Climate of Darjeeling
The hilly tract of Darjeeling offers unique environmental Eco-perception. Essentially featured by cold and humid weather, the amount of rainfall in Darjeeling plays a significant role in causing instability of slopes. Although the high intensity of rainfall within a short period is not alien to Darjeeling hill areas, the old records show that this natural phenomenon has happened only for 42 times during the long period from 1891 to 1975.
However, the isohyets maps prepared in accordance with the average annual rainfall during last 25 years in 3 subdivisions in Darjeeling hill areas, reveals that the value increases from west to east, while the maximum concentration of landslides fall between 210cm and 410cm of Isohyets.
Apart from the basic phenomenon of season change, orographic factor also plays a pivotal role in determining the climatic features in Darjeeling. It induces the vertical zonation of temperature and declining percentage of precipitation. In this way, the mountain front is exposed to heavy rainfall, however, the middle parts of the southern hills gets the maximum amount of rainfall. During the rainfall, the mean annual temperature fluctuates from 24ºc in the plains but drops below 12ºc on the ridge. While the ridges experience the mean temperature of 160 to 170 centigrade during the summer months, the temperature drops at 5º-6º during the winters.
However, there is no distinct relation between total rainfall and altitude. The southern slopes of the ridges experience higher (4000-5000mm) precipitation than the leeward sides (2000-2500mm). One of the most important ridges along the Tiger Hill gets 3000mm while to the north the Great Rangit valley receives about 2000mm of rainfall. Generally, the annual total rainfall in Darjeeling town fluctuates between 1870-3690mm.
No discourse about the climatic features of Darjeeling completes, without talking about the landslide hazards. Reckoning the duration of rainfall is very important, when it comes to landslides, since rainfall is the associated feature of landslides. Long rainfall duration coupled with heavy down pour can result deeper infiltration and overland flow, which accentuates the occurrence of landslides on weaker slopes.
Contemplating the older weather and climate charts of Darjeeling, long continued down pours has occurred for a number of times. However, the widely known ones occurred in 1787, 1789, 1827 (493 mm in one day) and in June 1950 (965 mm). The last such rainfall recorded during 1968 (second and 5th Oct – about 1780 mm). In the following years, the rainfall recoded was 358 mm in Oct 1973, 382 mm in June 1983, 457 mm in September 1986 and 350mm in 1990.