"One of the most devastating Natural calamities"
Tropical Cyclones
“Violent storms 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."
AKA
- Cyclones - Indian Ocean
- Hurricanes - Atlantic
- Typhoons - western pacific & south china sea
- willy-willies - western Australia
Originate and intensify over warm tropical oceans.
Conditions favourable for the formation and intensification of tropical storms:
- Large sea surface with temperature higher than 27° C
- Presence of the Coriolis force
- Small variations in the vertical wind speed
- A pre-existing weak-low-pressure area or low-level-cyclonic circulation;
- Upper divergence above the sea level system.
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 the moisture supply is cut off and the storm disappear .
- The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
- The cyclones, which cross 20d degree north latitude generally, recurve and they are more destructive.
Characteristics:
1.Centre -Eye
- Calm region
- Winds revolve around Eye -Diameter - 150 -250 kms
2.Eye wall:
- Around the eye is the eye wall,
- where there is a strong spiralling ascent of air to greater height reaching the tropopause.
- wind reaches maximum velocity in this region, reaching as high as 250 km per hour.
- Torrential rain occurs here.
Scenario around India:
- Diameter of the storm over the Bay of Bengal, Arabian sea and Indian ocean is between 600 - 1200 km.
- System moves slowly about 300 - 500 km per day.
Extra Tropical Cyclones
"Systems developing in the mid and high latitude, beyond the tropics are called the extra tropical cyclones."
Extra tropical cyclones form along the polar front..
AKA,
- Extra Tropical Cyclones
- Middle lattitude cyclones
Cause:
- passage of front causes abrupt changes in the weather conditions over the area in the middle and high latitudes.
How it forms?
- Initially, the front is stationary
- In the northern hemisphere, cold air from the north of the front & warm air blows from the south
- pressure drops along the front
- warm air moves northwards and the cold air move towards south
- setting in motion an anti clockwise cyclonic circulation
- warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation…
How it dissipates?
- Cold front approaches the warm air from behind and pushes the warm air up.
- As a result, cumulus clouds develop along the cold front
- cold front moves faster than the warm front ultimately overtaking the warm front.
- warm air is completely lifted up and the front is occluded and the cyclone dissipates..
| Extra Tropical Cyclone | Tropical Cyclone |
| have a clear frontal system | not present in the tropical cyclones |
| can originate over the land and sea. | originate only over the seas and on reaching the land they dissipate. |
| Cover large area | Small as compared to Extra tropical cyclone |
| Compared to tropical cyclones ,wind velocity is low | wind velocity in a tropical cyclone is much higher |
| Less destructive than Tropical Cyclone | More destructive |
| move from west to east | east to west. |