No let-up in heat until Eid

The hot and dry weather during the day and plummeting mercury during the night are highly unusual phenomena in Bangladesh, said experts, blaming urbanisation and a set of weather conditions for the heatwave.

Dhaka experienced the hottest day since 1965 (40.5 degrees Celsius) yesterday.

The temperature was 40.4 degrees Celsius during the day on Saturday and 28 degrees that night, said Bangladesh Meteorological Department (BMD).

The highest temperature in the country that day was in Chuadanga, 42.2 degrees Celsius, but the night-time temperature there was a pleasant 23.5 degrees Celsius.

Meteorologists and experts said such differences between day and night temperatures are usually seen in desert areas.

They also said April is the hottest month in Bangladesh.

However, Abdul Mannan, a former meteorologist of BMD, said heatwaves are usually created by hot winds, but this year it has happened due to cloud-free skies over the region.

Explaining the situation, Ashraf Dewan, professor at the School of Earth & Planetary Sciences at the Curtin University of Australia, said, "It is because of high pressure in and around West Bengal and a low-pressure system in the Bay of Bengal. Because this high pressure is not allowing moisture to penetrate from the Bay of Bengal, people are experiencing low humidity inland than on the coast."

As a result, parts of Bangladesh including Dhaka are experiencing high temperatures. Apparently, a kind of heat dome has been created that does not allow moisture transfer from the Bay of Bengal to the low troposphere (the lowest layer of the Earth's atmosphere), he said.

Researcher of Canada's Saskatchewan University, Mostofa Kamal, who regularly monitors the country's weather forecasts, said the summertime weather in the subcontinent is controlled by a jet stream (a narrow band of fast-flowing wind that transports water vapour, heat, and momentum from the west to east).

"A jet stream ridge is stagnant over the Indian subcontinent like a bus that is standing still on the Dhaka-Tangail highway during Eid rush causing tailbacks," he said.

Kamal said that under a ridge, a high-pressure system is created that prevents cloud formation.

The upper air ridge acts like a pressure cooker valve. Solar radiation cannot escape the troposphere, which adds heat. The troposphere over Bangladesh and the West Bengal of India is building a column of hot air like a pressure cooker, he said.

He said that a disturbance is currently over Iran and Pakistan and it is expected to reach Bihar, West Bengal, and Bangladesh -- bringing nor'westers, rain, hail, and severe lightning from April 23.

The second reason for the current heatwave is another weather phenomenon called Mason-Julian Oscillation, Kamal said, adding that it periodically creates suppressing conditions for cloud formation and favourable conditions for cloud formation over the Bay.

Unfortunately, for the last 15 days, Mason-Julian Oscillation has been going through a suppressing phase and it is expected to be in a favourable condition for forming clouds from April 20, he said.

He said clouds will start flowing over Bangladesh and rainfall may start from April 21 over the Sylhet division. Rain is expected across the country after April 23.

Mohan Kumar, executive director at National Oceanographic and Maritime Institute (NOAMI), said there is almost no cloud and moisture content support from the Bay and the winds are too weak.

Meteorologist Mannan said the weather and heatwave patterns this month are very unusual.

He said the heatwave is gradually expanding. Usually, heatwaves sweep over the western region of the country like Khulna, Rajshahi, and some parts of Rangpur. But this year Barishal, Chattogram, and Dhaka regions have also been affected.

Experts, however, also blame rapid urbanisation, and depletion of forest and water bodies for the gradual rise of temperature in the country.

Researcher Mostofa Kamal said the smaller the green space and water bodies in an area, the faster it becomes hotter.

Prof Dewan said the issue of urban temperatures, especially in Dhaka, has become a serious concern and this is a result of the increase in air conditioner usage, rapid loss of green-blue space, construction of high-rises and increased industrial activities.