Nature took an ominous turn over Calcutta on Friday afternoon. For about an hour beginning 1.55 pm, the whole of the city and a radius of about 30 km from its centre was in pitch darkness. It was as if a total eclipse of the sun had taken place.
The sun had been “eclipsed,” not by the moon but by clouds — dense, thick clouds that were between six and 14 km tall. In a rare situation, four cumulonimbus cloud formations, or ‘CB cells’ in meteorological parlance, were positioned close together over the city, blocking off sunlight completely.
Cumulonimbus clouds are a combination of cumulus, or rounded cloud masses heaped on each other above a horizontal base, and nimbus or rain clouds. They are commonly seen during the monsoon season.
Senior officials at the Alipore weather office said that such cloud formation can also be seen during the nor’wester season in April-May. “But Friday’s formation did seem uncanny. As far as I remember, the last time such a dense formation took place was during the rains of September 1978,” a weather official said.
“A CB cell occurrence always happens, especially during the the monsoon,” explained Alipore Meteorological department director K.K. Chakraborty. What happened on Friday was that a number of cells were close together, blocking the sun’s light from coming on to the earth’s surface.”
The Met officials plotted data from the radar positioned on top of the New Secretariat building off Strand Road. The plotting showed four cloud formations, one of which was as much as 14 km in depth, or equivalent to the distance from Esplanade to Salt Lake! The second was 10 km, the third 13 km and the fourth 6 km tall (not in that order). “If such thick, rain-bearing clouds stand between the sun and the earth’s surface, darkness is inevitable,” the director said.
“Whenever such formations take place, it is very difficult and dangerous for aircraft to land or take off,” Chakraborty said. The Air Traffic Control tower at Netaji Subhas International Airport also has radar to “diagnose” cloud formations.
“In such situations, pilots are either told to circumnavigate the cloud mass or told to circle above it till the density dissipates,” he added. However, airport officials said that there were no disruptions in flights during the day.
Explaining how such a build-up had taken place, the Met office director put forward two main reasons, leading to what is called a “meso-scale phenomenon” where a convective cloud formation takes place.
One reason was the existence of a low-pressure area over the central Bay of Bengal and the adjoining North Bay region. On Friday afternoon, the low-pressure area lay about 350 km south of Calcutta.
“Under its influence, and in association with upper-air circulation that extended up to six km above sea level, rain clouds had developed over the city. The low-pressure area, which was moving in a westerly direction, is likely to intensify into a depression by Saturday morning or afternoon,” Chakraborty said.
As a result, overcast skies and rain, sometimes heavy, has predicted for the morrow.
The second reason for such dense cloud formation was the transitional phase of the monsoon. “The south-west monsoon is gradually withdrawing. It has already done so in some parts of north-west India. During this phase, there is a change in air mass as well, from monsoon to post-monsoon. Moist air mixes with dry air and this interaction leads to cloud formation.”
The clouds opened up over most parts of the city about 20 minutes into the darkness. The rainfall was heavy. At 5.30 pm on Friday, the Alipore weather office had recorded 98.5 mm of rain, most of which had fallen in a one-hour period.
The eastern region’s Met department deputy director-general, R.N. Goldar, agreed that Friday’s cloud formation was a rare development over Calcutta.