This August was special for Chennai when it came to rainfall. Showers during evening or night was almost a daily affair. But, not all areas were lucky to get rainfall on the same day.
Weather experts say that it is typical during Southwest monsoon between June and September when thundershowers may deliver several centimetres of rainfall in one area while leaving another area, a few kilometres away, dry. On Wednesday night too, places around Sriperumbudur and Kancheepuram received 7cm of rainfall whereas areas like Nungambakkam got only 1cm.
K.V.Balasubramanian, secretary, Indian Meteorological Society, said it is a common phenomenon when there is a difference in the heating of land and sea between March and September. Land would get heated up faster than the sea. During these months when Southwest monsoon prevails, land is already hot due to heat in the daytime and moisture-laden winds from the Bay of Bengal enters the land. As moisture evaporates into the atmosphere due to intense heating of land, it forms into cumulus clouds that brings thundershowers due to convective activity.
Unlike monsoon rains, these rain spells last for a short time and clouds may form again to bring more rainfall.
“The penetration of moisture-laden winds that can cover up to 100-150 km decides where it will bring rain,” he said. These clouds are steered by westerly winds prevailing in this season. This is why it rains only in some pockets while other localities remain dry.
Y.E.A.Raj, former deputy director general of meteorology said such clouds are formed at a height of 12-14 km from the surface. Clouds forming over a higher altitude and assisted by weather systems like upper air cyclonic circulation bring widespread rains. In coastal places like Chennai, sea breeze too triggers convective activity. The friction between cool easterly and hot westerly winds brings in an instability in the atmosphere. Rains over a particular area depends on the way rain clouds form and the direction of their movement, he added.
This August was special as many parts received showers simultaneously in the State. This year, more factors contributed to thunderstorms.
The forming of weak weather systems over the Bay of Bengal as Southwest monsoon took a break in northern parts of India coupled with convective activity brought some widespread rains, noted weather experts.
