West Bengal has a tropical climate. The plains are hot except during the short winterseason.
The monsoon arrives by a middle of June. Its scouts start arriving about two weeks before its normal onset. This is called the Chhota monsoon which breaks the hot spell of summer. The monsoon rains in west Bengal are caused solely by the current of wind from the Bay of Bengal.
Variability is a characteristic feature of the monsoon in west Bengal as well as Bangladesh and Orissa which all receive the impact of the south-west Bay current. Breaks in the continuity of rain are not unusual, the resultant thoughts of low pressure develop into cyclone storms especially towards the end of the season and in early autumn.
A welcome change in the weather begins to be distinctly felt towards the end of September. Autumn in West Bengal is the season for festivity in the fields the golden grain of paddy starts ripening and is harvested towards the end of the season. The conclusion of the round of the festivities marks the onset of the winter in mid-November.
Winter, which lasts about three months, is mild over the plains, the average minimum temperature not falling 15o C. It is attended by a cold and dry northern wind, substantially lowering the humidity level. Winter is the season for the rabi crops-pulses, potato and vegetables and citrus fruits that grow on the Darjeeling hills. There occurs a short interregnum of clouds and rain usually the last week of December and the first week of January, caused by the incursion of the western monsoon coming all the way from the Arabian Sea. The cold is severe on the hills and there are sometimes sleet and snow on the higher reaches during the days of rain.
The weather gets warmer by the middle February, which heralds a brief spring season lasting about a month during which the deciduous trees break out in young green leaves and flowers. But this mellow season is too short-lived and the heat is turned on until with the coming of April, clammy summer comes in full blast and the annual cycle of seasons rolls on once again.