The Met office has warned of a muggy start to the week as an atmospheric system that was expected to trigger downpours following the onset of the monsoon has passed over Calcutta too fast to have the desired effect.
The monsoon arrived in the city on June 18, over a week behind schedule. The onset was followed — as well as preceded — by spells of shower but the “heavy rain” forecast by the Met office remained elusive.
“A low-pressure area that was drawing moisture from the Bay of Bengal has moved north. Heavy rainfall is expected in the northern districts, such as Darjeeling, Jalpaiguri and Cooch Behar, in the next two days. On the other hand, the clouding over south Bengal will lessen,” said G.C. Debnath, director, India Meteorological Department, Calcutta.
A weather scientist said the low-pressure belt passed over the city too fast to bring the predicted “heavy rain”.
Pre-monsoon rain started in Calcutta a couple of days before the northern limit of the southwesterly monsoon arrived in Calcutta and the daily shower count varied between 2.6mm and 38.5mm since (see chart).
In meteorological terminology, rainfall between 7.6mm and 35.5mm a day is considered moderate, and between 35.6mm and 64.4mm, “rather heavy”. Rainfall beyond 64.4mm is called heavy.
As the low-pressure belt moved north, districts like Purulia, Birbhum, Burdwan and Murshidabad received “heavy” rainfall.
While some people waiting for the torrential downpour are disappointed, not everybody is complaining.
“This is just fine because the weather is cool and there is no waterlogging that results in commuters’ nightmare,” said Anuradha Basu, 33, an executive in a Salt Lake Sector V firm.
The monsoon flow, according to Debnath, is still “quite strong” but the low-pressure area was pulling moist air from the Bay in a direction that won’t impact Calcutta’s weather.
The minimum relative humidity, which indicates the saturation of moisture in the air during the day, rose to 89 per cent on Thursday, the day after the monsoon arrived. On Sunday, it dropped to 58 per cent.
There can, however, be a drizzle or a small thunderstorm in Calcutta in the next two days as some moisture is still reaching the city from the sea and some more is trapped in the air.
If the Met office’s prediction turns out to be true, the sun will make its appearance in Calcutta’s skies after five days. A sunny sky will push up the mercury, which remained below normal from Wednesday to Saturday.
The cloud cover and rain dragged the mercury down to 28.1 degrees Celsius, five notches below normal, on Saturday. On Sunday the maximum temperature was 33 degrees, which was normal for this time of the year.
“The sunny phases would make staying outdoors uncomfortable. Sweat will stick to the skin because of high humidity,” said a Met department official.
Calcutta has received a cumulative rainfall of 179.5mm so far this month. Going by the June mean of 283.5mm, this month’s shower deficit has been pegged at a little over 13 per cent.
When are the showers likely to resume in Calcutta? “For clouds to come back to Calcutta, there has to be a favourable system close to the Bengal shore on the Bay or a trough of low pressure extending from the low-pressure area in the north to Calcutta. The trough will draw moisture from the Bay of Bengal, resulting in cloud formation and rain,” said a senior official at the Alipore Met office.