KOLKATA: A dust-free monsoon sky and insufficient rain have combined to push up the mercury in Kolkata, making it unusually warm and humid. Since Wednesday, the city’s maximum temperature have climbed three notches above normal and touched 36.1°C on Thursday.According to the Met office, apart from a dry monsoon, lack of heat-absorbing dust particles in the lower levels of the atmosphere is also responsible for the sudden spurt in temperature. Respite, though, is in store during the weekend when the city is set to receive rain, triggered by a low-pressure trough over north-west Bay of Bengal.Dust particles in the troposphere – which is close to the earth’s surface – lessen during monsoon. Since they can absorb or reflect solar radiation, their absence leads to a rise in temperature, according to Regional Meteorological Centre (RMC) director GK Das. “In monsoon, rain washes away the dust particles in the lower levels of atmosphere. These particles tend to absorb and reflect sunlight, thereby keeping the temperature in check. Without them, the surface receives more direct sunlight, which pushes the mercury up,” explained Das.He added that the heat has been accentuated by lack of rain in July, usually the rainiest monsoon month. Since July 1, Kolkata has received 51% less rain than normal. The deficiency is set to continue unless a major system forms in the region. There is none yet, barring a low-pressure trough over north-west Bay of Bengal that is set to intensify over the weekend. But it is likely to trigger moderate rain, which may not be enough to neutralize the deficit or lower the temperature appreciably.“Unless we have heavier showers more regularly, the mercury will keep spiraling. Monsoon rain is driven by lowpressure systems. The more they form, the more rain we will receive. They have been rare in the region so far. And the mercury has been climbing quickly in the absence of the dustparticle shield,” added Das.Even though showers have been light this month, the occasional drizzles are enough to wash down the dust particles, explained an atmospheric scientist. “It has been drizzling almost every morning. This is ideal to remove the dust in the troposphere and let the sunrays in directly. This is a natural phenomenon in monsoon,” he said.

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