VAPOUR ACTION

MOISTURE IS A SILENT FORCE THAT AMPLIFIES GLOBAL WARMING. YET SCIENTISTS UNDERSTAND LITTLE ABOUT THIS GREENHOUSE GAS AND HOW IT INFLUENCES WEATHER SYSTEMS

INDIA WITNESSED a meteorological phenomenon this August that has left climate scientists perplexed: cyclone Asna. What set the cyclone apart was not its path or intensity but the source of its strength. Usually cyclones form over warm waters in the tropical oceans, where they draw energy through continuous supply of evaporating water, and weaken when the moisture supply is cut off after making landfall or passing over cold waters. But Asna defied convention. It originated as a low-pressure area in the Bay of Bengal on August 16. Over the next 14 days, it travelled westwards across six states, including the arid expanses of Rajasthan. Rather than dissipating, it gained momentum, gathered moisture and caused heavy rainfall and floods in areas it passed through. By the time it reached the Arabian Sea on August 30, Asna had intensified into a full-fledged cyclone.

"The question here is how the deep depression managed to get its moisture supply. Was it recycling soil moisture from earlier precipitation, or transporting the moisture from the oceans?" asks Raghu Murtugudde, professor of climate studies at the Indian Institute of Technology, Bombay, and emeritus professor at University of Maryland, US. While Murtugudde suggests that a moisture transport calculation would help decode the puzzle, there is no denying the fact that rising global temperatures are adding moisture or water vapour to the atmosphere.