From UPSC perspective, the following things are important :
Prelims level : Cyclone Fani
Mains level : Tropical Cyclones in India
- A powerful cyclonic storm named Fani (pronounced Foni) is headed towards the Odisha coast..
- It is not just a severe cyclone but an “extremely severe cyclone”.
- Expected to generate storms with wind speeds as high as 200 km per hour, it has the potential to cause widespread damage in Odisha and neighbouring states.
- The last time such a powerful cyclonic storm had emerged in the Bay of Bengal at this time of the year, in 2008, it had killed more than 1.25 lakh people in Myanmar.
- However India has impressively managed disasters caused by cyclones, most remarkably during Cyclone Phailin of 2013, which was even stronger than the approaching Fani.
- Fani is, thus, unusual, and that is mainly because of the place it originated, very close to the Equator, and the long route it has taken to reach the landmass.
How are they formed?
- Cyclones are formed over slightly warm ocean waters.
- The temperature of the top layer of the sea, up to a depth of about 60 metres, need to be at least 28°C to support the formation of a cyclone.
- This explains why the April-May and October-December periods are conducive for cyclones.
- Then, the low level of air above the waters needs to have an ‘anticlockwise’ rotation (in the northern hemisphere; clockwise in the southern hemisphere).
- During these periods, there is a ITCZ in the Bay of Bengal whose southern boundary experiences winds from west to east, while the northern boundary has winds flowing east to west.
- This induces the anticlockwise rotation of air.
- Once formed, cyclones in this area usually move northwest. As it travels over the sea, the cyclone gathers more moist air from the warm sea, and adds to its heft.
What strengthens them?
- A thumb rule for cyclones is that the more time they spend over the seas, the stronger they become.
- Hurricanes around the US, which originate in the vast open Pacific Ocean, are usually much stronger than the tropical cyclones in the Bay of Bengal, a relatively narrow and enclosed region.
- The cyclones originating here, after hitting the landmass, decay rapidly due to friction and absence of moisture.
Cyclone Fani in Odisha: In situ origins
- A big difference between the strengths of cyclones in April-May and October-December is that the former originate in situ in the Bay of Bengal itself, barely a few hundred kilometres from the landmass.
- On the other hand, cyclones in October-December are usually remnants of cyclonic systems that emerge in the Pacific Ocean, but manage to come to the Bay of Bengal.
- They are considerably weakened after crossing the southeast Asian landmass near the South China Sea.
- These systems already have some energy, and gather momentum as they traverse over the Bay of Bengal.
- April-May is not the season for typhoons in the west Pacific Ocean. Most of the typhoons in west Pacific in northern hemisphere form between June and November.
- That is why almost all the cyclones in the Bay of Bengal in April-May period are in situ systems.
What’s unusual with Fani?
- The in situ cyclonic systems in the Bay of Bengal usually originate around latitude 10°, in line with Chennai or Thiruvananthapuram.
- Fani, on the other hand, originated quite close to the Equator, around latitude 2°, well below the Sri Lankan landmass.
- The forecast landfall on the Odisha coast is at a latitude of almost 20°.
- It has traversed a long way on the sea, thus gaining strength that is unusual for cyclones originating in the Bay of Bengal in this season.
- It was initially headed northwestwards, towards the Tamil Nadu coast, but changed course midway, and swerved northeast away from the coastline to reach Odisha.
- If it had remained on its original course, and made a landfall over the Tamil Nadu coastline, Fani would only have been a normal cyclone, not the extremely severe cyclone it has now become.
Tropical Cyclones in India
- The eastern coast of India is no stranger to cyclones.
- On an average, five to six significant cyclonic storms emerge in the Bay of Bengal region every year.
- The months of April and May just before the start of the monsoon, and then October to December immediately after the end of the monsoon, are the prime seasons for tropical cyclones.
- Cyclones emerging in April-May usually are much weaker than those during October-December.
- There have been only 14 instances of a “severe cyclone” forming in the Bay of Bengal region in April since 1891, and only one of them, which formed in 1956, touched the Indian mainland.
- The others all swerved northeast to hit Bangladesh, Myanmar or other countries in the southeast Asian region. Since 1990, there have been only four such cyclones in April.
Grading of Cyclones
- Tropical cyclones in the Bay of Bengal are graded according to maximum wind speeds at their centre.
- At the lower end are depressions that generate wind speeds of 30 to 60 km per hour, followed by:
- cyclonic storms (61 to 88 kph)
- severe cyclonic storms (89 to 117 kph)
- very severe cyclonic storms (118 to 166 kph)
- extremely severe cyclonic storms (167 to 221 kph) and
- super cyclones (222 kph or higher)