 Tropical cyclones are violent storms -originate over oceans in tropical

areas -move over to coastal areas -bring large scale destruction -violent
winds (squalls), -heavy rainfall (torrential rainfall) and storm surge.
 are irregular wind movements- involve closed circulation of air- around
a low pressure center-
 . closed air circulation (whirling motion) - result of rapid upward
movement of hot air - subjected to Coriolis force.
 low pressure at center -responsible for wind speeds.
 anti-clockwise in northern hemisphere -clockwise in southern
hemisphere ( due to Coriolis force).

 Conditions Favourable for Tropical Cyclone Formation

 Large sea surface -temperature higher than 27° C,-source of moisture-
release latent heat of condensation
 Coriolis force -to create a cyclonic vortex-zero at the equator (no
cyclones at equator- 65 per cent of cyclonic activity -between 10° and
20° latitude.
 Temperature contrast between air masses-convergence of air masses of
different temperatures-resulting instability-prerequisites for origin and
growth of cyclones
 Small differences in the vertical wind speed-
 pre-existing weak low-pressure area or low-level-cyclonic circulation,
 Low-level Disturbances-in form of Easterly wave disturbances in ITCZ
o Upper divergence above sea level system,-so that rising air
currents within cyclone -pumped out -and low pressure
maintained at center.
 High humidity-in mid-troposphere-since moist air leads to formation
of cumulonimbus cloud.

 Origin and Development of Tropical Cyclones

 tropical cyclones - thermal origin- develop over tropical seas -late
summers (August to mid-November).
 strong local convectional currents -acquire whirling motion-Coriolis
force
 After developing- cyclones advance- till they find- weak spot in trade
wind belt.

 Origin
 Under favorable conditions- multiple thunderstorms originate
over oceans.
 These thunderstorms merge and create -intense low pressure system
(wind is warm and lighter).

 Process-[img-tropical cyclone formation]

 In thunderstorm- air warm and light- uplifted -
 At certain height- lapse rate and adiabatic lapse rate-temperature of air
falls - undergoes condensation.
 Condensation releases latent heat of condensation - air more warmer-
further uplifted.
 space filled by fresh moisture laden air- Condensation occurs in this air
-cycle repeated- as long as -moisture supplied.
 Due to excess moisture over oceans-thunderstorm intensifies- sucks in
air at faster rate
 air from surroundings rushes in -undergoes deflection due to Coriolis
force -creating a cyclonic vortex (spiraling air column. Similar to
tornado).
 Due to centripetal acceleration (centripetal force pulling
towards center)- air in vortex -forced to form - region of calmness -
called an eye at center of cyclone.
 inner surface of vortex forms eye wall, -most violent region of cyclone
 All wind-carried upwards loses its moisture -becomes cold and dense. -
descends to surface - at edges of cyclone.
 Continuous supply of moisture from sea - major driving force behind
every cyclone. -On reaching land -moisture supply cut off - storm
dissipates.

 Mature stage
 At this stage-spiraling winds create multiple convective cells -with
successive calm and violent regions.
 regions with cumulonimbus cloud formation (rising limbs of convective
cell)-called rain bands- intense rainfall occurs.
 ascending air -lose moisture at some point -descends (subsides) back to
surface- that exist between two rain bands.
 Cloud formation -dense at center-cloud size decreases from center to
periphery.
 Rain bands -mostly made up of cumulonimbus clouds. - ones
at periphery -made up of nimbostratus and cumulus clouds.
 dense overcast at upper levels of troposphere -due to cirrus clouds-
mostly made up of hexagonal ice crystals.

 Modification and Decay

 begins to weaken- in terms of central low pressure, internal warmth and
extremely high speeds- as soon as source of warm moist air -begins to
ebb -or is abruptly cut off.
 his happens after its landfall or when it passes over cold waters.

 Structure of a tropical cyclone

 Eye-center of tropical cyclone.-no precipitation-lowest surface pressure
and warmest temperatures aloft-eye temperature -10°C warmer or more-
than surrounding environment
 Eye wall-circular ring of deep convection-area of highest surface winds-
fastest winds-occur along the eye wall region.-occasionally strong –
updrafts and downdrafts-wind reaches maximum velocity in this region
 Spiral bands-Convection in tropical cyclones-long, narrow rain bands-
same direction as horizontal wind-spiral into center of a tropical
cyclone-called “spiral bands”.-plays a role in forming and maintaining
the eye

 Vertical Structure of a Tropical Cyclone

 lowest layer-up to 3 km-responsible for driving the storm.
 middle layer-from 3 km to 7 km- where main cyclonic storm takes
place
 outflow layer lies above 7 km - 12 km-movement of air
is anticyclonic in nature.

 Categories of Tropical Cyclones

 Category one (tropical cyclone):strongest winds are GALES with
typical gusts over open flat land -90-125kph,-
 Category two (tropical cyclone):DESTRUCTIVE winds -typical gusts
over open flat land -125-164kph,
 Category three (severe tropical cyclone):VERY DESTRUCTIVE
winds- 165-224kph,
 Category four (severe tropical cyclone):225-279kph
 Category five (severe tropical cyclone):280kph.

 Distribution of Tropical Cyclones[img]

 South-east Caribbean region -called hurricanes.
 Philippines islands, eastern China and Japan- called typhoons.
 Bay of Bengal and Arabian Sea- called cyclones.
 Around south-east African coast and Madagascar-Mauritius islands.
 North-west Australia.

 Characteristics of Tropical Cyclones

 Size and Shape-symmetrical elliptical shapes + steep pressure
gradients.
 Wind Velocity and Strength-more in poleward margins than at
center/more over oceans than over landmasses

 Path of Tropical Cyclones

 cyclones start with a westward movement —- because earth rotating
from west to east + zone of cyclone formation -under influence of
easterlies.
 b) turn northwards around 20° latitude — Coriolis force deflects path
towards right.
 c) turn further north-eastwards around 25° latitude — Coriolis force
deflects it further right.
 d) then eastwards around 30° latitude — Because of westerly winds.
 e) then lose energy and subside — Ocean water at 30 ° latitude -not
warm enough to sustain a cyclone. + increasing wind shear due to
westerlies -not facilitate formation of cyclonic vortex.
 Why tropical cyclones form mostly on western margins of oceans? OR
Why tropical cyclones don’t form in eastern tropical oceans?
 depth of warm water (26-27°C)- should extend 60-70 m from ocean
surface- so that deep convection currents within water -do not churn
and mix cooler water -below with warmer water near surface.
 above condition- occurs only in western tropical oceans- because of
warm ocean currents (easterly trade winds )- that flow from east
towards west -forming a thick layer of water- with temperatures greater
than 27°C.
 supplies enough moisture to storm.
 cold currents lower surface temperatures of eastern parts of tropical
oceans -making them unfit for breeding of cyclonic storms.
 [One Exception: During strong El Nino years- strong hurricanes occur
in eastern Pacific- due to accumulation of warm waters in eastern
Pacific- due to weak Walker Cell]
 Why cyclones occur mostly in late summers?
 Whirling motion -enhanced when doldrums (region within ITCZ) over
oceans -farthest from the equator.
 happens during autumnal equinox (August-September).
 At this time- two advantages— air is overheated and sun is exactly
over equator.
 [Due to high specific heat of water, and mixing, -ocean waters in
northern hemisphere attain maximum temperatures in August.
(Continents attain maximum temperatures in June-July)]

 Warning of Tropical Cyclones

 Detection of cyclones -three main parameters:
 fall in pressure,
 increase in wind velocity,
 direction and movement (track) of storm.
 Monitoring mechanism
 network of weather stations- monitoring pressure fall and wind
velocities -in all countries-including Arctic and Antarctic regions.
 In India-detection radars along both coasts.
 Monitoring -also done by aircraft- which carry-number of instruments
including a weather radar.
 Cyclone monitoring by satellites -done through -high resolution
radiometers, -obtain infra-red regions of spectrum- to obtain image
of cloud cover and its structure.
 possible to detect cyclone right from its genesis in high seas + follow
its course-, giving a warning at least 48 hours prior to a cyclone strike.

 helps in taking advance steps in following areas:

 closing of ports and harbours,
 suspension of fishing activities,
 evacuation of population,
 stocking of food and drinking water, and
 provision of shelter with sanitation facilities (safety homes).


 Why do ‘tropical cyclones’ winds rotate counter-clockwise (clockwise)
in the Northern (Southern) Hemisphere?
 earth’s rotation- sets up an apparent force (called Coriolis force) -
pulls winds to right in Northern Hemisphere (and to left in the
Southern Hemisphere).
 So- when low pressure starts -form over north of equator-surface
winds flow inward- trying to fill in the low - will be deflected to right-
and a counter-clockwise rotation will be initiated.
 The opposite (a deflection to the left and a clockwise rotation) will
occur south of equator.

 Why both clockwise and anti-clockwise flowing drains?

 Coriolis force -too tiny to effect rotation in drains of sinks and toilets.
 rotation in those -determined by geometry of container and original
motion of water.
 Thus,- can find both clockwise and counter-clockwise flowing drains-in
both North and South hemispheres

 Why there are fewer cyclones over the Arabian Sea as compared to the
Bay of Bengal?

 frequency more in Bay of Bengal than in Arabian Sea-ratio being 4:1.

 More low-level disturbances in Bay of Bengal
 Cyclones formed over Bay of Bengal -either those develop that insitu
over southeast Bay of Bengal and adjoining Andaman Sea- or remnants
of typhoons over Northwest Pacific - that move across south China sea
to Indian Seas.
 As frequency of typhoons over Northwest Pacific -quite high ( 35 %
of global annual average)-Bay of Bengal -gets its increased quota.
 cyclones over the Arabian Sea -originate insitu over southeast Arabian
Sea ( includes Lakshadweep area also) or remnants of cyclones
from Bay of Bengal -that move across south peninsula.
 As majority of Cyclones over Bay of Bengal weaken over land after
landfall-frequency of migration into Arabian Sea is low.

 Surface temperature
 surface temperature of Bay of Bengal is higher-usually between 22 °C
and 31 °C-cooler by 1-2 °C in Arabian Sea because of monsoon winds.
 Arabian Sea -relatively colder (mosnsoon winds) than Bay of Bengal -
hence inhibits formation and intensification of system.

 Salinity
 Arabian Sea surface has higher salinity-because evaporation
over Arabian Sea - greater than precipitation + river runoff ( loses more
freshwater than it receives)
 • Salinity in Bay of Bengal -can be as low as 31 ppt- because -
receives
 freshwater from Ganga, Brahmaputra, Irrawaddy, Godavari, and others.
• Salinity near the surface in the Arabian Sea is much
higher than in the Bay of Bengal because evapouration
 and

 Higher stratification in Bay of Bengal-

• Freshwater less dense compared to saline water- Hence
vertical mixing -inhibited in Bay of Bengal.
 • On the other hand-high evapouration + low inflow of fresh water -
increases salinity in Arabian Sea-increases vertical mixing.

 Monsoon winds in Arabian sea drive away moisture froom India


 Why there are very few Tropical Cyclones during southwest monsoon
season?
 Large vertical wind shear
 southwest monsoon -characterized by presence of strong westerly
winds- in lower troposphere (below 5 km) + very strong easterly winds
in upper troposphere (above 9 km).
 results in large vertical wind shear.
 Strong vertical wind shear inhibits cyclone development.
 Less time for development-
 potential zone for development of cyclones -shifts to North Bay of
Bengal -during southwest monsoon season.
 During this season-low pressure system-form along monsoon trough
(ITCZ), -extends from northwest India to north Bay of Bengal.
 Depression forming over this area -crosses Orissa – West Bengal coast
in a day or two. -These systems have shorter oceanic stay-make landfall
very quickly-reasons for their non-intensification into intense cyclones.

 Storm Surge?
 abnormal rise of sea level as cyclone crosses the coast.
 Sea water inundates coastal strip causing loss of life, property & crop.
 Increased salinity in soil - land unfit for agricultural use
 Storm surge depends on intensity of the cyclone
 pollute drinking water sources
 uprooting of trees, damage to dwellings, overhead installations,
 intense & heavy precipitation
 storm tide?
 combination of storm surge and astronomical tide.

 Naming of Cyclones[pyq]
 WMO divided world Oceans into Basins
 assigned responsibility of naming Cyclones to respective regional
bodies.
 Each regional body - own rules in naming cyclones.
 In most regions pre-determined alphabetic lists of alternating male and
female names used.
 In north-west Pacific -majority of names used -not personal names. -
majority are names of flowers, animals, birds, trees, foods or descriptive
adjectives.


 How are cyclones named in Northern Indian Ocean Region

 WMO/ESCAP (World Meteorological Organisation/United Nations
Economic and Social Commission for Asia and the Pacific)-comprised
of Eight countries in the region - Bangladesh, India, Maldives,
Myanmar, Oman, Pakistan, Sri Lanka and Thailand -
o After each country sent in suggestions, -WMO/ESCAP Panel on
Tropical Cyclones (PTC) finalised the list.
o not allocated in alphabetical order-but are arranged by name
of country which contributed the name.
 usual practice for a storm to be named -when it reaches tropical storm
strength (winds of 34 knots).
 (IMD) which issues cyclone advisors- to eight countries- has a list of
names contributed by each of them.
 Every time a cyclone occurs- name is picked in order of names that are
already submitted.
 Each country- gets a chance to name a cyclone. -After all the countries
get their turn, -next list of names is followed.
 Hudhud, Titli, Phethai, Fani, Vayu and Amphan -among names of
cyclones in Indian Ocean region.
 Why is this system of uniformity in naming a cyclone in the
region/Benefits of naming :
 Tropical cyclones named -to provide ease of communication between
forecasters and general public regarding forecasts and warnings.
 Since storms -often last a week or even longer- and more than one
cyclone can be occurring in same region at same time- names can
reduce confusion- about what storm is being described
 Naming them after a person/flower/animal etc. -it easier for media to
report on tropical cyclones + increases community preparedness + quick
information exchange between faraway stations, ships etc.
 Identify each individual cyclone.
 Create awareness of its development.

 Polar or Arctic Cyclones
 Arctic or polar cyclones occur in Antarctic regions and can reach up to
1,200 miles wide.
 differ with others because they are not seasonal. [Tropical Cyclones are
seasonal]
 can occur at any time of year.
 can also form quickly (sometimes less than 24 hours),
 direction or movement cannot be predicted.
 can last from a day up to several weeks. [Tropical Cyclones doesn’t for
more than a week]
 Most frequently-develop above northern Russia and Siberia.

 Upper Tropospheric cyclone/Upper Air Disturbance /upper tropospheric
westerly trough
 upper tropospheric cyclone -moves slowly from east to west -is
prevalent in summer
• circulations -do not extend below 6000 m in altitude
• remains of this cyclone (upper tropospheric westerly
trough ) from westerlies -move deep into tropical
latitude regions
• These troughs can assist tropical cyclogenesis and
intensification- by providing additional forced ascent
• As divergence prevails (upper tropospheric divergence)
on eastern side of troughs, - rising motion occurs at the
surface- leads to development of thunderstorms or
intensification of existing storms.
 Furthe-these remnants of temperate cyclones- usually have cold cores-
suggesting
 steeper environmental lapse rate - Such instability encourages
thunderstorms.
• An upper tropospheric westerly trough -important for
tropical cyclone forecasting

 This is because
 Fast moving upper tropospheric westerly trough-s can create large
vertical wind shear over tropical disturbances and tropical cyclones-
which may inhibit their strengthening.
 Slow moving upper tropospheric westerly troughs- can drive tropical
cyclones eastward or northeastward.

 Cyclone prone states in India [img]
 WB-Odisha-AP-TN-Kerela-Karnataka_Maharashtra Goa-Gujrat

 4-stage warning system for Tropical Cyclones?[pyq]
 IMD and Cyclone Diasster Management
 1999, IMD introduced a 4-Stage warning system to issue cyclone
warnings

 Pre-Cyclone Watch
 Issued when a depression forms over the Bay of Bengal irrespective of
its distance from coast.
 issued at least 72 hours in advance of commencement of adverse
weather.
 issued at least once a day.

 Cyclone Alert (Colour code Yellow)

 Issued at least 48 hours before the commencement of bad weather
 when cyclone is located beyond 500 Km from coast.
 issued every three hours.

 Cyclone Warning (Colour code Orange)

 Issued at least 24 hours before commencement of bad weather
 when cyclone located within 500 Km from coast.
• Information about time/place of landfall -indicated
in bulletin.
• Accuracy in estimation increases as cyclone comes
closer to coast

 Post-landfall outlook (Colour code Red)

 issued 12 hours before cyclone landfall
 when cyclone is located within 200 km from coast.
 More accurate information about time/place of landfall and associated
bad weather indicated in bulletin.

 How are Tropical Cyclones monitored by IMD?

 network of meteorological observatories (both surface and upper
air) operated by IMD,-covering entire coastline and islands.
 supplemented by observational data from automatic weather stations
(AWS), radar and satellite systems.
 INSAT imagery obtained at hourly intervals

 Positive impacts of cyclones
 Break up Red Tide-winds and waves mix-breakup patches of bacteria
 Replenish Barrier Islands-pick up substantial amounts of sand, nutrients
and sediment on ocean’s bottom
 Seed dispersal to faraway locations- blow spores and seeds further
inland from where they would normally fall;

 Negative impacts
 Floods
 Winds
 Storm Surge
 loss of lives and damage to property
 Economic losses: Loss of livelihood for the fishermen
 Damage to marine birds and animals. Chilika lake,India’s largest coastal
lagoon-also at high risk zone.
 Unreliable forecasts:

 Which sector of the cyclone experiences strongest winds?
 In general-strongest winds in a cyclone - right side of the storm.
 “right side -if cyclone moving to the west- right side would be to north
of storm- if cyclone moving to north-right side would be to east
of storm, etc.
 strongest wind on right side- mainly due to fact that -motion
of cyclone -contributes to its swirling winds.
 cyclone with a 145 kmph winds -while stationary -would have winds
up to 160 kmph on right side -only 130 kmph on left side -if began
moving at 16 kmph.
 What is the normal movement of a Tropical Cyclone?
 cyclones, which cross 20° N latitude- generally, recurve - are more
destructive.
 Tropical Cyclones -casually move west-northwestwards or
northwestwards in northern hemisphere.
 average speed -15-20 kmph (360-480 km per day). -may change their
direction of movement towards north.
 During this change -speed of movement decreases to 10 kmph or even
less.
 later turn towards northeast and move northeastwards very fast at a
speed of 25 kmph or more.


 mitigation measures[do from disaster management]

 Climate change and Cyclones

 Role of Climate Change in Formation

 Temperature rise:produces more evaporation-ocean’s surface condenses
into storm’s rain.
 warmer atmosphere-hold more moisture,-lead to Increase in rain
 eg-Recent studies -rainfall rates in hurricanes increase by at least 7% per
degree of warming.
 Constant Inflow of Fresh Water:makes it impossible for warm water to
mix with cooler water-making it ideal for a cyclonic depression.
 Intensification of storms:increase in a warming climate-means stronger
winds.-storms that intensify into powerful Category 4 or 5 storms will
increase.
 Sea level rise:causing shifts in landfall patterns
 Early formation of cyclones-eg-In BOB -cyclone formation prior to
usual April and May
 Rise in frequency of cyclones-Climate warming-creating conducive
condition for cyclones-for larger part of the year
 merger between two large tropical storms-leading to formation of a
mega cyclone
 Change in impact area:maximum intensity is moving poleward.