Climate:
Darjeeling-Sikkim Himalayas experience a climate that varies right from Tropical to Tundra. This change can be felt in a matter of 5 to 6 hours journey. The temperatures in the lower foothills vary from 40 C in winter up to 350 C in summer. Regions above 1500M up to 2500 M have an average temperature of Minimum 200 C and Maximum 120 C in winter and Minimum 120 C and Maximum 250 C in summer. The region above 6000m in Northern, Eastern and Western Sikkim remains under permanent snow throughout the year. Winter period in the hills Mid October to March and summer period is from May to July. Monsoon starts from July and continues till Mid September. Occasional rainfalls occur in early May and June. It is also one of the rainiest regions with an average rainfall of 350cm per annum. Best time to visit this region from September to December and March to June.
 
A possible chart of Temperature in 0 Celsius and Rainfall in mm with Clear Sky possibilities in % in 2 major towns:
 
Gangtok
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Max Temp
14
15
19
22
22
23
23
23
23
22
19
19
Min Temp
4
5
9
12
14
16
17
17
16
12
9
6
Rain Fall
44
56
142
222
493
644
663
588
476
152
35
15
Darjeeling
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Max Temp
9
11
15
18
19
19
19
20
20
19
15
12
Min Temp
3
4
8
11
13
15
15
15
15
11
7
4
Rain Fall
22
27
52
109
187
522
713
573
419
116
14
5
Clear sky possibility
90
90
75
40
30
10
1
2
5
50
98
98