Brahamaputra Basin

Brahamaputra Basin
1.3 Climate
The basin experiences four distinct seasons in a year : winter, summer, monsoon and autum/postmonsoon.
The winter season begins in December and continues to the end of February. In the high altitude
places of Arunachal Pradesh and Sikkim, winter lasts longer-from November to March. Light northeasterly
winds blow down the Brahmaputra valley in Assam and light northerly to north westerly
winds in West Bengal. The weather is occasionally changed by the passage of western disturbances
across the region, light rainfall occurs in January and February along the hills, increasing towards
North-east Assam. Thunder storms are rare in December and January and occur only on one or two
days in February, these may occasionally be accompanied by a dust or hail storm.
From March onwards, the hot weather starts and continues up to the last week of May. In this
season the basin is under the sway of three air streams a deep north-westerly current from Uttar
Pradesh and Bihar, a shallow southerly stream from the head of the Bay of Bengal over the Orissa
and Bengal coasts, and a shallower current of North easterly to easterly winds from the
Brahmaputra valley blowing over the plains of North Bengal and Bihar. With the interactions
between these air streams, this is a season when marked instability develops in the atmosphere and
severe thunderstorms occur extensively, sometimes preceded by dust-raising squalls. Rainfall
increases both in amount and frequency as the season advances, being generally associated with
afternoon or evening thunderstorms and squalls. They are generally more concentrated in Upper
Assam. Hail storms occur on one to three days in the season, especially in the Assam hills. The
humidity is relatively high.
The monsoon sets in by the last week of May or in early June, being usually ushered in by a
depression in the Bay of Bengal. Subsequently, a series of such depressions, forming at the head of
the Bay and moving inland, give spells of continuous and moderate to heavy rain generally over the
region. July and August are the rainiest months. Almost all the rain in this season is associated with
thunder.
The monsoon withdraws in the last week of September or the first week of October. After the
withdrawal of the monsoon winds, light unsteady winds are experienced by the middle of October.
Occasionally, in October, cyclonic storms from the Bay cross the Bengal coast bringing clouds and
rain with them.The weather during the autumn remains very comfortable with relatively clear sky
and moderate temperature. On average these two months experience 3% to 6% of the average
annual rainfall.
1.3.1 Rainfall
The distribution of rainfall in the Brahmaputra basin is different at different parts of the basin.
Monsoon rains from June to September account for 60-70 % of the annual rainfall in the basin, while
the premonsoon season covering the period March through May produces 20-25 % of the annual
rainfall caused primarily by depressions moving from the west and by local convectional storms.
During the post-winter months, the north-east monsoon finds its way into the Brahmaputra valley
through a saddle in the high Himalayas, at their eastern end. The Assam range of hills gradually rises
in height eastward from 300 m in the Garo hills to about 3,000 m in the Naga Hills. The low clouds
brought in by the south-west monsoon get interrupted on the southern face of the Khasi and Jaintia
hills by a 1,830 m high ridge and cause extremely heavy rainfall along the Cherrapunji-Mawphlang-
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