Assam experiences tropical - warm and humid climate or "Tropical Monsoon Rainforest Climate" (summer max. at 35o - 39oC and winter min. at 5o – 8oC) with four well defined seasons in a year viz., winter, summer, monsoon and spring. The state receives substantial precipitation and the summer temperature stays around 38oC. The state receives large part of the annual precipitation during the monsoon season which reduces summer temperature and enables formation of foggy nights and mornings in winters. Before the arrival of the monsoon the state witnesses two months of cyclonic activity. Precipitation occurs other times of the year as well.
The warm and humid climate and the topography of Assam are conducive for plants and growth of diverse pattern of vegetation. The state houses as many as 51 forest and sub-forest types.
The hill districts of the state generally have salubrious climate and the plain districts are comparatively warm in summer and cold in winter. Accordingly, the climate of the state portrays alternate warm and cool periods with high humidity especially from May to November. During March to May when precipitation in northern India is least, Assam receive fair amount of rainfall from the Norwesters which keeps the temperature low in spring season. In the plains of Assam the temperature usually does not rise beyond 32oC in summer and in winter the minimum temperature experienced is about 8oC.
The climate of the plains and the sub-montane region becomes unpleasant in summer due to the high humidity carried by the southwest monsoon which showers an average annual rainfall of 120 inches or more on the Brahmaputra valley and the surrounding region. The monsoon is the lifeline of the region and its moist tropical climate creates treasure trove of biodiversity as rich as the equatorial rain forests.
Indian Meteorological Department (IMD) has analysed weather data from 1951 to 2010 of 282 stations for temperature and 141 stations for rainfall across the nation. For Assam, the analysis was based on 6 stations for temperature and 12 stations for rainfall showed that the mean temperature has increased by +0.01oC per year. There is also an increase in seasonal temperatures across seasons with pronounced warming in post monsoon and winter temperatures. The annual rainfall has also decreased by 2.96 mm per year during the same period. In addition, when station wise data are analysed for a period of at least 25-30 years, significant variations were observed across seasons in number of rainy days and in 24 hour maximum rainfall.
Assam is very much part of the regional warming trend, the summer monsoons are found to decreasing over this region at a rate of 11 mm per decade during the last century. During 2005 and 2006 several districts of Assam were severely affected by drought and IPCC has vindicated these droughts as signatures of climate change. The intense drought situation of Assam in the summer monsoon months of 2006 owing to below normal rainfall i.e. 40% has affected 15 districts at large along with the remaining districts has caused loss of Rs. 100 crores due to crop failure rendering 75% of the 26 million people’s agrarian livelihood. Another spell of drought during October 2008 to July 2009 had severely affected agriculture and production of hydropower in Assam and neighbouring states.
Climate projections in the state action plan also predict that extreme rainfall events will increase by 38%. The poor are more vulnerable to extreme climate events and the drastic climate change projections are particularly worrisome for Assam as almost 32% of its population lives below the poverty line. Further, a majority of this population is dependent for its income on agriculture, which in turn is highly dependent on climatic factors such as precipitation and weather, and is frequently disrupted due to damage from floods and droughts.
