Bangladesh has a tropical monsoon climate with significant variations in rainfall and temperature throughout the country. There are four main seasons: the pre-monsoon (March-May) has the highest temperatures and experiences the maximum intensity of cyclonic storms, especially in May; the monsoon (June-September) when the bulk of rainfall occurs; the post-monsoon (October-November) which, like the pre-monsoon season, is marked by tropical cyclones on the coast; and the cool and sunny dry season (December-February).
The mean annual temperature is about 250 C, with extremes of 4 and 430 C. Ground frosts can occur in the hills. Humidity ranges between 60% percent in the dry season and 98 percent during the monsoon.
About 80% percent of the total rainfall occurs in the monsoon, and the average annual rainfall over the country is 2666 mm. Precipitation varies from 1110 mm in the west to 5690 mm in the northeast. The country is regularly subjected to drought, floods and cyclones.
The internal renewable surface water resources are estimated at 105 km³/year. This includes 84 km³ of surface water and about 21 km³ of groundwater resources produced within the country, although part of the groundwater comes from the infiltration of surface water with an external origin. The total renewable water resources are therefore estimated at 1 210.6 km³.