April and May have always been Bangladesh's months of intense heat, but the season is stretching. Midday sun in September now brings the shimmer of a hot wind, and the cool that once arrived in October and November comes later. Against that background, meteorologists have gone back through the records to answer two questions: when and where was the country hottest, and does the highest temperature mean the greatest discomfort?

The Bangladesh Meteorological Department (BMD) runs 48 stations and holds maximum and minimum temperature records going back to 1947. The highest reading in that archive is 45.1 degrees Celsius at Rajshahi on 18 May 1972, still the national record. The second highest was also in Rajshahi, 44.4 degrees in April 1980. Third is 44 degrees at Bogura in April 1989, matched at Ishwardi in May 1970. Then come 43.9 degrees at Bogura in May 1958 and 43.8 degrees several times, at Jashore in 1963, Rajshahi in 1985 and Jashore again in 2024, all in April. Chuadanga's 43.7 degrees in April 2024 is the highest of recent decades.

Meteorologist Abul Kalam Mallik says the relief from extreme heat is wind: gusts and storms. "In the years when the highest temperatures were recorded, there were fewer kalbaishakhi storms, or Delhi, Uttar Pradesh and West Bengal may have been very hot and hot winds entered Bangladesh from there," he explained. He adds a caveat: a lowland station covers 18-20 kilometres and a hill station 8-10, so places with few trees, many brick kilns and factories, dense housing and diesel traffic may be hotter still without ever being measured.

The records cluster in Rajshahi, Chuadanga, Jashore, Bogura and Ishwardi for geographical reasons. Chuadanga recorded 43.5 degrees on 1 May 1995, 43 degrees on 25 April that year and 43.3 degrees on 4 May 1989. Meteorologist A K M Nazmul Haque points to three factors: the wide plains of Chuadanga, Jashore, Meherpur and Khulna, and of West Bengal to their west, let heat travel directly by conduction; water vapour from the Bay of Bengal enters through the south-west and raises the temperature; and during the sun's northward passage from 21 March to 21 June its strongest rays fall around the 23-degree parallel, with Bangladesh lying between 22 and 26 degrees north.

A high reading is not the same as feeling hot. Mr Mallik says wind speed and humidity matter most. In 2024 a record 35-day heatwave ran from 1 April to 5 May, breaking a 76-year record, yet Chuadanga and Rajshahi were not the most uncomfortable places because the air was dry and moving; coastal Patuakhali, Barishal, Barguna and Cox's Bazar felt worse at lower temperatures because of marine humidity. When the gap between day and night temperatures falls below 10 degrees the heat is severe, at five degrees it is suffocating, and below five degrees heatstroke becomes a real risk. Eight to ten hours of cloudless sun also raises the temperature, hence the proverb that sun after cloud is hottest.

Mymensingh and Sylhet stay tolerable because of their haors and hills: marine moisture is forced up by the hills, forms cloud and falls as rain, and warm and cool air mix in the valleys. Regular rain and forest cover do the same for Chattogram and Sylhet.

The BMD classifies 36-38 degrees as a mild heatwave, 38-40 as moderate and 40-42 as severe; above 35 degrees the body's cooling mechanism begins to fail, a direct hazard for anyone working in the fields. Forecasters expect this year to be warmer than normal, although April should be near normal. "The years 2022 to 2027 will be warm worldwide and Bangladesh is no exception," Mr Mallik said, but with more kalbaishakhi storms this year the heat will not exceed 2024. Between April and June the country may see six to eight heatwaves, three or four of them severe.

Source: BBC Bangla