Several factors merged to cause thunderstorm

NEWDELHI:The freakish thunderstorm activity that resulted in nearly 100 deaths in north and northwest India on Wednesday night was caused by unusually high temperatures, availability of moisture, and an unstable atmosphere which, in turn, led to the formation of a chain of thunderstorms.

“It can be called a freak incident,” Mahesh Palawat, chief meteorologist at Skymet Weather, a private forecaster, said. “Dust storms are usually not this intense nor do these systems cover such a large area.”

The bad news: experts say climate change will likely fuel more such storms.

Storms need three things: Moisture, warm temperatures, and an agitated atmosphere to push the warm moist air upwards. The unusually high temperatures in northwest India, especially Rajasthan, and a cyclonic circulation over Haryana and adjoining areas meant that air close to the land surface was pushed upwards where it formed storm clouds.

The moisture was provided by easterly winds blowing in from Bay of Bengal, especially in the eastern parts of UP and Bihar. Simultaneously, a western disturbance, a system of low pressure, brought moisture from Eurasian water bodies.

In the absence of moisture, the strong upward movement of air would only carry dust and cause dust storms. However, the incursion of moisture because of the western disturbance brought thunderstorms even to areas which would normally only experience dust storms.

Heating of the landmass causes an updraught or the movement of air upwards. If moisture is present, the updraught will carry the moisture upwards. When this air reaches the colder parts of the upper atmosphere, water vapour condenses to form dense deep clouds. When clouds cannot contain moisture, it rains, creating a downdraught, a movement of air downwards.

The important feature of this storm was that the updraught and downdraught was happening in different places due to the wind shear. And the downward draught itself created another updraught that led to another thunderstorm — creating a chain of thunderstorms.

The storms were being continuously formed along a trough line running from east to west, an extended low pressure area where winds blow in the anti clockwise direction and rise upwards. Higher surface temperatures mean warmer near surface air that allow greater retention of moisture and formation of deeper storm clouds capable of bringing more rain and generating stronger winds.

“Local thunderstorm formation is impacted by temperatures,” Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, said. “All over India, temperatures are abnormally high, even if they are not the drivers, they will aggravate the situation by causing the atmosphere to become more unstable.”

Local thunderstorm formation is impacted by temperatures. All over India, temperatures are abnormally high, even if they are not the drivers, they will aggravate the situation by causing the atmosphere to become more unstable. ROXY MATHEW KOLL, climate scientist at Indian Institute of Tropical Meteorology