The climate of the district has been roughly divided into the tropical, temperature and alpine zones. For most of the period in a year, the climate is cold and humid as rainfall occurs in each month. The area experiences a heavy rainfall due to its proximity to the Bay of Bengal. The general trend of decrease in temperature with increase in altitude holds good every where. Pre-monsoon rain occurs in April-May and monsoon (south-west) operates normally from the month of May and continues up to early October.
The mean temperature in the lower altitudinal zone, it varies from 1.5 degree centigrade to 9.5 degree centigrade. Temperature varies with altitude and slope. The maximum temperature is recorded usually during July and August, and minimum during December & January. Fog is a common feature in the entire state from May to September. Biting cold is experienced at high altitude places in the winter months and snowfall is also not uncommon during this period.
An examination of available rainfall data shows that the mean annual rainfall is minimum at Thangu (82 mm.) and maximum at Gangtok (3494 mm.) . An isohyatal analysis of these data reveals that there are two maximum rainfall areas (i) South-East quadrant, including including Mangan, Singhik, Dikchu, Gangtok, Rongli etc. (ii) South-West corner including Hilley. Rainfall is heavy and well distributed during the months from May to early October. July is the wettest month in most of the places. The intensity of rainfall during South-West monsoon season decreases from south to North, while the distribution of winter rainfall is in the opposite order. The highest annual rainfall for the individual station may exceed 5000 mm. and average number of rainy days ( days with rain of 2.5 mm. or more) ranges from 100 at Thangu to 184 at Gangtok.