List of Tables
Chapter 3 Study Area
3.4 Variations of seasonal weather
The climate of the Brahmaputra valley is pre-dominantly humid subtropical with hot, humid summers, severe monsoons and mild winters. India as a whole, not to speak of
Brahmaputra valley and its neighboring areas, is a good example of monsoon climate. On the basis of precipitation received, India Meteorological Department has specified four seasons for the country. They are (a) Hot weather season (from March to May), (b) Southwest monsoon season (from June to September), (c) Retreating Southwest monsoon or post–monsoon season (from October to November) and (d) Northeast monsoon or winter season (from December to February). The divisions hold good in case of NE India too. But instead of calling the period from March to May ‘Hot weather seasons’ it would be appropriate to call it ‘Pre-monsoon season’, because in this region the hottest months are from June to August unlike in other parts of India.
3.4.1 Pre-monsoon season
The pre-monsoon starts by early March along with the gradual rise in temperature coupled with vanishing fog, pleasant mornings and hot afternoons, occasional dust storms and thunder showers. The valley gets a good amount of rain (about 25% of annual total) falling during this season due to “Nor’westers” which makes the climate cool even during spring.
In contrast, there is very little rainfall from March to May in the rest of northern India (Das 1992). This occasional rain in the Brahmaputra valley is highly significant for the cultivation of jute, autumn rice and for the budding of tea plants. The Brahmaputra valley including adjacent eastern Gangetic plains gets affected by severe thunderstorms during pre-monsoon months, in particular during April–May. Thunderstorm activity over the Brahmaputra valley is highest in the country. Thunderstorm produces heavy rain showers, lightening, thunder, hail-storms, dust-storms, surface wind squalls and tornadoes (DST 2005) causing extensive damage to winter wheat, summer paddy and tea crop. Nearly 28 severe thunderstorms occur in the valley during the period of two months (DST 2005).
Maximum frequency of thunderstorms is along the Brahmaputra river (Chakravarti et al.
2008) in a west to east decreasing gradient. Average number of thunderstorm events over Guwahati was 28, which was much higher than Kolkata (on an average 15) during 1998–
2008 (Chaudhury and Middey 2012). On an average (1971–2000), 10 hailstorms generally occur during this season in the Brahmaputra valley. Hailstorms cause damage to tea bushes (defoliation, breakage of young stems) depending on severity of storms, its time of occurrence and stage of growth of tea bushes. Unpruned tea bushes suffer less damage from hail than young tea and bushes in the process of recovery from pruning (Barua 1989).
3.4.2 Monsoon season
The normal date of onset of monsoon in NE India is within the first week of June.
Monsoon season is mainly characterized by cloudy weather, high humidity, heavy rainfall, higher temperature and weak variable surface wind. Temperature and rainfall slowly increases with advancement of the season. This is the longest season, during which the rising temperature is brought down considerably by intermittent rainfall. The undulated surface configuration deflects the monsoon winds and forces the moisture-laden air to rise along the slopes causing adiabatic cooling. This leads to heavy rainfall in the valley, except in a narrow rain-shadow belt to the east of the hills of Meghalaya. Rainfall is so frequent that about 18 to 20 days in a month are rainy. The monsoon season is the most important period for agricultural activities in the Brahmaputra valley, since the cultivation of winter rice (sali rice), the principal cereal crop, takes place during this period. Monsoon withdraws normally around 15th October from the region (Dhar and Nandargi 2000).
3.4.3 Post-monsoon season
With the retreat of monsoon wind, the temperature drops and morning mist and fogs appear. The post monsoon months of October and November are characterized by fair weather with fall of temperature and rainfall. The bright sunny weather with a tendency to raise atmospheric heat in the afternoon and rapid fall of temperature at night, makes the weather the most endurable and pleasant over the entire year.
3.4.4 Winter season
The winter is characterized by fall of night temperature, low rainfall with occasional cool breeze from the north and morning fog. The cool, fair and pleasant weather is interrupted by driving showers, associated with western disturbances which lower the temperature and bring cold spells. Ground fog is very common during winter on the southern bank of the Brahmaputra. The northerly or northeasterly winds drive the fog towards the southern plateaus of Karbi hills and the Meghalaya. Chilly wind from the Tibetan region is obstructed by the Himalayas and as a result, the valley is protected from unbearable cold.
Winter is the driest period in the valley. Local circulations and western disturbances bring some precipitation even during the winter season (Atri and Tyagi 2010). Even a little rain during this season is of great help for tea and other rabi crops. Tea crop in parts of central
Brahmaputra valley is susceptible to drought than upper parts which normally gets few showers during the dry part of the year (Barua 1989).