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CYCLONES
WHAT IS CYCLONE?

A cyclone is a storm system characterized by a low-pressure center, strong

winds, and heavy rain. The rotation direction depends on the hemisphere:

Northern Hemisphere
rotates
counterclockwise Southern Hemisphere
rotates clockwise

TYPES OF CYCLONES:

1. Tropical Cyclones form over warm ocean waters (e.g., hurricanes,

typhoons)
2. Extratropical Cyclones form outside the tropics, usually linked to cold and
warm fronts.
3. Subtropical Cyclones have characteristics of both tropical and
extratropical systems.

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HOW ARE CYCLONES CLASSIFIED?

Cyclones are classified on the basis of wind speed by the Indian Meteorological
Department (IMD):

Depression: Wind speeds of between 31–49 km/h

Deep Depression: Between 50-61 km/h

Cyclonic Storm: Between 62–88 km/h

Severe Cyclonic Storm: Between 89-117 Km/h

Very Severe Cyclonic Storm: Between 118-166 Km/h

Extremely Severe Cyclonic Storm: Between 166-221 Km/h

Super Cyclonic Storm: Above 222 Km/h

WHAT IS THE PROCESS OF NAMING OF CYCLONES?

• There are five tropical cyclone regional bodies in the world:

ESCAP/WMO Typhoon Committee,

WMO/ESCAP Panel on Tropical Cyclones,

RA I Tropical Cyclone Committee,

RA IV Hurricane Committee,

RA V Tropical Cyclone Committee.

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• WMO/ESCAP Panel on Tropical Cyclones is responsible for naming of

cyclones in the Indian Ocean.

PROCESS

• The process of naming cyclones involves several countries in the region

and is done under the aegis of the World Meteorological Organization
(WMO).

• For the Indian Ocean region, a formula for naming cyclones was agreed
upon in 2004.

• Eight countries in the region - Bangladesh, India, Maldives, Myanmar,

Oman, Pakistan, Sri Lanka and Thailand - all contributed a set of names
which are assigned sequentially whenever a cyclonic storm develops.

• Hudhud, Titli, Phethai, Fani, Vayu and Amphan are among the names of
cyclones in the Indian Ocean region.

WORLDWIDE TERMINOLOGY OF TROPICAL CYCLONES

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4
Cyclones in Hurricanes in Typhoons in Willy-willies
the Indian the Atlantic the Western in the
Ocean Pacific and Western
South China Australia.
Sea

World Meteorological Organization (WMO)

It is a specialized agency of the United Nations responsible for promoting
international cooperation in meteorology, climatology, hydrology, and related
fields.

It was established in 1950 and currently has 193 member countries and
territories. The primary goal of the WMO is to facilitate the exchange of
weather and climate information among member countries.

It fosters partnerships and facilitates the development and implementation

of international frameworks and agreements, such as the Intergovernmental
Panel on Climate Change (IPCC) reports and the Global Framework for
Climate Services (GFCS).

MOVEMENT OF CYCLONES

ROLE OF WINDS IN GUIDING CYCLONES:

• Winds guide cyclones, influenced by the global wind pattern known as the
“general circulation,” which divides surface winds into three main belts.
• These include the Polar Easterlies (60-90 degrees latitude), Prevailing
Westerlies (30-60 degrees latitude), and Tropical Easterlies (0-30 degrees
latitude).

FORMATION OF THE INTERTROPICAL CONVERGENCE ZONE:

• The convergence of easterly trade winds from both hemispheres forms
the Intertropical Convergence Zone (ITCZ), characterized by a band of
clouds and thunderstorms encircling the globe near the equator.

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INFLUENCE OF WIND BELTS:

• A cyclone’s path is determined largely by the wind belt it is situated in.

• Example: Cyclones originating in the eastern tropical Pacific are driven
westward by easterly trade winds before turning northwestward around
the subtropical high and moving into higher latitudes.
• However, local steering winds may sometimes deviate from this pattern.
SPECIAL CYCLONES

TWIN CYCLONES:

The interplay of the wind and the monsoon system combined with the Earth
system produces twin cyclones.
They are basically caused by equatorial Rossby waves.

• Rossby waves are named for famous meteorologist Carl-Gustaf

Rossby who was the first to explain that these waves arose due to the
rotation of the Earth.

• They are huge waves in the ocean with wavelengths of around 4,000–
5,000 km.

• This system has a vortex in the northern hemisphere and another in the
southern hemisphere, and each of these is a mirror image of the other.

• The vortex in the north spins counterclockwise and has a positive

spin, while the one in the southern hemisphere spins in the clockwise
direction and therefore has a negative spin.

• Both have a positive value of the vorticity which is a measure of the

rotation.
Cause: Equatorial Rossby Waves.
Characteristics: Positive vorticity, counterclockwise and clockwise rotation.
Favorable Conditions: Positive vorticity, airlifting, warm ocean (>27°C), low wind
shear.
Development: Starts as tall vortices, intensifies and moves westward.

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HOW ARE TWIN CYCLONES FORMED?

• When the vorticity is positive in both Northern and Southern

hemispheres, as is the case with Rossby waves, the air in the boundary
layer, which is moist, is lifted slightly. That is enough to set off a feedback
process.

• When the air is lifted slightly, the water vapor condenses to make clouds.
As it condenses, it lets out the latent heat of evaporation.

• The atmosphere warms, this parcel of air rises, and positive feedback is
set off by this process.

• The warmer parcel of air can rise further because it is lighter than the
surrounding air, and it can form deeper clouds.

• Meanwhile, moisture comes in from both sides. This leads to the

formation of a cyclone if certain conditions are present.

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Madden-Julian Oscillation or MJO:

Karim and Asani are “twin” cyclones not only because they formed at the
same time in the same general area but also because they were formed
primarily from the same “parent” circulation: the Madden-Julian
Oscillation or MJO.

• The MJO is a large cluster of clouds and convection, around 5000-

10,000 km in size.
• It is composed of a Rossby wave and a Kelvin wave, which is a type of
wave structure that we see in the ocean.
• On the eastern side of the MJO is the Kelvin wave, while on the
western, trailing edge of the MJO, is the Rossby wave, once again with
two vortices on either side of the equator.
• However, not all tropical cyclones are born from the MJO. Sometimes
it is a mere Rossby wave with two vortices on either side

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FUJIWHARA EFFECT:

• The “Fujiwhara effect,” where two cyclones (or hurricanes) spinning in the
same direction interact in a dance-like manner around a common centre.
If one cyclone is stronger, it can absorb the weaker one.
• When of similar strength, they might merge or rotate around each other.
In some rare instances, the two cyclones could merge into a mega-cyclone
with significant destructive potential.
• The Fujiwhara effect was first described by Japanese meteorologist
Sakuhei Fujiwhara in 1921.

ANTI CYCLONE

An anticyclone is a weather phenomenon defined as a large-scale circulation of

winds around a central region of high atmospheric pressure.
It rotates clockwise in the Northern Hemisphere and counter clockwise in the
Southern Hemisphere (opposite to a cyclone).
Effects of surface-based anticyclones include clearing skies as well as cooler,
drier air. Fog can also form overnight within a region of higher pressure.

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KEY FEATURES OF ANTICYCLONES:

• High pressure system (opposite of a cyclone/low pressure).

• Clear skies and calm weather: Air is descending, which inhibits cloud
formation.

• Winds spiral outward and are clockwise in the Northern Hemisphere due
to the Coriolis effect.

• Often associated with dry, settled weather especially in temperate

regions.

• Can lead to cold weather in winter (due to lack of cloud insulation) or

heatwaves in summer (due to persistent clear skies).

TYPES OF ANTICYCLONES:

COLD-CORE ANTICYCLONES

• Also known as: Thermal high or polar high

• Location: Common in polar and continental interiors (especially in winter)

• Temperature: Cold at the center and throughout the column of air

• How it forms: Cold, dense air sinks and increases surface pressure

• Weather effects:
o Very cold, dry conditions
o Often leads to frost and fog in winter
o Little to no precipitation

• Example: Siberian High (Eastern Russia during winter)

WARM-CORE ANTICYCLONES

• Also known as: Subtropical high

• Location: Found in subtropical zones, over oceans or deserts (e.g., the

Sahara or Pacific Ocean)

• Temperature: Warmer air at the core, especially aloft

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• How it forms: Warm air rises in the tropics, moves poleward aloft, then
sinks in the subtropics

• Weather effects:
o Clear skies, hot, and dry weather
o Associated with deserts and droughts
o Can persist for a long time (blocking systems)

• Example: Bermuda High (affects eastern U.S. summers)

ANTICYCLONIC WEATHER CONDITIONS:

WINTER ANTICYCLONES SUMMER ANTICYCLONES

Dry and bright with very little cloud. Very little cloud

Sun low in the sky, so cold conditions. Dry with light winds

Clear evening skies mean that nights

Sun high in the sky, so hot and sunny
can be very cold.

Early morning frost and fog may last Cloudless skies at night allow heat to
all day. escape, so nights can be cool.
Extensive low cloud or fog may
Risk of thunderstorms at end of 'heat
produce overcast or 'gloomy'
wave' conditions
conditions.

Early morning dew and mist.

BLOCKING ANTICYCLONES

• These anticyclones form when something blocks the air circulation in the
upper atmosphere over mid-latitudes.
• They are called “blocking” because they stop the movement of
temperate cyclones in these areas. Anticyclones usually bring steady
weather.

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• As air descends from above, it creates clear and dry conditions due to the
stability from the sinking wind.
• Anticyclones are common in Canada, the United States, and northern
Eurasia.

CHARACTERISTICS OF ANTI CYCLONE

FEATURE DESCRIPTION

Pressure High atmospheric pressure at the center

Air Movement Air descends (sinks) in the center

Wind Direction
Winds move clockwise and outward
(Northern Hemisphere)
Wind Direction
Winds move counterclockwise and outward
(Southern Hemisphere)
Generally clear skies, due to sinking air inhibiting
Cloud Cover
cloud formation
Dry, calm, and stable weather; can lead to
Weather Conditions
heatwaves in summer or frost in winter

Rainfall Very low precipitation (if any)

Often good, but fog can form in cold, still

Visibility
conditions
Can vary: cold in winter, hot in summer,
Air Temperature
depending on region and type of anticyclone

SIGNIFICANCE OF ANTICYCLONES

WEATHER STABILITY

• Bring clear, settled weather sunny days with little to no cloud or rain.

• Often lead to long periods of calm, especially in summer or winter.

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IMPACT ON AGRICULTURE

• Positive: Dry, sunny spells can be good for harvesting and outdoor farming
activities.

• Negative: Prolonged dry periods can cause droughts, damaging crops and
reducing water supply.

TEMPERATURE EXTREMES

• In summer: Can cause heatwaves due to persistent sunshine and high

temperatures.

• In winter: Can cause cold snaps or frost, especially at night when skies are
clear and heat escapes.
AIR POLLUTION AND HEALTH

• Calm winds + sinking air trap pollutants near the surface.

• Leads to poor air quality, especially in cities—can worsen asthma and

respiratory issues.
CLIMATE INFLUENCE

• Subtropical anticyclones shape the location of deserts (e.g., Sahara,

Australian Outback).

• Influence global wind patterns and ocean currents, playing a role in long-
term climate systems.

BLOCKING SYSTEMS

• Strong anticyclones can block other weather systems (like storms or rain
fronts) from moving in.

• This can cause extended periods of extreme weather—either very dry or

very cold/hot.

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DIFFERENCE BETWEEN CYCLONES AND ANTI-CYCLONES

CYCLONES ANTI CYCLONES

It is a low-pressure system with It is a high-pressure system with

surroundings of high pressure. surroundings of low pressure.

It blows anti-clockwise in the It blows clockwise in the Northern

Northern Hemisphere. Hemisphere.

It blows clockwise in the Southern It blows anti-clockwise in the

Hemisphere. Southern Hemisphere.

It is associated with cloudy skies, It is associated with clear skies, mild

heavy rainfall with stormy winds. winds, and dry conditions.

It can cause great damage to lives

and property if precautions are not The weather is settled and pleasant.
taken.

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