GUWAHATI, March 2 - The conflict of the relatively warm, humid, southerly wind coming from the Bay of Bengal in its lower air with the dry and cold continental air that conjoins with the remnants of the winter air, in its upper air, turns the northeastern region of India into a theatre of lightning, thunderstorm and hailstorm during the pre-monsoon season.
This is the observation made by Yusuke Yamane, an associate professor in meteorology in Tokoha University, Japan. Yamane, who is doing a research on the phenomena of thunderstorm, hailstorm and lightning in the Indian northeastern region, West Bengal and Bangladesh since 2006, jointly with Cotton University and three other institutions, was speaking to this newspaper during a recent tour to Guwahati.
He was accompanied by one of his associates Dr Hideyuki Kamimera, a researcher in radar hydrology and disaster risk management at the National Research Institute for Earth Science and Disaster Resilience (NIED), Japan.
Yamane said a lot of incidents of severe thunderstorm take place frequently in the March-May pre-monsoon season in the Northeast India, West Bengal and Bangladesh. This is because of the very much favourable atmospheric situation in this part of the globe during this period.
The favourable situation results from the big difference in the temperatures of the lower atmosphere and the upper atmosphere due to wind circulations. The upper level air becomes heavy and the lower level air becomes light. The upper level air hence descends under the impact of its weight and the light lower atmosphere air ascends, which is known as convection.
This convection leads to formation of clouds, which contains raindrops, and sometimes, hails that result in rainfall and hailstorms. These are unique phenomena to Northeast India, West Bengal, Bihar and Bangladesh. However, similar situations are also observed in Central America.
But, in case of Central America, the lightning activities occur in the period between April and June, while such activities in Northeast India, West Bengal and Bangladesh occur in the period between March and May. In case of Central America, a lot of study has been done, whereas on this portion of the globe, hardly any detail study was done, said Yamane.
Explaining further the science behind the stormy pre-monsoon season in eastern parts of India and Bangladesh, Yamane said between March and May, sunlight heats up the water of the Bay of Bengal in a significantly. Sunlight also heats up the subcontinental land region more during this period. This makes the lower air atmosphere of the region warm, while the continental air and the remnant of the winter air together make the upper atmosphere cold, providing the ideal condition for the thunderstorm, etc., developments, he said, adding, lightning also accompanies the thunderstorms on occasions.
Sometimes, tornadoes and big hails of the size of potatoes, also lash Northeast India and Bangladesh and at times West Bengal, too, during this period. All these may come together in a composite manner in this part of the globe, he said.
He also attributed the thunderstorms in the northeastern region to its unique hilly terrains. These hilly terrains also play a major role in this part of the globe to catapult thunderstorms, Yamane said.
The research group has installed several high-resolution digitised rain gauges in Northeast India, West Bengal and Bangladesh to collect rainfall data on an hourly basis. When pointed to the India Meteorology Department�s rain gauges in Northeast India and West Bengal, he said the data generated by the IMD equipment are not high-resolution ones and they are available only at daily scales.
The Japanese university has a plan to install devices with high-time resolution in Northeast India to observe temperature, humidity, air-pressure, wind direction and wind speed, he informed.
In reply to a question, he said before predicting such weather events, one should have a good understanding of the mechanism of severe storm formation. �Our understanding of this factor is not enough at this moment,� he said.