Nights in the late monsoon this year, and past couple of years, were too humid and hot. Keeping the living room's windows open at bedtime hardly gave a relief for sound sleep – unless it rained at night.
A hypothesis suggests that monsoon-time temperature increases due to excess humidity in the air. However, a study on daytime asymmetry in temperature, cloud cover, humidity and precipitation published in the scientific journal Global Change Biology in August this year found that prolonged cloud coverage over land is the reason for monsoon-time temperature increase.
Bangladesh, as a tropical country, too, observes an increasing trend of both daytime and night-time temperatures, according to Bangladesh Meteorological Department officials.
Heat from the earth's surface is emitted freely into space if the sky is clear, decreasing temperature. On the other hand, if clouds are present, heat emitted from the surface gets trapped in the atmosphere as it is re-emitted back towards the earth, increasing temperature.
The study said clouds lessen shortwave radiation, resulting in lowering of daytime temperatures, whilst at night, they re-emit long wave radiation downwards, warming the earth's surface.
The scientists led by Daniel Cox, a research fellow at the University of Exeter, compared the rises in daytime and night-time temperatures over 35 years up to 2017. The collected data set were of near-surface temperature, cloud cover, specific humidity and precipitation.
The scientists came to the conclusion that there has been a global trend of night-time temperatures increasing at a faster rate than that of daytime.
Minimum temperature has been increasing slightly compared to the daytime, particularly in urban areas
Meteorologist Muhammad Abul Kalam Mallik at the Bangladesh Meteorological Department said a statistical analysis of data recorded in 1948 shows an increasing trend of night-time temperature in Bangladesh's weather. Minimum temperature refers to night-time temperature.
"Minimum temperature has been increasing slightly compared to the daytime, particularly in urban areas," he said.
According to the Meteorological Department's latest data, minimum temperature shows a 2.1°C increase in every hundred years.
Mallik explained the reasons. In recent monsoons, the presence of excess water vapour or moisture particles has been found up to the mid troposphere. Water vapour retains temperature.
Meanwhile, there are also some anthropogenic causes.
"Diesel-run vehicles plying at night as well as air coolers release greenhouse gases in the lower surface. The gases, actually the climate pollutants, also retain heat, resulting in an increase of surface temperatures," Mallik said.
Diesel-run vehicles plying at night as well as air coolers release greenhouse gases in the lower surface. The gases, actually the climate pollutants, also retain heat, resulting in an increase of surface temperatures
A joint study by the Meteorological Department and the Norwegian Meteorological Institute published in 2016 found that temperatures, both maximum and minimum, increased during 1981-2010.
The minimum temperatures of almost all 44 weather stations showed an increment between 2.1°C and 3.8°C per hundred years.
The study also revealed that the rates of temperature increments of the weather stations located across the southern part of the country were higher than those located across the north and north-western parts.
Scientists led by Daniel Cox have examined the proportional change in the leaf area index (LAI) as a potential biological response to the 24-hour day-night warming asymmetry.
The LAI of a plant canopy is defined as its leaf area per unit of ground area. In other words, it tells us how many layers of leaves would be on the ground if they would all fall down and be arranged exactly side by side.
Their study found that night-time warming was more prevalent in wetter regions, where increased cloud cover reduces photosynthesis and drives a negative correlation between precipitation and vegetation growth. The findings showed "profound consequences" for wildlife in their ability to adapt to climate emergencies.
The scientists have warned that species that are only active at night or during the day will be particularly affected by the warming asymmetry.
Any change in climatic conditions such as temperature, humidity and rainfall will impact wildlife. Wildlife will adapt to the changes by rearranging their activity period in case the changes are mild. If the changes are humiliating, wildlife will migrate to a suitable ecosystem or become extinct
Professor M Monirul H Khan of the zoology department at Jahangirnagar University said climatic condition controls behaviour, occurrence and activities of wildlife.
"Any change in climatic conditions such as temperature, humidity and rainfall will impact wildlife. Wildlife will adapt to the changes by rearranging their activity period in case the changes are mild. If the changes are humiliating, wildlife will migrate to a suitable ecosystem or become extinct," he said.
Warmer nights harm crops and livestock. Like humans, livestock need lower night-time temperatures to restore health and release excess heat built up during the day. Increments of night-time temperatures also increase transpiration from some crops, thus drying them out, introducing health problems and lowering yield.
Professor Monirul emphasised further research on how climatic changes impact flora and fauna, and broadly, the ecosystem on which the humans are dependent on.