April is into its second week, but the city proper is yet to see the first squall of the season.
Metro spoke to Met officials to find out why.
Missing in action
A squall is a storm that usually brings rain. A Nor’wester is a squall that originates over the Chhotanagpur Plateau in the late afternoon during summer and sweeps through parts of eastern India over the next three to five hours at a wind speed of over 45kmph. It is usually followed by a brief spell of rain. Some of the squalls that Calcutta experiences during this time of the year are Nor’westers.
A trough or wind discontinuity over Jharkhand, Bihar or adjoining areas and a high-pressure area over the Bay of Bengal are the two important factors needed for a Nor’wester.
The moisture-laden winds, which then flow from the sea to the land, lead to the formation of thunderclouds that make up a squall line (a series of thunderclouds). The squall line moves from northwest to southeast (towards the Bay of Bengal), triggering gusts of wind, streaks of lightning and rain, as it moves. If Calcutta falls in the line of their passage, the city receives a squall.
The basic ingredients for any thunderstorm, local or originating elsewhere, are heat and humidity — in the form of moist winds from the Bay of Bengal. A mechanism to lift the moist air and a vertical wind shear are the two other requirements.
“In the first week of April, for a significant time, there was sufficient heating. But the moisture was minimal. For the first few days, hot and dry winds from northwestern India dominated the atmosphere,” said Somenath Dutta, the deputy director-general of the India Meteorological Department, Calcutta.
“Now, there is a lack of sufficient heating,” he said.
On Friday, the maximum temperature was 35.2 degrees in Calcutta, a notch below normal.
A cyclonic circulation persists over north Odisha. “The moisture is enough to send clouds and keep the day temperature down,” said a weather scientist.
The process
When there is enough heating and sufficient supply of moist winds from the Bay, the air closer to the surface of the earth heats up. Moist air is much lighter than dry air and moves up. It reaches the atmosphere and condenses into water droplets. The clouds storing the droplets become heavier and start descending, causing rainfall.
“At the early stage of a thunderstorm there is an updraft. In the mature stage, there is both an updraft and downdraft. That means a simultaneous upward movement of warm moist air and downward movement of cold and denser air. Towards the end, there is only the downdraft,” said Dutta.
The moisture incursion is dependent on the position of an anti-cyclone over the Bay. The moisture incursion went up from April 7 and several districts got thunderstorms. In Dum Dum, the Met office recorded a wind speed of over 60kmph on April 7.
“In Calcutta, there was not enough moist air close to the surface of the earth,” said Dutta.
Met data compiled over 30 years between 1981 and 2010 suggest that Calcutta gets a couple of thunderstorms, on an average, in April. May’s quota is 10.
Last year, a spell of thundershower on March 30 ended a long dry spell in Calcutta, one that began at the end of October. The next time the city got uniform rain and gusts of wind was April 24. May received a series of squalls.
There is little chance of uniform thunderstorm activity in Calcutta over the next few days, according to the Met forecast. “On April 14 and 17, there are chances of thunderstorms in some districts,” said an official.