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Decoded: the weather double whammy

- Brace for weekend spurt in discomfort

The heat-humidity assault Calcuttans have been enduring over the past 10 days is likely to intensify over the weekend, with the Met office warning of a heatwave-like condition. The Celsius could touch 40 degrees by Sunday, which would be five degrees above normal.

Metro explains why it is so humid and what can bring relief to the city.

Why are we feeling so uncomfortable?

The moisture content in the air — expressed in “relative humidity” — is usually very low during the hottest part of the day, because moisture tends to evaporate.

But over the past few days, the heat has been unable to suck the water droplets out because of excessive moisture flow from the sea. So the relative humidity in the afternoons — when the Celsius is at the day’s high — has been ranging between 60 and 70 per cent.

With air being saturated with moisture, sweat is not evaporating, making people feel uncomfortable.

The discomfort index, which is a measure for the effect of heat and humidity on the body, read 59.4 degrees Celsius, five notches above the comfort level, at 5.30 in the morning. At 11.30am, it was 67 degrees Celsius, 12 degrees above the comfort level.

On the hottest day of this year, May 20, when the mercury touched 41.5 degrees Celsius, the minimum relative humidity was 18 per cent. It was 56 per cent on Thursday and 51 per cent on Friday.

Why is the humidity so high?

A cyclonic circulation over north Bengal and Bangladesh is drawing moisture from the Bay of Bengal to the city. Besides, the general direction of the wind — south-westerly — is aiding the moisture flow inland.

If it is humid, why isn’t there more clouding and rain?

The cyclonic circulation is not strong or close enough for the upper air over the city to be packed with moisture, resulting in prolonged spells of rain. Currently, there is excess moisture over the surface but not in the upper air.

The thundershowers in the city on Wednesday and Thursday were caused by rain-bearing clouds that had formed over the western districts of Bengal and were blown towards the city.

Why did the evening rain on Wednesday and Thursday fail to cool the city?

Lack of moisture in the upper air over Calcutta and its surroundings did not allow enough rainfall despite the squalls clocking 50kmph on both days.

On Wednesday, the Alipore Met office had recorded just 0.7mm of rain. Thursday’s count was 2.1mm.

“The moisture in the upper air fuels the progress of a thunderstorm. In its absence, the squalls fizzled out and the rainfall was minimal. As a result, even after the rain there was enough moisture in the air to torment Calcuttans,” said G.C. Debnath, director, India Meteorological Department, Calcutta

The situation will change after the arrival of the monsoon. As the monsoon flow gains in strength, moisture accumulation occurs at a higher level, resulting in widespread formation of rain-bearing clouds.

When is the monsoon expected to reach Calcutta?

Cyclone Nanauk on the Arabian Sea, which has been holding the monsoon winds back, weakened into a deep depression on Friday. It is weakening further and moving away from India’s cost. The northern limit of the monsoon is likely to start moving again.

There is no official word yet on when the monsoon would arrive in the city but indications are that it could do so by the middle of next week. The monsoon usually comes to Calcutta between June 8 and 10.

How would the weekend be?

Brace for a further rise in the mercury and the discomfort level. If the Met office’s prediction of the maximum temperature touching 40 degrees by Sunday comes true, it would mark the start of the season’s fourth heat spell.