Extra-Tropical Cyclones

• The passage of the front causes abrupt changes in the weather conditions over the area in the middle and high latitudes.
• The systems developing in the mid and high latitudes, beyond the tropics are called the middle latitude or extratropical cyclones. 
• Extratropical cyclones form along the polar front. Initially, the front is stationary. 
• In the northern hemisphere, warm air blows from the south and cold air from the north of the front.
• When the pressure drops along the front, the warm air moves northwards and the cold air moves towards the south, setting in motion an anticlockwise cyclonic circulation. 
• The cyclonic circulation leads to a well developed extratropical cyclone, with a warm front and a cold front. 
• There are pockets of warm air or warm sector wedged between the forward and the rear cold air or cold sector. 
• The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation. 
• The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop
• along the cold front. 
• The cold front moves faster than the warm front, ultimately overtaking the warm front. 
• The warm air is completely lifted up and the front is occluded and the cyclone dissipates.
• The processes of wind circulation both at the surface and aloft are closely interlinked.
• The extratropical cyclone differs from the tropical cyclone in a number of ways. The extratropical cyclones have a clear frontal system which is not present in the tropical cyclones.
• They cover a larger area and can originate over the land and sea. 
• The tropical cyclones originate only over the seas and on reaching the land they dissipate. 
• The extratropical cyclone affects a much larger area as compared to the tropical cyclone. 
• The wind velocity in a tropical cyclone is much higher and it is more destructive. The extratropical cyclones move from west to east but tropical cyclones move from east to west.

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.
• They are known as:
• Cyclones in the Indian Ocean

• Hurricanes in the Atlantic
• Typhoons in the Western Pacific and the South China Sea
• Willy-willies in Western Australia.

• Tropical cyclones originate and intensify over warm tropical oceans. 
• The conditions favourable for the formation and intensification of tropical storms are: 

(i) Large sea surface with a temperature higher than 27° C

(ii) Presence of the Coriolis force

(iii) Small variations in the vertical wind speed

(iv) A pre-existing weak low-pressure area or low-level-cyclonic circulation

(v) Upper divergence above the sea level system.

• The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. 
• With a continuous supply of moisture from the sea, the storm is further strengthened. 
• On reaching the land the moisture supply is cut off and the storm dissipates. 
• The place where a tropical cyclone crosses the coast is called the landfall of the cyclone. 
• The cyclones, which cross 20^o N latitude generally, recurve and are more destructive.
• The wind reaches maximum velocity in this region, reaching as high as 250 km per hour.
• Torrential rain occurs here. From the eyewall 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. The cyclone creates storm surges and they inundate the coastal lowlands. 
• The storm peters out on the land.