The hot and humid spell of the past few days may take a break over the weekend as a low-pressure trough over Bengal has raised hopes of rain.

The weatherman expects clouding in the Calcutta skies to start on Saturday afternoon and the showers to arrive in the evening. If the trough behaves as predicted, the rain is likely to continue till Sunday.

“A low-pressure trough originating from a cyclonic circulation over Assam is extending up to the southern districts of north Bengal, till Malda. The trough is likely to make a southerly movement, which is why we are expecting rain in Calcutta and some other parts of Bengal over the next two days,” said Gokul Chandra Debnath, director, India Meteorological Department, Calcutta.

The weather office expects light to moderate rain in the city on Saturday and Sunday.

According to meteorological terminology, that means anything between 2.5mm to 35.5mm a day.

On Friday, the city’s maximum temperature was 35.3 degrees Celsius, normal for this time of the year, but the humidity was on the higher side.

At 2.30pm, the temperature was 34.6 degrees and the relative humidity, a torturous 61per cent. The discomfort index, a measure of the combined effect of heat and humidity on the body read 64.4 degrees at that time, more than nine notches above the comfort level.

Experts predict there would be a cloud cover on Sunday, because of which the solar radiation received by the earth surface would reduce, resulting in a fall in the temperature. So, despite high humidity people would sweat less and feel less uncomfortable.

“The cyclonic circulation is likely to weaken on Saturday but the trough of low pressure is expected to stay active till Sunday,” said a weather scientist.

“Patches of clouds seen on Friday afternoon were the effect of the trough, which is pulling moist air from the Bay of Bengal.”

The moisture flow from the sea is likely to intensify on Saturday following the southward movement of the trough, raising the possibility of thunderstorms towards the evening.

The morning and early afternoon on Saturday, however, are expected to be sultry because of the moisture incursion.

“Once the rain starts, the city is likely to cool down.... The heating in the morning and early afternoon is necessary for the formation of thunderclouds and rain in the latter part of the day,” said an expert.

The low-pressure trough is expected to lose strength on Monday.