5A A Y , A A · violent winds, very heavy rainfall and storm surges. Storm Surge is an abnormal rise of sea level as the cyclone crosses the coast. Most of the tropical cyclones develop

NEO IAS 0484-4030104, 9446331522, 9446334122 Page 1 www.neoias.com | www.youtube.com/neoias | www.facebook.com/neoias | www.twitter.com/neoias

CLIMATOLOGY

MODULE – 5A

TROPICAL CYCLONES, THUNDERSTORMS AND TORNADOES

TROPICAL CYCLONES Tropical cyclones are violent storms that originate over oceans in tropical areas

and move over to the coastal areas bringing about large scale destruction caused by

violent winds, very heavy rainfall and storm surges. Storm Surge is an abnormal

rise of sea level as the cyclone crosses the coast. Most of the tropical cyclones

develop in a belt of 8o to 15o north and south latitudes. They are known as Cyclones

in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific

and South China Sea, and Willy-willies in the Western Australia.

Tropical Cyclones are characterized by circular and asymmetrical isobars. They have

very low pressure at the centre, and the pressure gradient is very steep and winds

are very strong; their velocity ranges between 120-200km per hour. Rainfall is

torrential and is evenly distributed around the centre. Area covered is relatively

small in the range of 150-500 km diameter. They move from east to west with the

trades.


The energy that intensifies the storm, comes from the condensation process in the

towering cumulonimbus clouds, surrounding the centre of the storm. With

continuous supply of moisture from the sea, the storm is further strengthened. On

reaching the land, a tropical cyclone begins to weaken rapidly because the storm

lacks the moisture and heat sources that the ocean provided. This depletion of

moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near

the storm centre. Without this convection the cyclone cannot survive.

The place where a tropical cyclone crosses the coast is called the landfall of the

cyclone. A tropical cyclone is classified as making landfall when the center of the

storm moves cross the coast; in strong tropical cyclones this is when the eye moves

over land. This is where most of the damage occurs within a mature tropical

cyclone as most of the damaging aspects of these systems are concentrated near the

eyewall. Such effects include the peaking of the storm surge, the core of strong

winds comes on shore, and heavy flooding rains. These coupled with high surf can

cause major beach erosion. The cyclones, which cross 20o N latitude generally,

recurve and they are more destructive.

FORMATION AND INTENSIFICATION In the tropics, weak pressure waves move from east to west. These are called

easterly waves. Under favourable situation, a low pressure area forms over the area

of an easterly trough. This gives rise to low level convergence. If the sea is warm

(sea surface temperature ≥ 26.50 C) and there is sufficient upper level divergence

i.e air is blown off at higher levels from the area of low pressure, the pressure

gradually falls. Low level convergence coupled with upper level divergence gives rise

to vertical motion taking moist air upwards. These moistures condense at higher

levels (middle troposphere) and give out latent heat of condensation. Due to release

of heat of condensation the area warms up resulting into further fall in pressure. This

process continues and a low pressure system gradually intensifies into a cyclonic

storm. Hence, for tropical cyclogenesis, there are several favourable

environmental conditions that must be in place.

FACTORS NECESSARY FOR THE FORMATION OF TROPICAL CYCLONES

Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown

how deep, but at least on the order of 50 m). Warm waters are necessary to fuel

the heat engine of the tropical cyclone.

An atmosphere which cools fast enough with height such that it is potentially

unstable to moist convection. It is the thunderstorm activity which allows the

heat stored in the ocean waters to be liberated for the tropical cyclone

development.

Relatively moist layers near the mid-troposphere (5 km). Dry mid-levels are not


conducive for allowing the continuing development

of widespread thunderstorm activity.

A minimum distance of at least 500 km from the

equator. For tropical cyclogenesis to occur, there

is a requirement for non-negligible amounts of the

Coriolis force. Without the Coriolis force, the low

pressure of the disturbance cannot be maintained.

This is the reason why the narrow corridor of width

of about 300 km on either side of the equator is

free from cyclones. Because of this there is no

inter-hemispheric migration of tropical cyclones

across the equator.

A pre-existing near-surface disturbance with

sufficient vorticity (rotation) and convergence. Tropical cyclones cannot be

generated spontaneously. To develop, they require a weakly organized system

with sizable spin and low level inflow. Over the Bay of Bengal region, the

remnants of a typhoon from North-West Pacific, which has weakened may

provide the initial disturbance for the regeneration of a tropical cyclone.

The intertropical convergence zone (ITCZ) can provide the low-level

convergence and the upper-level divergence necessary for the formation of a

tropical cyclone.

Low values (less than about 10 m/s or 20 knots) of vertical wind shear between

the lower (1.5 km) and the upper troposphere (12 km). Vertical wind shear is the

magnitude of wind change with height. Large values of vertical wind shear

disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical

cyclone has already formed, large vertical shear can weaken or destroy the tropical

cyclone, by interfering with the organization of deep convection around the

cyclone centre, because it affects the vertical transport of latent heat. This is

the chief reason, why no tropical cyclone forms during the Indian summer

monsoon season (June to September), over the Indian seas since the

monsoonal winds are characterized by strong vertical wind shears with westerly

winds at lower levels and strong easterlies at upper levels.

STRUCTURE

A mature tropical cyclone is characterised by the strong spirally circulating wind

around the centre. The diameter of the circulating system can vary between 150 and

250 km. The circular eye or centre of a tropical cyclone is an area characterised by light

winds, fine weather and often clear skies with subsiding air. The eye is the region

of lowest surface pressure. The size of the eye varies from one cyclone to the next

ranging from 10 km to over 100 km.


The eye is surrounded by a dense ring of cloud, known as the eye wall, where

there is a strong spiralling ascent of air to greater height reaching the tropopause.

This marks the most dangerous part of the cyclone having the strongest winds and

heaviest rainfall. The wind reaches maximum velocity in this region, reaching as

high as 250 km per hour. Torrential rain occurs here. From the eye wall rain bands

may radiate and trains of cumulus and cumulonimbus clouds may drift into the

outer region. The diameter of the storm over the Bay of Bengal, Arabian sea and

Indian ocean is between 600 - 1200 km. The system moves slowly about 300 - 500

km per day.

DISTRIBUTION Tropical cyclones are most prominent in the Pacific Ocean, (eastern coast of China,

Taiwan, southern part of Japan). In India, their intensity is felt more in the Bay of

Bengal along the eastern coast. In Africa, they are recorded in Ethiopia and Kenya.

In North America, they are prominent in Texas, coastal parts of Mexico and

Florida and West Indies. In the South Atlantic regions tropical cyclones does not

originate owing to strong vertical wind shear and lack of upper level divergence

due to absence of inter-tropical convergence zone (ITCZ) over the ocean. The

South Eastern Pacific is just about as void of activity as the South Atlantic, largely

due to the tendency of ITCZ to stay near or north of equator and low sea-surface-

temperature.

DIFFERENCE BETWEEN TROPICAL CYCLONES AND EXTRA-TROPICAL CYCLONES

The extra tropical cyclone differs from the tropical cyclone in number of ways. The

extra tropical cyclones have a clear frontal system which is not present in the tropical


cyclones. They cover a larger area compared to tropical cyclones and can originate

over the land and sea. Whereas the tropical cyclones originate only over the seas

and on reaching the land they dissipate and typically have little to no temperature

differences across the storm at the surface. Their winds are derived from the release

of energy due to cloud/rain formation from the warm moist air of the tropics. The

wind velocity in a tropical cyclone is much higher and it is more destructive. The

extra tropical cyclones move from west to east but tropical cyclones, move from

east to west.

Often, a tropical cyclone will transform into an extra-tropical cyclone as it

recurves poleward and to the east. Occasionally, an extra-tropical cyclone will lose its

frontal features, develop convection near the centre of the storm and transform into a

full-fledged tropical cyclone. Such a process is most common in the north Atlantic

and northwest Pacific basins. The transformation of tropical cyclone into an extra-

tropical cyclone (and vice versa) is currently one of the most challenging forecast

problems.

TEMPERATE TROPICAL

Much larger Relatively smaller

Form in temperate zones Form in tropical regions

Form over both land and sea Form only over sea

Produced in winter Produced in late summer and autumn

More regular in their formation Vary greatly from year to year

Derive their energy from air mass

temperature contrast

Derive their energy from latent heat of

condensation

Wind speed is 10-20 ms-1 Wind speed is over 30 ms-1

Continuous rainfall for many days Violent and Torrential rainfall for few

hours to few days

Rate of movement is faster Slower movement

Generally moves west to east Generally moves east to west

NAMING OF CYCLONES In the beginning, storms were named arbitrarily. Then the mid-1900's saw the start of

the practice of using feminine names for storms. In the pursuit of a more organized

and efficient naming system, meteorologists later decided to identify storms using

names from a list arranged alphabetically. Thus, a storm with a name which begins

with A, like Anne, would be the first storm to occur in the year. Before the end of

1900's, forecasters started using male names for those forming in the Southern

Hemisphere. Since 1953, Atlantic tropical storms have been named from lists

originated by the National Hurricane Center. They are now maintained and updated

by an international committee of the World Meteorological Organization. It is


important to note that tropical cyclones /hurricanes are named neither after any

particular person, nor with any preference in alphabetical sequence.

The tropical cyclone/hurricane names selected are those that are familiar to the

people in each region. Obviously, the main purpose of naming a tropical

cyclone/hurricane is basically for people easily to understand and remember the

tropical cyclone/hurricane in a region, thus to facilitate tropical cyclone/hurricane

disaster risk awareness, preparedness, management and reduction.

IDENTIFYING TROPICAL CYCLONES OVER NORTH INDIAN OCEAN

The WMO (World Meteorological Organization)/ESCAP (Economic Social

Commission for Asia and the Pacific.) Panel on Tropical Cyclones at its twenty-

seventh Session held in 2000 in Muscat, Sultanate of Oman agreed in principal to

assign names to the tropical cyclones in the Bay of Bengal and Arabian Sea. After

long deliberations among the member countries, the naming of the tropical cyclones

over north Indian Ocean commenced from September 2004.

The RSMC (Regional Specialized Meteorological Center) New Delhi gives a tropical

cyclone an identification name from the below given name list. The identification

system covers both the Arabian Sea and the Bay of Bengal. These lists are used

sequentially, and they are not rotated every few years as are the Atlantic and

Eastern Pacific lists.


IMPORTANT RULES FOLLOWED ARE:

• The Panel member’s name are listed alphabetically country wise.

• The name will be used sequentially column wise.

• The first name will start from the first row of column one and continue

sequentially to the last row in column eight. Example, this will be as Onil, Hibaru,

Pyar, Baaz …………. Amphan

(The names which have been already used from the list are in italics)

PANEL

MEMBER

COLUMN

ONE

COLUMN

TWO

COLUMN

THREE

COLUMN

FOUR

COLUMN

FIVE

COLUMN

SIX

COLUMN

SEVEN

COLUMN

EIGHT

NAMES NAMES NAMES NAMES NAMES NAMES NAMES NAMES

Bangladesh Onil Ogni Nisha Giri Helen Chapala Ockhi Fani

India Agni Akash Bijli Jal Lehar Megh Sagar Vayu

Maldives Hibaru Gonu Aila Keila Madi Roanu Mekunu Hikaa

Myanmar Pyarr Yemyin Phyan Thane Nanauk Kyant Daye Kyarr

Oman Baaz Sidr Ward Murjan Hudhud Nada Luban Maha

Pakistan Fanoos Nargis Laila Nilam Nilofar Vardah Titli Bulbul

Sri Lanka Mala Rashmi Bandu Viyaru Ashobaa Maarutha Gaja Pawan

Thailand Mukda

Khai

Muk Phet Phailin Komen Mora Phethai Amphan

TROPICAL CYCLONES IN NORTH INDIAN OCEAN The genesis of cyclonic disturbances varies with respect to season. While genesis

mostly takes place in lower latitudes in association with inter-tropical convergence

zone (ITCZ) during pre-monsoon (March-May) and post-monsoon (October-

December) seasons, it occurs over northerly latitude during monsoon season

(June-September).

The Bay of Bengal TCs (Tropical Cyclones) more often strike Odisha-West Bengal

coast in October, Andhra coast in November and the Tamilnadu coast in

December. Over 60 percent of the TCs in the Bay of Bengal strike different parts of

the east coast of India, 30 percent strike coasts of Bangladesh and Myanmar and

about 13 percent dissipate over the sea itself.

The frequency of tropical cyclones is very less over the west Arabian Sea and it is

mainly due to colder Sea surface temperature (SST). A significant number of

cyclones dissipate over the sea itself before making any landfall (about 48.5%).

Gujarat coast is the most prone for the cyclones developing over the Arabian Sea,

with about 23% of total cyclones developing over Arabian Sea cross Gujarat coast

and 11% each cross Pakistan and Oman coasts.


CYCLONES OVER THE ARABIAN SEA ARE FEW COMPARED TO THE BAY OF BENGAL

Cyclones that form over the Bay of Bengal are either those develop insitu over

southeast Bay of Bengal and adjoining Andaman Sea or remnants of typhoons

over northwest Pacific and move across south China sea to Indian Seas. As the

frequency of typhoons over northwest Pacific is quite high (about 35% of the global

annual average), the Bay of Bengal also gets its increased quota. The cyclones over

the Arabian Sea either originate insitu over southeast Arabian Sea (which includes

Lakshadweep area also) or remnants of cyclones from the Bay of Bengal that

move across south peninsula. As the majority of Cyclones over the Bay of Bengal

weaken over land after landfall, the frequency of migration into Arabian Sea is low. In

addition to all the above the Arabian Sea is relatively colder than Bay of Bengal and

hence inhibits the formation and intensification of the system.

TROPICAL CYCLONES ARE FEW DURING SOUTHWEST MONSOON SEASON

The southwest monsoon is characterized by the presence of strong westerly

winds in the lower troposphere (below 5 km) and very strong easterly winds in the

upper troposphere (above 9 km). This results in large vertical wind shear. Strong

vertical wind shear inhibits cyclone development. Also the potential zone for the

development of cyclones shifts to north Bay of Bengal during southwest monsoon

season. During this season, the low pressure system upto the intensity of

depressions form along the monsoon trough, which extends from northwest India to

the north Bay of Bengal. The Depression forming over this area crosses Orissa –

West Bengal coast in a day or two. These systems have shorter oceanic stay

which is also one of the reasons for their no intensification into intense cyclones.


CLASSIFICATION OF LOW PRESSURE SYSTEM IN INDIA

The low-pressure systems over Indian region are classified based on the maximum

sustained winds speed associated with the system and the pressure deficit/ number

of closed isobars associated with the system. The pressure criteria is used when

the system is over land and wind criteria is used, when the system is over the sea.

The system is called as low if there is one closed isobar in the interval of 2 hPa. It is

called depression, if there are two closed isobars, a deep depression, if there are

three closed isobars and cyclonic storm if there are four or more closed isobars.

Considering wind criteria, the system with wind speed of 17-27 knots is called as

depression and the low pressure system with maximum sustained surface winds

between 28-33 knots is called a deep depression. The system with maximum

sustained surface winds of 34 knots or more is called as cyclonic storm.

ANTICYCLONE Anticyclone is large-scale circulation of winds around a central region of high

atmospheric pressure, clockwise in the Northern Hemisphere, counter-

clockwise in the Southern Hemisphere. They form from air masses cooling more

than their surroundings, which causes the air to contract slightly making the air

denser. Since denser air weighs more, the weight of the atmosphere overlying a

location increases, causing increased surface air pressure. The strongest

anticyclones occur over snow covered portions of Asia and North America in the

winter when clear, dry air masses cool from loss of radiation, while little sunlight is not

absorbed. The average velocity of anticyclones is 30-50km per hour. They can also

form when winds descend from above towards the centre. Thus weather becomes

clear and rainless because the descending winds cause atmospheric stability.

THUNDERSTORMS AND TORNADOES Other severe local storms are thunderstorms and tornadoes. They are of short

duration, occurring over a small area but are violent. These violent storms are the

manifestation of the atmosphere’s adjustments to varying energy distribution. The

potential and heat energies are converted into kinetic energy in these storms and the

restless atmosphere again returns to its stable state.

THUNDERSTORMS A thunderstorm is a well-grown cumulonimbus cloud producing thunder and

lightning. When a lightning bolt travels from the cloud to the ground it actually opens

up a little hole in the air. Once then light is gone the air collapses back in and creates

a sound wave that we hear as thunder. When the clouds extend to heights where

sub-zero temperature prevails, hails are formed and they come down as hailstorm.

Thunderstorms are caused by intense convection on moist hot days. If there is

insufficient moisture, a thunderstorm can generate dust-storms. A thunderstorm is


characterised by intense updraft of rising warm air, which causes the clouds to grow

bigger and rise to greater height. This causes precipitation. Later, downdraft

brings down to earth the cool air and the rain.

CONDITIONS NECESSARY FOR FORMATION

For a thunderstorm to form, the following conditions need to be satisfied. Firstly, there

must be plenty of moisture available in the lower to middle levels of the troposphere

together with large amount of insolation. Furthermore, the atmospheric conditions

must be conducive for the establishment of instability. In addition, it is necessary to

have a mechanism for initiating an updraft in a moist unstable atmospheric

environment. The above mechanism for lift can either be mechanical or dynamical.

The source of lift is mechanical when the moist air is forced to flow up, say, over a

mountain. In addition to the above, advancing cold fronts, outflow boundaries, dry

lines and sea breeze fronts may act as triggers to lift the moist low level air.

Dynamical effects become important for lifting, with the existence of an upper-level

divergence region, which can provide for low-level convergence and dynamical lifting.

The existence of upper-level jet streams can initiate lifting from low levels through

dynamical uplift.

EFFECTS

Thunderstorms play an important role in transporting heat and moisture from the

lower levels to the upper levels of the atmosphere. Strong winds and intense

precipitation including hail as well as lightning and thunder usually accompany

thunderstorms. Associated with the passage of a thunderstorm, rapid and marked

changes in surface pressure and also changes in air temperature are generally

seen. As the thunderstorm approaches a station, an abrupt fall in the surface

pressure is noticed at the station. While the thunderstorm is directly overhead the

station with the station experiencing rain showers, there is an abrupt rise in the

surface pressure. However, as the thunderstorm moves over from the station and the

rainfall ceases, the surface pressure returns to its normal value. If the rainfall

associated with the thunderstorm reaches the surface, the air temperatures will star

to decrease due to evaporative cooling. If the rainfall lasts long enough, the air

temperature may cool to its dew point temperature.

THUNDERSTORMS IN INDIA

Severe thunderstorms occur in large numbers and in their most violent form over

India during the pre-monsoon months of April and May as well as during the

month of June. The above months are characterized by hot and humid conditions

and hence the above characteristics are conducive for the formation of

thunderstorms. The most intense of the pre-monsoon thunderstorms as well as the

highest number of thunderstorms occur over the eastern parts of India comprising


the state of West Bengal and Orissa and the north-eastern parts of India. Pre-

monsoon thunderstorms over Indian subcontinent are usually caused by western

disturbances. By the time western disturbance reaches India, it would have lost

much of its vigour but sometimes it pipes in moisture from the Arabian Sea and flare

up just before pushing its way in. This moisture will get pushed up to cool and

condense and interact with the air over heated-up land to set up violent

thunderstorms.

TORNADOES

From severe thunderstorms sometimes spiralling wind descends like a trunk of an

elephant with great force, with very low pressure at the centre, causing massive

destruction on its way. Such a phenomenon is called a tornado. While some

tornadoes appear as a very thin rope-shaped column of air, most tornadoes have the

familiar funnel shape that narrows from the cloud base to the ground. Tornadoes

exhibit a very wide range of sizes with diameters varying from about 100 m to 1500

m. Most of the tornadoes rotate cyclonically (counter-clockwise) in northern

hemisphere. While typically tornadoes move from south-west to north-east, they

have been known to move in any direction. Over India, tornadoes occur very rarely

and can occur only over the north-eastern parts of India. The maximum numbers of

tornadoes, which occur over the whole globe, are seen over the United States of

America and they amount to about 750 tornadoes in a year. The months of May and

June account for the largest number of tornadoes, which occur in USA, while the

number is somewhat less in April and July.

When tornadoes occur over warm water bodies they are known as waterspouts.

Waterspouts are much smaller than tornadoes and typically have diameters between

5 m and 100 m and are considered somewhat weaker as compared to the


tornadoes. The visible water seen in the waterspout is not associated with the

sucked-up water from the ocean below, but chiefly arises from the water vapour in

the air.

PREVIOUS YEAR QUESTIONS (PRELIMS)

1. In the South Atlantic and South-Eastern Pacific regions in tropical latitudes,

cyclone does not originate. What is the reason? (2015)

(a) Sea surface temperature are low

(b) Inter-Tropical Convergence Zone seldom occurs

(c) Coriolis force is too weak

(d) Absence of land in those regions

2. During a thunderstorm, the thunder in the skies is produced by the (2013)

1. meeting of cumulonimbus clouds in the sky.

2. lightening that separates the nimbus clouds.

3. violent upward movement of air and water particles.

Select the correct answer using the codes given below:

(a) 1 only (b) 2 and 3

(c) 1 and 3 (d) None of the above produces the thunder

3. Consider the following names (2009)

1. Ike 2. Kate 3. Gustav

Which of the above are the names of hurricanes that had occurred very recently?

(a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3

4. Consider the following statements: (2001)

1. Anti-cyclonic conditions are formed in winter season when atmospheric

pressure is high and air temperatures are low.

2. Winter rainfall in Northern India causes development of Anticyclonic conditions

with low temperatures.

Which of the statement(s) given above is/are correct?

(a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2

5. Assertion (A): The surface winds spiral inward upon the centre of the cyclone.

Reason (R): Air descends in the centre of the cyclone. (2002)

6. Consider the following climatic and geographical phenomena: (2002)

1. Condensation 2. High temperature and humidity

3. Orography 4. Vertical wind

Thunder cloud development is due to which of these phenomena?

(a) 1 and 2 (b) 2, 3 and 4 (c) 1, 3 and 4 (d) 1, 2, 3 and 4

7. Which one of the following weather conditions is indicated a sudden fall in

barometer reading? (2001)

(a) Stormy (b) Calm weather

(c) Cold and dry weather (d) Hot and sunny weather

8. Willy Willy is: (1995)


(a) a type of tree grown in temperate regions

(b) a wind that blows in a desert

(c) a tropical cyclone of the north-west Australia

(d) a kind of common fish found near Lakshadweep islands

PREVIOUS YEAR QUESTIONS (MAINS)

1. Tropical cyclones are largely confined to South China Sea, Bay of Bengal and Gulf

of Mexico. Why? (2014)

2. The recent cyclone on east coast of India was called ‘Phailin’. How are the tropical

cyclones named across the world? Elaborate. (2013)

Follow us on our Prelims Telegram Channel Economy Telegram Channel https://goo.gl/W8kAmH Watch our Videos on our YouTube Channel Visit our Website www.neoias.com

https://t.me/joinchat/AAAAAEABQk5IsIgmOepQOQ

https://t.me/joinchat/AAAAAEABQk5IsIgmOepQOQ

https://goo.gl/W8kAmH

https://www.youtube.com/channel/UCbZaVhpVcsmVU3t5qB6naIw/videos

5A A Y , A A · violent winds, very heavy rainfall and storm surges. Storm Surge is an abnormal rise of sea level as the cyclone crosses the coast. Most of the tropical cyclones develop

Documents