Lecture 21 & 22 : Geography

Topics Covered in this Lecture

  1. Conditions Required for the Formation of Tropical Cyclones.
  2. Stages of Formation of Cyclones.
  3. Structure of Cyclones and Associate Weather Conditions.
  4. Global Distribution of Tropical Cyclones.
  5. Tropical Cyclones in India.
  6. Naming of Tropical Cyclones.
  7. Comparison between Tropical and Temperate Cyclones.

Cyclones- Temperate or Extratropical Cylone

It is defined as a low pressure centre with closed isobars having outside high pressure and therefore, winds move from high pressure to low pressure that they are anti clockwise in northern Hemisphere and clockwise in Southern Hemisphere.

There are two types of cyclones that are a tropical cyclone that originates from 5-degree to 30-degree North and South latitude and temperate cyclones which originate 35-degree to 65-degree North and south latitudes.

Temperate Cyclones

  • They are also known as extra tropical cyclones (outside the tropics) or depressions or V-cyclones or Wave cyclones. They originate due to convergence of two different air masses that is warm westerlies and cold polar easterlies.

  • The pressure difference between the central low pressure and outside high pressure is low and therefore the temperate cyclones do not blow with great speed (30km/hour to 50km/hour).

  • Frontogenesis is the cause for the origin of the temperate cyclone while frontolysis results in the death of temperate cyclone.

Characteristics of temperate cyclone

  • They are wave cyclones. The average diameter of the temperate cyclone is around 300 kms but they can be as large as 1000 kms.

  • The pressure difference between the central low pressure and outer high pressure is very less and hence winds move in an anticlockwise direction in the northern hemisphere and vice versa with low velocities.

  • Velocity is less because the pressure gradient is less.

  • After their origin, they travel from west to east under the influence of Jet Streams and westerlies.

  • Different parts of the temperate cyclones have different temperature conditions.

  • Precipitation in temperate cyclones occurs only along the fronts and it is divided into warm front precipitation– light drizzle, long-duration covering a larger area. In cold front precipitation– Heavy rain (Cu-ni), lightning, hail, short duration, and lesser area.

  • Some of the temperate cyclones after originating over the Mediterranean Sea, Black Sea, and the Caspian Sea enter India and gives winter rainfall known as Western disturbances.


The source of energy for temperate cyclones is the convergence of two different air masses and hence the temperate cyclones can originate in both land and water body. They can originate in any season.

Weather conditions associated with temperate cyclones

  • As temperate cyclones move from west to east, the arrival of the temperate cyclones is marked by a sudden drop in temperature due to the arrival of the cold sector. The weather is cloudless and clear.

  • The arrival of warm front- Cirrus clouds marks this as they are high in the sky. Temperature increases and there is light to medium precipitation due to nimbostratus clouds.

  • The arrival of Warm sectors– There is an increase in the temperature but cloudless and clear skies promoting atmospheric stability.

  • Cloud front precipitation due to the presence of Cu-Ni clouds in the sky gives very heavy precipitation along with hail, lightning, and thunder.

  • Cold sector– Temperature suddenly decreases. The sky becomes cloudless and clear. Promoting atmospheric stability.

Tropical Cyclones

-depression/disturbance – 120km/hour.

Cyclone – 120 km/hour.

Super cyclone – 350 km/hour.

  • A high energy central low pressure which originated on warm oceans or sea surfaces (>27°) which has a greater difference between central low pressure and outside high pressure which causes winds to deflect in such a way that they become anticlockwise in the northern hemisphere and clockwise in Southern Hemisphere. Ore tropical cyclones are surrounded by closed isobars.

TYPHOONS – China, Japan, south China Sea, north west Pacific.

HILLY WILLY – Australia or South West Pacific.

HURRICANES – North Atlantic, Carribean, Gulf of Mexico, CYCLONES USA (south east USA) .

CYCLONES – Indian Ocean.

There are proper names depending upon the oceans. Phillip, Mathew Katrina, Niluphen, falin, okhi Bangladesh.

Characteristics of tropical cyclones

  1. They originate on warm ocean surfaces due to the convergence of the same or warm air masses.

  2. Tropical cyclones do not originate in equatorial regions because Coriolis force is minimum ( zero at 0 °).

At equator, tropical cyclones do not originate because Coriolis force is zero.

  • After originating the tropical cyclones generally move from east to West because they are high energy systems therefore, they moved in a curved manner/ path.


  1.  Warm sea surfaces temperature greater than 27° C therefore Tropical cyclones do not originate above 35°North and south latitudes.

  2. Presence of Coriolis force which will be enough to cause deflection of the winds. In other words, it requires a minimum value of the Coriolis force, and hence in spite of the high temperatures, tropical cyclones do not originate in the equatorial regions.

  3. The presence of ITCZ will favor the development of tropical cyclones. That is when the ITCZ is away from the equator the tropical cyclones will develop because warm air masses will coverage at ITCZ and Coriolis force will also be enough to cause deflection of the winds.

  4. As the ITCZ does not shift in the areas of cold currents. Therefore, tropical cyclones do not originate on the Western Coast of the continent except North Indian.

  5. Presence of strong upper air anticyclonic circulation causing upper air diversions.


  • The latent heat of condensation is the source of energy to drive tropical cyclones.

  • A warm sea surface temperature (27°C or more) up to a depth of 60m. Will provide abundant water vapor, latent heat through continuous evaporation causing the development of cyclonic depressions.

  • This low-pressure center will attract the winds from all the sides and these winds will be thrown upwards Developing Culumbinous clouds.

  • As the low-pressure center in the upper atmosphere, there is a divergence of winds which ensures that more air from the ground is pulled upwards.

Structure of tropical cyclones

A fully developed tropical cyclone consist of-

  1. EYE- a Rainless cloudless center having a high temperature is known as an eye from the upper atmosphere, the eye looks like a black hole or a dot surrounded by thick clouds. Above the eye, there is a presence of anticyclonic in the eye.

  2. Eyewall- numerous air or bands of thick Culumbinous clouds is known as an eyewall can be of numerous types – inner eyewall, outer eyeballs, and many move.

  3. In the tropical cyclones, rainfall, high-velocity winds occur only along the eyewall.

  4. The most dangerous part of the tropical cyclone is very high-velocity waves that hit the coast known as storm surges.

Mangroves on eastern coast

  • Shock absorbers.
  • It helps from cyclones.
  • Saltwater crocodiles.

Death of a tropical cyclone

The tropical cyclone dies as soon as the eye touches the land or when it makes landfall.

One why eastern coast is more prove to tropical cyclone?

  1. The Bay of Bengal is warmer as compared to the Arabian sea.

  2. Tropical cyclone moves from east to West in a curved manner. Therefore tropical cyclones originate in the Bay of Bengal and Andaman sea come towards the eastern coast of India while the tropical cyclone which originates in the Arabian sea will move away from the coast of India.

  3. The Bay of Bengal is warmer because it is connected to the Pacific Ocean, it transfers it’s excess heat to – Malacca, surela, Java straits China Sea to the Bay of Bengal and Andaman sea.

Naming of Tropical Cyclones–

The practice of naming of cyclones is used for clear, effective, fast reporting and thus helps in disaster planning, mitigation (reduction in the impact of disaster) as well as early warming and response.


  • The naming initially started in Australia whose idea was taken officially by US weather Service in 1953 where women’s names from alphabets A to W were used.

  • In 1978, World Metrological Organization (WMO) started using male names along with the female name due to the wave of feminism.

  • For the Indian Ocean regions, the naming of cyclones started in 2000 and 8 counties in these regions are included in the naming of cyclones-

  • Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Srilanka, Thailand.

  • These countries are arranged in alphabetical order and they contribute the list of 8 names in total 64 names.

  • The country whose name is first in alphabetical order gives the names to the first cyclone. The entire naming is done by regional meteorological center located in the New Delhi. Example – ohkhi was named by Bangladesh, Nilopher by Pakistan, phallin by Thailand.

  • ‘GITA’ – originated it Tonga island (Pacific) named by new Zealand.

  • The Indian names on the list include Agni, Akash, Sagar, Vayu, leher, etc. If a cyclone is associated with death and disaster its name is dropped from the list as a respect to those who died. Example- Katrina, Mathew


As the pressure gradient between the central High pressure and outside low pressure is very less. Winds move with very low velocity. The anticyclones are so large that they may cover a diameter of 1 lakh square kilometer. The anticyclones move units very low velocity and sometimes they become stationary over an area for many days.

There are two permanent anticyclones belts in the world-

  1. Subtropical high-pressure belt.
  2. Polar high-pressure belt.

Weather conditions

In anticyclones, air descends promoting dry conditions and atmospheric stability.

The sky is cloudless and clear and because of stable conditions, anticyclones are known as Weatherless phenomenon.

Topics Covered in this Lecture

  1. Weather and Climate
  2. Controls of Climate
  3. Koepperi’s Classification of Climate
  4. Global Climate of the World.
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