The widely variable topography of Sikkim and windflows has resulted in Sikkim having many microclimates.
The temperatures that a particular place experiences varies considerably with altitude. At places of low altitude, like Singtam, Rangpo and Jorethang, the temperatures vary between 4 oC to 35 oC. Places like Gangtok with moderate altitudes of about 1800 metres (6000ft) experience temperatures between 1 oC and 25 oC; it is very rare that Gangtok gets snowfall. At altitudes above 4000 metres (13100 ft), the temperature never rises above 25 oC and remains much below the freezing point during the winters and great part of the spring and autumn. At higher altitudes, temperatures can change for instance from + 25 oC to -25 oC within minutes
Sikkim is one of the rainiest regions in India. Because of the proximity of Sikkim to the Bay of Bengal and the fact that the mountains of the state come directly in the path of the monsoon clouds, most parts of Sikkim experience torrential rains during summers. Clouds weighed down with moisture gingerly lumber up the mountains, jettisoning barrels of rain. So much so that even a small depression over the Bay of Bengal triggers off a downpour in Sikkim. Even during spring moisture laden clouds formed due to local evaporation, continue to batter a greater part of Sikkim. By the last week of September the monsoon disappears but there is a nip in the morning air. It presages the onset of autumn and cooler days to come. It is only during October to March that there is hardly any rain and the weather remains more or less clear.
Rainfall however varies considerably from place to place because of the hill features. The northern border of Sikkim experiences comparatively low rainfall because the monsoon clouds dry out by the time they hit the northern barrier. For the sake of comparison, Gangtok registers an average of 325 cm rainfall per annum whereas Muguthang in the extreme north experiences an average rainfall of only 60 cm per annum. Similarly Namchi in South District gets hardly 100 cm of precipitation annually as the Darjeeling hill feature obstructs the monsoon clouds.
Most of Sikkim does not experience high intensity winds.However, at many hill tops and passes, winds and blizzards having high speeds blow. Given below is the average maximum-minimum temperature and average rainfall over the last ten years at Gangtok and the probability of seeing clear skies.
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Max 13.5 11.9 16.6 20.7 22.3 21.5 22.5 22.5 21.8 19.9 16.1 13.0
Min 2.0 3.6 10.0 12.1 15.0 17.4 17.7 17.7 16.9 13.0 9.4 6.7
Rainfall 2.7 6.5 10.3 30.4 54.0 57.0 66.0 57.0 49.8 13.7 4.7 2.7
Chance of 90% 90% 75% 40% 30% 10% 1% 2% 5% 50% 98% 98%
Although the figures above pertain to Gangtok which is at an altitude of about 2000 metres it could be considered representative of the whole area after you make corrections based on the altitude. The rainfall would decrease at higher altitudes and so would the maximum-minimum temperature.