate.
The 90th percentile is the value for which 90% of the data points are smaller.
The 90th percentile is a measure of statistical distribution, not unlike the median.
The median is the middle value. The median is the value for which 50% of the values were bigger, and
50% smaller.
The 90th percentile tells you the value for which 90% of the data points are smaller and 10% are
bigger.
Statistically, to calculate the 90th percentile value:
1. Sort the transaction instances by their value.
2. Remove the top 10% instances.
3. The highest value left is the 90th percentile.
Example:
There are ten instances of transaction "t1" with the values 1,3,2,4,5,20,7,8,9,6 (in sec).
1. Sort by value — 1,2,3,4,5,6,7,8,9,20.
2. Remove top 10 % — remove the value "20."
3. The highest value left is the 90th percentile — 9 is the 90th percentile value.
Available from:
https://www.researchgate.net/post/What_is_the_significance_of_expressing_values_as_percentile_in_
climate_statistics [accessed Sep 4, 2017].

5. In contrast, the incidences of cold days and nights (10th percentile of temperatures) have
decreased. From IPCC AR5 Figure 2.32
3. Warmest day of the year
Between 1951 and 2010 there was a robust rise in the temperature value of the warmest day of the year
in many areas of the World. The trend was up to >1 °C per decade. This means up to 10 °C in some
areas. From IPCC AR5 Box 2.4, Figure 1.

6. Heat waves / Cold Wave
 Heat wave: When the air temperature rises more than or equal to 50C above normal
temperature is termed as heat wave.
 Cold wave : If the temperature falls more than or equal to 50C below normal temperature
then is termed as cold wave.
 Heat waves are most deadly in mid latitude regions where they concentrate extremes of
temperature and humidity over a period of few days in the warmer month.
 The oppressive air mass in an urban environment can result in many deaths especially among
the very young, the elderly and the infirm.
 In Australlia and USA, casualties due to heat waves have been reported than that of any other
weather related hazards (Guest, 1996 and Andrews, 1994).
 In France, Italy, the Netherlands, Portugal, Spain and the United Kingdom, they caused some
40000 deaths.
 In northwest parts of India, heat waves prevail in summer months (May and June).
 The hot and dry westerly wind in this region locally termed as ‘Loo’. It affects human life and
also causes loss of life under extreme conditions.
 In 1998, the nearly 1300 deaths had occurred due to heat wave in country, out of which 650
were from Orissa (De and Mukhopadhyay, 1998).
 The livestock production is also affected by heat wave through reduction in their productivity
e.g. milk production is drastically reduced under extreme heat wave conditions.
 Loo also affects the health of livestock, causing water deficit if proper care is not taken.
A heat-wave in Maharashtra was declared in the end of march,2017 only.

7. Precipitation extremes
As can be seen in the Figure there is a positive trend in the annual amount of extreme
precipitation (expressed as days above the 95th percentile of precipitation distribution between
1951 and 2010). From IPCC AR5 Figure 2.33
Increased Heavy rainfall events in India
Climate change also influenced the weather of India. Here, you can see there is decrease in no. of light
to moderate rainfall events, and increase in heavy and very heavy rainfall events.

8. 1. Heavy Rainfall
Rainfall is considered exceptionally heavy when the rainfall amount at or near a given
rainfall station is highest among the past record for that particular month or seasonand
also amount is greater than 12 cm.
 Rainfall greater than or equal to 50 mm in past 24 hours.
(W.M.O.)
 By IMD, based on intensity of rainfall
DDeessccrriippttiivvee tteerrmm uusseedd RRaaiinnffaallll aammoouunntt iinn mmmm iinn aa ddaayy
NNoo RRaaiinn 00..00
VVeerryy lliigghhtt RRaaiinn 00..11--22..44
LLiigghhtt RRaaiinn 22..55--77..55
MMooddeerraattee RRaaiinn 77..66--3355..55
RRaatthheerr HHeeaavvyy 3355..66--6644..44
HHeeaavvyy RRaaiinn 6644..55--112244..44
VVeerryy HHeeaavvyy RRaaiinn 112244..55--224444..44
EExxttrreemmeellyy HHeeaavvyy
RRaaiinn
>>224444..55
EExxcceeppttiioonnaallllyy HHeeaavvyy
RRaaiinn
AAmmoouunntt nneeaarr hhiigghheesstt rreeccoorrddeedd rraaiinnffaallll aatt oorr nneeaarr tthhee ssttaattiioonn ffoorr tthhee mmoonntthh oorr
sseeaassoonn wwhhiicchh aallssoo eexxcceeeeddss 1122 ccmm..

9. Causes of Heavy Rainfall
 Cumulonimbus clouds (fig 1)
 Tropical cyclones or Hurricanes (fig 2)
 Windward side in mountain region (fig 3)
 Global warming and pollution

10. Heavy rain is caused by cumulonimbus clouds. They have high vertical growth extended upto
10 km. They are formed by convection leading to the tropical cyclones or hurricanes.
(Increased condensation nuclei, increased evaporation and moisture holding capacity of air
resulting in increased precipitation)
In Hawaii , Mount Waiʻaleʻale (Waiʻaleʻale), on the island of Kauai, is notable for its extreme
rainfall. When it moves north and lies close to the Himalayan foothills, the rains cease abruptly
increase equally rapidly in intensity over the foothills of northeast India monsoon rains. It leads
to a paradoxical situation when people in the plains while those living in the northeastern parts
of the country are distressed major river systems of India have their origin in the Himalayan
region.
2. Cloudburst
 Rainfall rate is greater than or equal to 10 cm/hr accompanied with strong winds and
lightening. (IMD)
 Extreme amount of precipitation, sometimes with hail and thunder, normally last for a few
minutes but is capable of creating flood conditions.
 The associated convective cloud can extend up to a height of 15 km above the ground.
 Suddenly dumps about 72,300 tons of water over one square acre.
 Usually small areas between 20 to 80 square kilometers are affected.
Cause of Formation of Cloudburst
 Occurs mostly in desert and mountainous regions.
 Occurs when a pregnant monsoon cloud drifts northwards, from the Bay of Bengal and
Arabian sea across the Ganges plains, then onto the Himalayas and bursts, bringing rainfall as
high as 75 mm per hour.
 In convective, cumulonimbus clouds, strong updrafts and high surface temperature restricts the
rain drops to fall, so, smaller raindrops collapse to form bigger one which falls under influence
of gravity to form cloudburst.
 Also occur due to sudden collision of two or more clouds.
 Lack of upper level winds prevents dissipation of thunderclouds.

11. Cloud burst formation facilitated by orographic effect in Himalayas leading to flash flood
3. Unseasonal Rainfall
In India, normal rainfall period for southwest monsoon is June to September and northeast
monsoon is October to December (south peninsula, mainly Tamil Nadu). Rainfall at any other
time except that period is termed as unseasonal
Causes of unseasonal rainfall
 Western disturbance:
Originated from Mediterranean sea as low pressure or depression and bring winter rain
through Iran and Pakistan in NW India mainly driven by westerlies
 Convectional rain:
Due to development of low pressure by extreme summer heating of land near the vast water
bodies resulting in vertical cloud formation, and, thus, precipitation with gusty winds. e.g.
Nor’westers shower or Kal Baisakhi which occurs in April and May in Jharkhand, Bihar, West
Bengal and Odhisa.

12.  Orographic Effect
Mountain ranges acts as barrier to the wind with moisture and deflected upward. After reaching
LCL, cloud formation and heavy rain takes place in windward side. Orographic effect aids the
western disturbance.
 Climate change
Climate change reduced the frequency of the rain events and increases the intensity of the
rainfall. Causes are not very clear and research is going on.
Unseasonal rainfall-Boon or Bane
BOON:-
 Kal Baisakhi -helpful for pre-Kharif crops like tea, jute, paddy , vegetables and fruits.
 Gives desired relief after mid-day heat and pours well on the thirsty soil for the development of
crops.
 Winter rainfall due to western disturbance-beneficial for Rabi crops, particularly wheat.
 Pre-monsoon showers in Kerela and Karnataka-beneficial for ripening of mangoes “Mango
Showers”
 In Karnataka, pre-monsoon showers -beneficial for coffee plantation.
BANE:-
 Damage to crop
 Increased risk of attack by insect-pests and diseases.

13. 4. Floods
 Heavier rains together with sea level rise, tides and storm surges drive the occurrence of
flood.
 Floods have become more frequent.
 Major floods that used to happen only once in 100 years now take place every 10 or 20
years.
Data from EM-DAT Data from EM-DAT: The OFDA/CRED International Disaster Database
– www.emdat.be – Université catholique de Louvain – Belgium.
Types of Floods
 Flash Flood: due to violent convection storms of short duration falling over a small area. Flash
floodisthe floodthatrisesand fallsquite rapidlywithlittle ornoadvance warning, usually as the
result of intense rainfall over a relatively small area. It mainly occurs in hilly parts of India.

14.  Fluvial (riverine) Flood: It occurs when the flow exceeds the capacity of stream channel and
spills over the natural banks and artificial embankments.
 Coastal Flood: arising from storm surge and high winds coinciding with high tides in coastal
areas.
 Urban Flood: decrease in ability of land to absorb water due to urbanization.

15.  Ice and debris-jam Flood: Floating ice or debris accumulate in natural or man-made
obstructions and stop flow of water thereby causing flood.
Causes of Flood
Flood is caused by various factors….. Out of which, the maximum contribution is of heavy
rain.
Source: http://www.dartmouth.edu/~floods/archiveatlas/cause.htm

16. Impact of Heavy Rainfall, Cloudburst and Flood
Heavy rainfall and cloudburst cause flood and landslides in hilly areas.
EFFECT ON AGRICULTURE
 HARMFUL
 Freshly sown crop fields have poor distribution, germination and emergence
 Water logging resulting in crop yield reduction (more severe in black soils)
 Crop lodging
 Shedding of grains, fruits and flowers
 Soil erosion leading to loss of nutrients
 Increased growth of fungus, moulds, etc. causing spoilage of produce
 Crop failure and heavy indebtedness results in increased farmer suicides or migration in search
of livelihood

17.  BENEFICIAL
 Flooding controls the termites and stem borers
 Recharging of ground water
EFFECT ON HUMAN LIFE AND ECOSYSTEM
 HARMFUL
 Increase in incidence of malaria, diarrhea and other water borne diseases
 Damage to infrastructure and buildings like houses, roads, bridges, etc.
 Landslides and heavy floods cause loss of human and animal lives
 Disrupt transport and communication
 Degradation of water quality by sewage overflow harming human health and ecosystem
 Elevated level of nutrients in water bodies cause algal outbreaks which reduces water oxygen
content and loss of aquatic life
 Increased population of disease carrying insects and rodents
 BENEFICIAL
 Periodic flooding causes water to overflow the banks and deposit nutrient-rich sediments onto
the floodplain called the silt, where they nourish the trees of the floodplain swamp
 Dilution of pollutants that enter water bodies.
PREVENTIVE MEASURES FOR AGRICULTURE
 Schedule sowing, harvesting, irrigation or pesticide application in accordance of weather
forecasting
 Ensure good drainage in the agricultural field
 Prevention of soil erosion by agronomic practices or conservation measures
 Application of soil conditioners like polyvinyl alcohol to prevent crust formation
 Cultivation of lodging, shattering or water-logging resistant varieties
 Rain cover systems for horticultural crops

18. RELIEF & RECOVERY
 Army
 Air Force
 Navy
 Police
 Public Works Department
 National Disaster Response Force (NDRF)
 Local administrations
 Non-Governmental Organizations
 Funds by Central and State Government
Food Distribution Evacuation

19. 5. DROUGHT
DEFINITION
 No universal definition
 WMO (1986) “drought means a sustained, extended deficiency in precipitation”
 UN Convention to Combat Drought and Desertification (1994) “drought means the naturally
occurring phenomenon that exists when precipitation has been significantly below normal
recorded levels, causing serious hydrological imbalances that adversely affect land resource
production systems.”
 FAO (1983) defines drought hazard as “the percentage of years when crop fails from the lack
of moisture.”
 Droughts are long period of extremely dry weather when there is not enough rain for the
successful growing of crops or the replenishment of water supplies.
 The primary cause of any drought is deficiency of rainfall. Drought is different from other
hazards in that it develops slowly, sometimes over years, and its onset can be masked by a
number of factors. In some cases, droughts are recognized too late for emergency measures to
be effective.
 Drought can be devastating: water supply dries up, crops fails to grow, animals die and
malnutrition and ill health become wide spread.

20. Drought can be classified into three types:
 Meteorological drought
 Hydrological drought
 Agricultural drought
 Meteorological drought: When the rainfall is 25 per cent below the normal value, it is defined
as meteorological drought. If the rainfall is 50 per cent below normal, then it is defined as
moderate drought and if the rainfall is 75 per cent below normal, then it is termed as severe
drought.
 Hydrological drought: It occurs when the surface and ground water resources are inadequate to
meet the demand of water. The surface water reservoir such as ponds, lakes, rivers and streams
are dried or insufficient to meet the demand of water. This condition is termed as hydrological
drought.
 Agricultural drought: When the water demand of crop is not met by soil moisture supply and
causes reduction in crop growth and yield. Such atmosphere condition is termed as agricultural
drought or it is the condition in which there is insufficient soil water available to crops. Some
times crop fails due to water deficit, under severe drought conditions.
Importance of Monitoring and Forecasting of Agricultural Drought
 The frequency of drought in India is increasing.
1950-1990 – 10 drought years
since 2000 – 5 drought years (2002, 2004, 2009, 2014 and 2015)
 68 % of the net sown area in India is prone to drought.
 Monitoring and forecasting are part of preparedness that helps in reducing the impact of
drought by following practices:
 Selection of crop management practices
 Contingency planning
 Policy formulation
 Crop/ Livestock insurance
 Watershed management
 Agro-advisories
 Monitoring helps in providing relief measures to severely affected areas.

21. 6. Frost
 Frost is the solid deposition of water vapor from saturated air in the form of small white ice
crystals on the ground or other surfaces.
 When the temperature of these solid surfaces falls below freezing point of water or below the
dew point of the adjacent air, , the water vapour directly converts into solid state (ice crystal)
and deposited on a surface as white ice layer.
 Frost crystals' size differ depending on time and water vapour available. Frost is also usually
translucent in appearance.
Two kinds of frosts are frequently problems in winter season in northern states.
1. Advection frost:
 results when the temperature at the surface in an air mass is below the freezing level.
2. Radiation frost:
 occurs on clear nights due to radiative cooling with a temperature inversion and usually results
in formation of ice crystals on cold objects.
 The frost causes a great damage to the plants as well as the grains. Frost-hazard is greatest in
north India in the winter months.
 The cold waves move towards southward or eastwards from north-west Himalayan ranges.
 These waves moves for 3 to 4 days continuously leading to widespread rain, cloudiness and
low temperatures in the region.

22. 7. Fog
 Fog is a low-lying cloud that forms at or near the surface of the Earth.
 It is made up of tiny water droplets or ice crystals suspended in the air and usually gets its
moisture from a nearby body of water or the wet ground.
 Fog is distinguished from mist or haze only by its density.
 In meteorological forecasts, the term “fog” is used when visibility is less than one kilometres
(one nautical mile in case of marine forecasts).
 If visibility is greater than that, but is still reduced, it is considered mist or haze.

23. 8. Storm
 A storm is any disturbed state of an environment or astronomical body's atmosphere especially
affecting its surface, and strongly implying severe weather.
 It may be marked by significant disruptions to normal conditions such as
strong wind, hail, thunder and lightning (a thunderstorm), heavy precipitation (snowstorm,
rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or
wind transporting some substance through the atmosphere as in a dust storm, blizzard,
sandstorm, etc.
 Storms have the potential to harm lives and property via storm surge,
heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and
vertical wind shear.

24. 9. Duststorm
 A storm is any disturbed state of an environment or astronomical body's atmosphere especially
affecting its surface, and strongly implying severe weather.
 It may be marked by significant disruptions to normal conditions such as
strong wind, hail, thunder and lightning (a thunderstorm), heavy precipitation (snowstorm,
rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or
wind transporting some substance through the atmosphere as in a dust storm, blizzard,
sandstorm, etc.
 Storms have the potential to harm lives and property via storm surge,
heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and
vertical wind shear.
 Pollution caused by suspension of dust particles is called dust pollution. The dust particles
created by anthropogenic activity are raised by high winds and dust storms.
 The size of dust particles ranges between 1.0 and 1000 µm. When the conditions over the
desert and semi-arid areas are unstable and humidity is low, convective clouds do not build up
to greater heights.
 The down drafts from these storms raise loose dust and cause dust storms.
 The mechanism of the formation of dust storms is same as that of thunder storms.
 The vertical growth is arrested due to low humidity aloft (Menon, 1997).
 The falling water drops evaporate quickly due to the high temperature and low humidity in low
levels; hence hardly any precipitation reaches the ground.
 Dust storms are common over North-West India. It is estimated that the quantity of raised dust
may be of the order of 50 to 500 kg/ha/day.

25. 10. Hailstorm
 A hailstorm is a particularly violet thunderstorm.
 Though short in duration, but precipitation forms and associated squalls are violent.
 It is one form of precipitation.
 Precipitation in the form of ice bolls pieces of diameter greater than 5 mm is called as hail.
 Hails are always associated with cumulonimbus clouds during transition period between
summer and winter seasons.
 A dissection of a hail resembles an onion with alternate layers of glaze ice and opaque rime.
 The glaze ice layers form during downward movement in the super cooled water droplets
region while the opaque rime layers form during upward movement of hail in higher levels
where small drops predominates in the cloud (Menon, 1997).
 It destroys the agricultural vegetation, the damage being predominantly mechanical.
 Hail storm causes a lot of damage to standing crops.
 Protection against violent hailstorms is difficult.
 Once the hails become too large to be supported by the updrafts, they escape from cloud and
falls on ground. The size of hail depends upon the strength of updraft within the cloud.
 But farmers who are forewarned, can harvest their crops if they are already ripe, or take other
protective measures.
 The largest hailstorms are about 5 inches in diameter and 1.5 lbs.

26. 11. Tornado
 It is the smallest but most violent form of all known weather storms.
 It seems to be a typically American storm, being most frequent and violent in United States.
 It also occurred in Australia and reported occasionally in other places in middle latitudes
(Strahler, 1975).
 It is also known throughout world in tropical and subtropical regions.
 It is a small, but intense cyclone in which the air is spiraling at extreme high velocity.
 It appears as a dark funnel cloud hanging from large cumulonimbus cloud. The funnel diameter
at its lower end may be from 90-400 m.
 The funnel appears dark because of the density of condensing moisture; dust and debris lift up
by the wind.
 Wind speeds in tornado exceed those in other storms and estimates show as high as 800
km/hour.
 The end of the funnel cloud may sweep the ground, causing complete destruction of anything
in its path and rise in air to leave ground below unharmed. Its destruction occurs both from
high wind speed and sudden fall of pressure in the vortex of cyclone spiral.
 Closed houses explode/collapse because of sudden increase in pressure gradient between
barriers and its immediate surrounding.
 It has been reported that even corks will pop out of empty bottles due to great difference in air
pressure.

27. 12. TROPICAL CYCLONE (Typhoons, Hurricanes and Tornadoes)
 Tropical cyclone is an almost circular storm center of extremely low pressure into which winds
spiraling with great wind speed in anti clock-wise direction in Northern hemisphere and clock
wise direction in southern hemisphere and accompanied by very high rainfall.
 The diameter of storm may range from 150 to 500 km and the wind velocity ranges from 120
to 200 km per hour and even sometime more.
 The air pressure at the center generally falls to 965 mb.
 During the day preceding the storm the air is generally calm, the air pressure somewhat above
normal and sky shows cirrus cloud in long streams.
 As the storm approaches the barometric pressure begins to fall.
 A great wall of dark cloud approaches with torrential rains and violent winds (Strahler, 1975).
 This continues for several hours and followed by clear and calm skies and pressure falls to
lowest in the central eye of storm.
 This calm period may last a half an hour.
 Again a wall of dark cloud approaches with heavy rains and violent winds but of reverse
direction to those of the first half of storm.

28. Tropical cyclone generation
 relies upon several meteorological conditions. Local sea surface temperatures must be around
26.5 °C.
 Evaporation from warm surface waters creates high humidity in the atmosphere, and then leads
to thunderstorm development.
 If multiple thunderstorm systems converge, a storm with a vortex movement develops.
 The vortex takes additional heat and water vapor from the surface of the ocean and releases it
into the atmosphere in the form of rainfall.
 High winds are created.
 The more heat available at the surface, the higher the potential winds.
 Once wind speeds exceed 55 km/h, the system becomes a tropical storm and is assigned a
name.
Tropical storm seen from Space

29.  On the basis of the role of sea surface temperature on the severity of the cyclone, it can be
expected that global warming may facilitate the incidence and/or severity of tropical cyclones.
 A 2005 study published in the journal Nature examined the duration and maximum wind
speeds of each tropical cyclone over the last 30 years (Nature. 436:686-688) and found that
their destructive power has increased around 70 percent in both the Atlantic and Pacific
Oceans.
 Another 2005 study (Science. 309: 1844-1846), revealed that the percentage of hurricanes
classified as Category 4 or 5 (the two strongest categories on the Saffir-Simpson scale) has
increased over the same period.
 A category 5 cyclone provokes ≥ 252 km/h wind speeds.
 The IPCC AR4 also concluded that there is no clear trend in the annual numbers of tropical
cyclones but that there is a significant increase in severity (expressed by the « power
dissipation index ») since 1970. Severity is directly proportional to sea surface temperature.
Harvey hit Texas on Friday (25th of August ) as a terrifying category 4 hurricane – one of
the biggest to batter the US mainland in more than a decade.

30. Lunar Cycle
 The lunar phase or phase of the moon is the shape of the illuminated portion of the Moon as
seen by an observer on Earth.
 The lunar phases change cyclically as the Moon orbits the Earth, according to the changing
positions of the Moon and Sun relative to the Earth.
 The Moon's rotation is tidally locked by the Earth's gravity, therefore the same lunar surface
always faces Earth.
 This face is variously illuminated depending on the position of the Moon in its orbit.
 Therefore, the portion of this hemisphere that is visible to an observer on Earth can vary from
about 100% (full moon) to 0% (new moon).
 The lunar terminator is the boundary between the illuminated and darkened hemispheres.
 Each of the four "intermediate" lunar phases are roughly seven days (~7.4 days) but this varies
slightly due to the elliptical shape of the Moon's orbit.
 Aside from some craters near the lunar poles such as Shoemaker, all parts of the Moon see
around 14.77 days of sunlight, followed by 14.77 days of "night". (The side of the Moon facing
away from the Earth is sometimes called the "dark side of the Moon", although that is
a misnomer.)
 Tidal locking (also called gravitational locking or captured rotation) occurs when, over the
course of an orbit, there is no net transfer of angular momentum between an astronomical
body and its gravitational partner.
 This state can result from the gravitational gradient (tidal force) between two co-orbiting
bodies, acting over a sufficiently long period of time.
 In the case where the orbital eccentricity is exactly zero, tidal locking results in one hemisphere
of the revolving object constantly facing its partner, an effect known as synchronous rotation.
 For example, the same side of the Moon always faces the Earth, although there is
some libration because the Moon's orbit is not perfectly circular.
 A tidally locked body in synchronous rotation takes just as long to rotate around its own axis as
it does to revolve around its partner.
 Usually, only the satellite is tidally locked to the larger body.
 This effect is employed to stabilize some artificial satellites.

31. PRINCIPAL AND INTERMEDIATE PHASES OF MOON

32.  When the Sun and Moon are aligned on the same side of the Earth, the moon is "new", and the
side of the Moon facing Earth is not illuminated by the Sun. As the moon waxes (the amount of
illuminated surface as seen from Earth is increasing), the lunar phases progress through new
moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The moon is then said
to wane as it passes through the gibbous moon, third-quarter moon, crescent moon and back to
new moon. The terms "old moon" and "new moon" are not interchangeable. The "old moon" is
a waning sliver (which eventually becomes undetectable to the naked eye) until the moment it
aligns with the sun and begins to wax, at which point it becomes new again. Half moon is often
used to mean the first- and third-quarter moons, while the term 'quarter' refers to the extent of
the moon's cycle around the Earth, not its shape.
 When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion
of the illuminated area that is visible will have a two-dimensional shape defined by the
intersection of an ellipse and circle (where the major axis of the ellipse coincides with a
diameter of the circle). If the half-ellipse is convex with respect to the half-circle, then the
shape will be gibbous (bulging outwards) whereas if the half-ellipse is concave with respect to
the half-circle, then the shape will be a crescent. When a crescent Moon occurs, the
phenomenon of earthshine may be apparent, where the night side of the Moon faintly reflects
light from the Earth.
 In the Northern Hemisphere, if the left side of the Moon is dark then the light part is growing,
and the Moon is referred to as waxing (moving toward a full moon). If the right side of the
Moon is dark then the light part is shrinking, and the Moon is referred to as waning (past full
and moving toward a new moon). Assuming that the viewer is in the northern hemisphere, the
right portion of the Moon is the part that is always growing (i.e., if the right side is dark, the
Moon is growing darker; if the right side is lit, the Moon is growing lighter).
 In the Southern Hemisphere the Moon is observed from a perspective inverted to that of the
northern hemisphere, and all of the images in this article, so that the opposite sides appear to
grow (wax) and shrink (wane).
 Nearer the Equator the Moon with its terminator will appear apparently horizontal during the
morning and evening. Since the above descriptions of the lunar phases only apply at temperate
or high latitudes and observers moving towards the Tropics from northern or southern latitudes
will see the Moon rotated anti-clockwise or clockwise with respect to the images in this article.
The crescent Moon can open upward or downward, with the "horns" of the crescent pointing
up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens
downward; when the Moon is above the Sun, the crescent opens upward. The crescent Moon is
most clearly and brightly visible when the Sun is below the horizon, which implies that the
Moon must be above the Sun, and the crescent must open upward. This is therefore the
orientation in which the crescent Moon is most often seen from the Earth's tropics. The waxing
and waning crescents look very similar. The waxing crescent appears in the western sky in the
evening, and the waning crescent in the east, in the morning.
 When the Moon, as seen from Earth, is a narrow crescent, the Earth as seen from the Moon is
almost fully lit by the Sun. Often, the part of the Moon that is not directly lit by the Sun is
sufficiently brightly lit by light reflected from the Earth to be easily visible from Earth. This

33. phenomenon is called "earthshine", and is sometimes picturesquely described as "the old moon
in the new moon's arms" or, as pictured here, "the new moon in the old moon's arms".
 Non-western cultures may use a different number of Moon phases, for example
traditional Hawaiian culture has a total of 30 different Moon phases (one per day).
Phases of the Moon, as seen looking southward from the northern hemisphere. The southern
hemisphere will see each phase rotated through 180°. The upper part of the diagram is not to
scale, as the Moon is much farther from the Earth than shown here.

34. SUNSPOT CYCLE
 The Sunspot/solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change
in the Sun's activity (including changes in the levels of solar radiation and ejection of solar
material) and appearance (changes in the number and size of sunspots, flares, and other
manifestations).
 They have been observed (by changes in the sun's appearance and by changes seen on Earth,
such as auroras) for centuries.
 The changes on the sun cause effects in space, in the atmosphere, and on Earth's surface.
 While it is the dominant variable in solar activity, periodic fluctuations also occur. One
complete magnetic cycle spans two solar cycles, or 22 years, before returning to its original
state. However, because nearly all manifestations are insensitive to polarity, the "11-year solar
cycle" remains the focus of research.
 Solar physicists monitor the solar cycle is by studying the surface of the sun for dark splotches
called sunspots.
 These short-lived patches are caused by intense magnetic activity and tend to cluster in bands
at mid-latitudes above and below the equator.
 The frequency and number of these mysterious dark spots on the solar surface act as indicators
of the sun's activity as it moves between solar minimum and maximum.
 The Sun's apparent surface, the photosphere, radiates more actively when there are more
sunspots.
 Sunspots sometimes erupt into powerful solar storms that shoot streams of charged particles
into space, occasionally in the direction of Earth. Some powerful solar storms can bombard
Earth's magnetic field and disrupt power grids or knock out satellites in orbit around the planet.
 As the sun reaches the end of a cycle, new sunspots appear near the equator, and a new cycle
begins with the production of sunspots at higher latitudes on the surface of the sun.
 As each cycle begins, sunspots appear at mid-latitudes, and then closer and closer to the
equator until solar minimum is reached.
 This pattern is best visualized in the form of the so-called butterfly diagram. Images of the Sun
are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots
calculated.
 This is plotted vertically as a color-coded bar, and the process is repeated month after month to
produce this time-series diagram.
The sunspotbutterflydiagram.This modern version is constructed (andregularlyupdated) by the solar group at NASA Marshall
Space Flight Center.

35.  In 1610, shortly after viewing the sun with his new telescope, Galileo Galilei, an Italian
astronomer, made the first European observations of Sunspots (or it may be Thomas Harriot, a
British astronomer, who made a drawing of the Moon through a telescope, on 26 July 1609,
over four months before Galileo.)
 The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, a German astronomer
who after 17 years of extended observations of sunspots at the Zurich Observatory noticed a
periodic variation in the average number of sunspots.
 Rudolf Wolf, a Swiss astronomer compiled and studied these and other observations,
reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest
observations of sunspots by Galileo and contemporaries in the early seventeenth century.
 Following Wolf's numbering scheme, the 1755–1766 cycle is traditionally numbered "1".
 Wolf created a standard sunspot number index, the Wolf index, which continues to be used
today.
Galileo Galilei (1564 – 1642) Thomas Harriot(1560 –1621)
Samuel Heinrich Schwabe (1789–1875) Rudolf Wolf (1816–1893)

36. RECENT SUNSPOT CYCLE
 Solar Cycle 24 is the recent solar cycle since 1755, when extensive recording of
solar sunspot activity began.
 It is the current solar cycle, and began in December 2008 with a smoothed minimum of 2.2
until early 2010.
 It reached its maximum in April 2014 with smoothed sunspot number only 116.4, the lowest in
over a century.
 Currently, the sun is in the midst of Cycle 24, and the star is swelling toward a maximum in
2013.
 The cycle featured a "double-peaked" solar maximum.
 The first peak reached 99 in 2011 and the second in early 2014 at 101.
 Several new studies are predicting that after this peak, the sun's activity could see a significant
drop in Cycle 25.
Cycle 23
 This cycle lasted 11.6 years, beginning in May 1996 and ending in January 2008.
 The maximum smoothed sunspot number (monthly number of sunspots averaged over a
twelve-month period) observed during the solar cycle was 120.8 (March 2000), and the
minimum was 1.7.
 A total of 805 days had no sunspots during this cycle.

37. ISES Solar Cycle 24 Sunspot Number Progression
Sunspot data
Start date December 2008
Max count 116.4
Max count month April 2014
Min count 2.2
Spotless days 51
International Solar Energy Society (ISES)
Total solar irradiance (TSI)/ Solar Constant
 The total solar irradiance (TSI)/ solar constant is the amount of solar radiative energy incident
on the Earth's upper atmosphere.
 TSI variations were undetectable until satellite observations began in late 1978.
 A series of radiometers were launched on satellites from the 1970s to the 200s.
 TSI measurements varied from 1360 to 1370 W/m2 across ten satellites.
 Solar irradiance varies systematically over the cycle, both in total irradiance and in its relative
components (UV vs visible and other frequencies).

38.  The solar luminosity is an estimated 0.07 percent brighter during the mid-cycle solar maximum
than the terminal solar minimum, even though sunspots are darker (cooler) than the average
photosphere at the time of solar maximum.
 The ratio of ultraviolet to visible light varies.
 TSI varies in phase with the solar magnetic activity cycle with an amplitude of about 0.1%
around an average value of about 1361.5 W/m2.
EFFECTS OF SUNSPOT CYCLE
1. CMEs (coronal mass ejections) produce a radiation flux of high-energy protons, sometimes
known as solar cosmic rays. These can cause radiation damage to electronics and solar
cells in satellites.
CME radiation is dangerous to astronauts on a space mission who are outside the shielding
produced by the Earth's magnetic field.
2. Energy changes in UV irradiance involved in production and loss of ozone have atmospheric
effects.
The amount of ultraviolet UVB light at 300 nm reaching the Earth varies by as much as 400%
over the solar cycle due to variations in the protective ozone layer.
In the stratosphere, ozone is continuously regenerated by the splitting of O2 molecules by
ultraviolet light.
During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a
decrease in the concentration of ozone, allowing increased UVB to reach the Earth's surface
which affects Terrestrial Organisms.
3. Sunspot activity has a major effect on long distance radio communications, particularly on
the shortwave bands although medium wave and low VHF frequencies are also affected.
High levels of sunspot activity lead to improved signal propagation on higher frequency bands,
although they also increase the levels of solar noise and ionospheric disturbances.
These effects are caused by impact of the increased level of solar radiation on the ionosphere.
10.7 cm solar flux could interfere with point-to-point terrestrial communications.
4. The cosmic ray changes over the cycle potentially have significant atmospheric effects.
Changes in ionization affect the aerosol abundance that serves as the condensation nucleus for
cloud formation.
During solar minima more cosmic rays reach Earth, potentially creating ultra-small aerosol
particles as precursors to Cloud condensation nuclei.

39. Clouds formed from greater amounts of condensation nuclei are brighter, longer lived and
likely to produce less precipitation.
A change in cosmic rays could cause an increase in certain types of clouds, affecting
Earth's albedo.
5. The sunspot cycle variation of 0.1% has small but detectable effects on the Earth’s climate.
Camp and Tung suggest that between solar maximum and minimum, solar irradiance correlates
with a variation of 0.18 K ±0.08 K with average global temperature
6. The current scientific consensus, most specifically that of the IPCC, is that solar variations
do play a smaller role in driving global warming, since the measured magnitude of recent solar
variation is much smaller than the forcing due to greenhouse gases.
Also, solar activity in the 2010s was not higher than in the 1950s, whereas global warming had
risen markedly.
Otherwise, the level of understanding of solar impacts on weather is low.
SOLAR-LUNAR TIDES
 People who have lived along the coast for a while know that no two tides are quite the same.
That’s because the tides are the result of a gravitational tug-of-war between Earth and two
other astronomical bodies i.e. Sun and the moon.
 The best-known tides are those caused by the Moon.
Gravitational attraction between two objects depends on the mass of the objects and the
distance between them. The closer two objects, the stronger the gravitational attraction
between them. Because of this, water closest to the Moon experiences more gravitational force
from the Moon than water further away. Water furthest from the Moon experiences the least
force.
 Its gravity pulls a little more strongly on the side of Earth that faces it, producing “bulges” in
the oceans -- one on the side of Earth that faces the Moon, the other on the opposite side.
When the bulges hit land, the water level rises, causing high tides.
And half-way between high tides, the water is at its lowest level in the cycle, causing low tides.
 The Earth rotates through these two water bulges as it spins on its own axis.
 If the Moon was stationary and the only movement was the rotation of the Earth, there would
be two high and two low tides every 24 hours.
 But the Moon is not stationary and during the 24 hours it takes the Earth to complete one
rotation, the Moon moves a small amount in the same direction.
 Because of the Moon’s orbital motion around Earth, the tides peak about 50 minutes later each
day. As a result, a spot on Earth has to rotate 50 minutes longer to 'catch up' to the Moon.

40. This periodicity means that on average:
• From a vantage point on the Earth, the Moon takes 24 hours and 50 minutes to complete one
full orbit.
• There is a high or low tide once every 6 hours and 12 minutes.
• High tides occur about 12 hours and 25 minutes after the previous high tide.
• Same for low tides, there is one about every 12 hours and 25 minutes.
• Every day, high and low tides occur 50 minutes later than they did on the previous day.
 Gravity from the Sun also influences water levels on the Earth.
 The solar tidal bulges are about half the size of those caused by the Moon.
 Like the Moon, gravitational attraction to the Sun creates one bulge towards the Sun and one
away from it.
 Unlike the Moon, solar tides do not vary on a daily basis.
 If there was no Moon, the daily tidal period would be exactly 24 hours.
 High tide would be at Noon and Midnight, and low tide at 6 PM and 6 AM every day.
 The orientation of the Moon and the Sun vary with respect to each other over the lunar cycle.
The different phases of the Moon are a manifestation of this variation.
 This change in orientation also causes tidal heights to vary predictably over the monthly cycle.
 The largest tides ranges are called Spring tides.
 These occur during full and new Moons when the gravitational influence of the Sun and the
Moon line up with each other.
 The smallest tides are called Neap tides.
 These occur during first and third quarter Moons when the gravitational influence of the Sun
and the Moon are at right angles to each other.
CHANDLERS COMPENSATION
 If you travel to the Arctic and attempt to find the axis of Earth’s rotation, you’ll notice
something odd.
 The position of this axis on Earth’s surface moves with a period of about seven years.
 This is the combined result of two effects.
 The one we’re interested today is called the Chandler Wobble, which has a period of 433 days
and was discovered by American astronomer Seth Carlo Chandler in 1891.
 Any sphere that is not perfectly spherical has a slight wobble when it spins, similar to the effect
you see as a spinning top is slowing down.
 The Chandler Wobble is a small variation in the Earth’s axis of rotation that causes latitude and
longitude to vary slightly, as the poles are displaced from their mean positions.
 The north pole of rotation rotates counterclockwise around its mean position.
 Because the axis passes through the polar regions, this motion is also called Polar Motion.
 Chandler’s detection of this effect was facilitated by his invention of the almucantar, a device
for measuring the positions of stars relative to a circle centred at the zenith rather than to
the meridian.

41.  The North Pole of Earth’s rotation axis wanders in an irregular, quasi-circular path with a
radius of about 8–10 metres (26–33 feet).
 So on an average, this wobble amounts to change of about 9 meters (30 ft) in the point at which
the axis intersects the Earth's surface and has a period of 433 days.
 Think of the wobble you see in a toy top when it first starts spinning or slows down.
 Its "poles" do not spin in a perfectly straight line.
 The displacement of the Chandler wobble is measurable –
 Imagine a gigantic ballpoint pen poked through the center of the earth, entering at the South
Pole and exiting at the North Pole.
 Imagine the pen is drawing on a scratch pad-equipped space station directly over the North
Pole.
 After a day (one full rotation of the earth on its axis) the ballpoint pen draws a circular path,
and not a dot, because of the "wobble" in the earth's rotation on its axis.
 Over 14 months the pen draws a spiral path similar to this drawing.

42. POLAR MOTION
 Polar motion, a periodic rotation of the Earth’s spin axis about a mean axis, somewhat like the
wobble of a spinning top.
 Slight variations in latitude and longitude result from this wobble because the poles are
displaced from their mean positions.
 The north pole of rotation rotates counterclockwise around its mean position.
Polar motion is primarily made up of two discrete periodic oscillations:
 One, called the Chandler Wobble, has about a 14-month period, and the other has a 12-month
period.
 The combination of these two wobbles causes the poles to trace spiral paths out of, around, and
eventually back into their mean positions over a period of about 6.5 years.
 The separation between the actual and mean poles was exceptionally large in about 1952, when
they were separated by 12 m (37 feet), or 0.37 arc second (0.37″).
 Their maximum separation during the 6.5-year period averages about 0.25″.
 Polar motion was first predicted by the Swiss physicist Leonhard Euler in 1765
using dynamical theory and a rigid model of the Earth.
 He predicted a 10-month oscillation period for the phenomenon.
 Observational proof for the postulated latitude variations was obtained in the mid-1880s, and
about that time the American astronomer S.C. Chandler analyzed these data and obtained both
the 14-month and the 12-month periods.
 Once the Chandler wobble was observed, The four-month difference between Euler’s predicted
period and the actual duration of the Chandler Wobble was explained by Simon Newcomb as
being caused by the non-rigidity of the Earth.
 The full explanation for the period involves the fluid nature of the Earth's core, elasticity of the
Earth’s mantle and the mobility of the oceans which together subtly affect the Earth’s response
to rotation, which Euler had not considered in his calculations.
 The wobble, in fact, produces a very small ocean tide with an amplitude of approximately
6 mm (1⁄4 in), called a "pole tide", which is the only tide not caused by an extraterrestrial body.
Despite the small amplitude, the gravitational effect of the pole tide is easily detected by
the superconducting gravimeter.
 The International Latitude Observatories were established in 1899 to measure the wobble.
These provided data on the Chandler and annual wobble for most of the 20th century, though
they were eventually superseded by other methods of measurement.
 Monitoring of the polar motion is now done by the International Earth Rotation Service.

43.  There are theories about the cause and the effect of the wobble.
 Some think tides and the liquid interior of the earth could play a part.
 Some include the constant winds over the oceans pushing varying amounts of water on the
earth at one time or even the effects of a major earthquake.
 Recent theories attributes most of the wobble to pressure changes in the ocean.
 Over the years, various hypotheses have been put forward, such as atmospheric phenomena,
continental water storage (changes in snow cover, river runoff, lake levels, or reservoir
capacities), interaction at the boundary of Earth's core and its surrounding mantle, and
earthquakes.
 One promising theory for the source of the wobble was proposed in 2001 by Richard Gross at
the Jet Propulsion Laboratory managed by the California Institute of Technology.
 He used angular momentum models of the atmosphere and the oceans in computer simulations
to show that from 1985 to 1996, the Chandler wobble was excited by a combination of
atmospheric and oceanic processes, with the dominant excitation mechanism being
ocean‐bottom pressure fluctuations.
 Gross found that two-thirds of the "wobble" was caused by fluctuating pressure on the seabed,
which, in turn, is caused by changes in the circulation of the oceans caused by variations
in temperature, salinity, and wind. The remaining third is due to atmospheric fluctuations.
 The amplitude and phase of the Chandler Wobble change over time, fluctuating noticeably
from one decade to another, which is widely believed to be caused largely by pressure
fluctuations at the bottom of the oceans. The pressure changes result from ocean
circulation currents, and variations in temperature and salinity. This theory explains small
changes in phase and amplitude, but in the 1920s the Chandler Wobble phase suddenly jumped
by 180 degrees, which cannot be explained by gradual variations in pressure.
 Now a new analysis of data on Earth’s rotation going back 160 years indicates that this 1920
event was not unique. In 2009, Zinovy Malkin and Natalia Miller analyzed time series data of
International Earth Rotation and Reference Systems Service (IERS) Pole coordinates from
January 1946 to January 2009 at the Russian Academy of Sciences Central Astronomical
Observatory in Pulkovo which say the phase has changed by small amounts on many occasions
during this time.
 But the big news is that the wobble underwent 180-degree changes in phase on two other
occasions: once in 1850 and the other in 2005. So three major phase reversals of the wobble
has been observed till now in 1850, 1920, and 2005.
 So why should the Chandler Wobble undergo these changes in phase? An interesting puzzle
for anybody with a few brain cycles to spare.

44. EFFECTS OF CHANDLERS WOBBLE
 The chandler wobble doesn't really have any effect on most people.
 The people who live with it on a daily basis are astronomers using earth-based telescopes and
people using various navigation systems.
 With telescopes, the wobble affects the ability to point at a star accurately.
 The Chandler wobble also affects celestial navigation, since the latitude does change over a
period of 14 months.
 Global Positioning Systems,(GPS), can overcome the effect of the wobble on navigation.
Navigators' star charts, however, still have to be updated to show the new reference point for
the geographic North and South Poles.
 The magnetic North Pole, used by a compass, is not affected.
 Astronomical observations of the Earth’s rotational position in space, which are used in
determining Coordinated Universal Time , must be corrected for slight variations in longitude
caused by polar motion.

45. BLOCKING HIGH
 Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly
stationary, effectively “blocking” or redirecting migratory cyclones.
 They are also known as blocking highs or blocking anticyclones.
 These blocks can remain in place for several days or even weeks, causing the areas affected by
them to have the same kind of weather for an extended period of time (e.g. precipitation for
some areas, clear skies for others).
 In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the
eastern Pacific and Atlantic Oceans.
TYPE OF BLOCKS
1. Omega Block
Omega blocks are named due to pattern they form which resembles the uppercase Greek letter
omega, Ω.
 An area of high pressure will be sandwiched in between two lows to the east and west, and also
slightly to the south.
 These best analyzed at 500-mbs.
 The region under the omega block experiences dry weather and light wind for an extended
period of time while rain and clouds are common in association with the two troughs on either
side of the omega block.
 Omega blocks make forecasting easier since you can pinpoint areas that will be dominated by
dry or rainy weather for several days.
 The right side of the omega block will have below normal temperatures while the region to the
left will have above normal temperatures in this case.
 These blocks frequently occur on the eastern edges of the Atlantic and eastern Pacific, and can
lead to easterly flows to the UK.

46. 2. Diffluent Block / Rex block/ Dipole blocks
 A split in the eastwards flow can lead to a Diffluent Block.
 Rex blocks consist of a strong high-pressure ridge situated to the north (more generally,
poleward) of a strong low-pressure trough.
 Very often both the high and the low are closed, meaning that the isobars (or
constant geopotential height lines) defining the high–low close to form a circle.
 Rex blocks are named after the meteorologist who first identified them.
 Air flow in the north Pacific curves around the ridge and then around the trough in the
Southwest US.
 This causes the air to swing in loops near the same longitude but forces the air to a much
lower latitude.
 The wind flow makes little progress to the east due to the "half figure 8" rotation the air must
endure.

47. 3. CUT-OFF HIGHS AND LOWS/ RING OF FIRE
 When an upper-level high- or low-pressure system becomes stuck in place due to a lack of
steering currents, it is known as being "cut off".
 The usual pattern which leads to this is the jet stream retreating poleward, leaving the then cut-
off system behind.
 If the block is a high, it will usually lead to dry, warm weather as the air beneath it is
compressed and warmed, as happened in southeastern Australia in 2006 and 1967 with
resultant extreme droughts.
 Rainy, cooler weather results if the block is a low.
 Precisely this situation occurred over the southern United States during late spring and early
summer of 2007, when a cut-off-low system hovering over the region brought unusually cool
temperatures and an extraordinary amount of rain to Texas and Oklahoma (see June 2007
Texas flooding), and a cut-off-high near the coast of Georgia that caused a drought in the
Southeast that same year on weakening of jet stream with a lack of Canadian cool fronts..