Why cyclones form

Did you know September to November is the season for cyclones in the Bay of Bengal? Every year a tropical cyclone forms in the Bay and affects India, leading to massive destruction to life and to crops. Last year it was cyclone Nilam and this year it was cyclone Phailin, which although reported minimal casualties, but caused an enormous loss to the agro sector in Odisha.

Lets’s try and understand a little more about these tropical cyclones:

The word cyclone has been derived from Greek the word ‘cyclos’ which means ‘coiling of a snake’. The terms "hurricane" and "typhoon" are region specific names for a strong "tropical cyclone". Tropical cyclones are called “Hurricanes” over the Atlantic Ocean and “Typhoons” over the Pacific Ocean.

Tropical Cyclones are low pressure systems that form over warm tropical waters. This includes everything from tropical depressions to massive category five hurricanes.

An important ingredient for storm development is a warm sea surface temperature. Tropical cyclones do not form unless the sea-surface temperature is above 26.5°C. One very important reason why September marks the peak of hurricane season is that at this point in the year the ocean has acquired all the summertime heating but has not started to cool down yet, so sea surface temperatures are warmest. Vertical wind shear and high relative humidity in the surrounding air also influence the development of tropical cyclones.

Further, as the earth's rotation sets up an apparent force (Coriolis force), pulls the winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In simple words, an unorganized group of thunderstorms needs the Coriolis force to turn them into a rotating mass of thunderstorms and ultimately a tropical cyclone.

These cyclones usually have gale force winds of around 100kmph. Though, when the sustained winds around the centre reach 118 kmph (gusts in excess, 165 kmph), then the system is called a severe tropical cyclone. However, most severe cyclones, gusts can exceed 280 kmph. (Phailin’s wind speed was over 220kmph)

The effects of a cyclone can have devastating impacts on agriculture. Strong winds and heavy rains can damage and destroy crops as well as livestock. A major concern for many farmers is the contamination of crops by flood waters. This heavy rains and flooding often fills the fields and causes them to overflow, contaminating nearby crops and farmlands. (Phailin has led to a loss of crops worth 2,400 crore)

“Accurate weather forecasts can go a long way in preparing for these cyclones that form every year in the sea. And this is what we try to achieve at Skymet”, says Jatin Singh, CEO of Skymet Weather Services.

It is accurate weather predictions that helped the Odisha government beat cyclone Phailin and avert a massive tragedy.