April this year was not only the driest of the decade but also saw the average maximum temperature for the month at 2.5 degrees Celsius above the “normal mean maximum”, calculated on the basis of meteorological data for 30 years.
The last time Calcutta got some rain was on March 25. The forecast for Thursday doesn’t include any rain either, extending the agony of April to the first day of May.
The city was either too dry or too humid in April, the one constant being high temperature. The average maximum temperature for the month has been pegged at 37.9 degrees Celsius, 2.5 degrees more than the normal mean maximum temperature of 35.4. The figure of 37.9 degrees was arrived at by adding the maximum temperature from April 1 to 30 this year and dividing the result by 30 to represent the number of days.
The Alipore Met office takes into account data for 30 years — 1971 to 2000 — to calculate the “normal” for a particular time of the year. A senior official said a new period — 2001 to 2030 — would be considered while calculating the normal from the year 2030.
In terms of rainfall in April, all that Calcutta got were a few drops on the very first day of the month. The rain gauge at the Alipore observatory measured the quantum of rainfall as 0.1mm.
In April 2009, which was equally hot, the city had received 0.2mm of rain, marginally more than this year. But in all other years from 2004 to 2013, it had rained between 20.6mm and 108.4mm (see chart).
Weather scientists blamed the lack of rain and the squalls that usually hit Calcutta between March and April for the hot and often unbearable weather over the past month.
“In April, the city gets about three squalls, accompanied by rain. These storms cool down the weather. There was none this year, so the period of hot weather lingered,” said G.C. Debnath, director of the India Meteorological Department, Calcutta.
The weather was at its hottest in the third week of the month with the Celsius hitting 40 degrees and relative humidity nosediving, prompting the weather office to declare a heatwave that continued for five days — an April record for at least 35 years since 1980.
In 2009, there was a heatwave for four days. The Met office had then said that there was no previous instance since 1980 of the April temperature surpassing 40 degrees Celsius for two days on the trot.
On April 25 this year, the city recorded a temperature of 42.1 degrees Celsius, which equalled the maximum recorded on April 19, 2009, for the highest temperature of the month in a decade.
The heatwave ended last Sunday when a high-pressure belt on the Bay of Bengal moved north and came close to the Bengal coast, because of which there was clouding and a fall in temperature. Another weather system, a trough of low pressure stretching across the state, started pulling moisture inland at the same time.
The moisture incursion led to a decrease in solar radiation received by the earth’s surface, but humidity soared to make the city more sweaty and uncomfortable. Rain has played truant despite the high humidity.
“For thunderstorms to be generated, we need both heat and humidity. We have had the heat but the moisture level has been low, which is why there have been no squalls in Calcutta,” Debnath said.
The one good news is that weather scientists expect the maximum temperature to drop a little over the next few days.