CLIMATE OF NORTH EAST INDIA

The North Eastern region lends itself favourably to the exposition of climate on a synoptic basis. Variability in climate both seasonally and as between the sub-regions is best understood if references are made to the distribution of surface features, periodic oscillation of pressure cells and local conditions, which displace each other in the area.

Moreover, the popular understanding of the region’s climate best fits the zone of heavy rainfall and even within this, is upset by the period of pronounced dryness as in the plain sections between the months of December and February. Variation and distribution of weather elements are affected by micro topographical differentiation and other local characteristics. These have brought striking between the windward and leeward sides.

These elements together constituted the basis of distinct regional pattern of climate for the area as a whole. The North East India has a sub-tropical climate with only the higher elevation in Arunachal Pradesh experiencing the Alpine conditions. Hills and mountains in general have salubrious climate while the plain sections have hot summers and cool winters. Sub regional variations exerted considerable influence in modifying the local weather patterns.

By far the trend and distribution of the mountain systems and sub-systems have remained crucial in the climatic pattern of the region. They prevented the central Asiatic or the cold Siberian airmass and the Pacific maritime airmass while permitted the tropical maritime airmass to penetrate the region in summer. Thus, along with the alignment of Meghalaya, the barrier functions of the Barail and Patkai ranges in the east and the Himalayan ranges in the north played a role of regulatory mechanism that controlled the weather elements in the region.

Considering the temperature, pressure and humidity conditions in their temporal distribution, the weather conditions of North east India in a year can be divided into the following four seasons:

1. Winter Season:

Winter conditions are dominated by the easterly jet streams located over Tibet and high pressure belt in Upper Myanmar. These along with the mountain rim prevented the cold Siberian airmass and maritime Pacific currents from penetrating the region. As a result, the retreating monsoons in the region became weak and ineffective. It also induced dry period between December and February. The occasional rains in the period were largely due to mechanical and thermal diversities attenuated by local conditions, thermal lowering of the westerlies and western depression.

The general anti-cyclonic conditions lowered the diurnal temperatures. Lower slopes were characterised by fog. However, in the Brahmaputra valley, parts of Meghalaya, Imphal valley, Cachar plains and parts of Tripura indicated a noticeable tendency of an increase in the day temperature. These areas also exhibited the development occasional formations of thermal lows. These along with the wind shear effect at the junction of the surface winds and the westerlies ensured a considerably long period of cloudy weather. These increase the intensity of cold in the hill and mountain section where the minimum temperature often falls below 0°C. However, the temperature varies from 6°C to 15ºC in the region during the winter season. The amount of rainfall is negligible but highest in Arunachal Pradesh of about 5 cm.

2. Pre-Monsoon Season:

The transition from winter to summer conditions is abrupt and highlighted by the onset of pre-monsoons. The weather patterns are characterized by a general rise of temperature all over the region. This period also synchronizes with the development of local convections and formation of depressions. The strong surface winds that blow in the region during this period are associated with the heating up of the Tibetan plateau as well as to the southward migration of the jet streams to its summer position over the Gangetic plains.

The ‘Northwesters’ bring at the end of April and may. These are the line squalls with lightening, thunder and intense rainfall, frequently accompanied by hails. (The initial turbulence are strong enough to regenerate series of turbulence). Local lows also encourage all the development of strong convectional activity over the region. All these have resulted in keeping the temperature from rising and averages between 19°C to 25ºC in the region. The amount of rainfall varies but ranges between 25 cm to 50 cm.

3. South West Monsoon Season:

The outbreak of the monsoons is more or less similar to the rest of the country. It is associated with the sub-continental low pressure systems and the consequent northward movement of the zone of Inter Tropical Convergence. The effectiveness and distribution of the south west monsoon is also influenced by the development of depressions in the Bay of Bengal. This latter aspect not only influences the continuity of the south west monsoon but also determines the quantum of rainfall. In fact, the south west monsoons blow into the region at an elevation of 4 to 5 km above the sea level and more or less follow the topographical alignment in the region. The windward side receive the maximum rainfall e.g. Cherrapunji and mawsynram. The season is characterized by extreme high humidity conditions and widespread low clouds. These aspects continue to keep the temperature from rising and on average ranges between 25°C to 30ºC in the region during this season.  

4. Season of Retreating Monsoon:

Towards the latter part of September the Sun crosses the equator. The temperature begins to decrease, the low pressure system developed over the northern India loosens its grip and the south west monsoon winds cease to be attracted. The sky slowly clears out with only the isolate patches of clouds. As the relative humidity remains high, even a slight fall of temperature is enough to make haze and mist appear in the distant horizon, especially in the morning and evening. The temperature comes down to 20°C to 25ºC in the plains and to 15°C to 20ºC over the hills. The period of the retreating monsoons particularly October and November experience unsteady winds. The low pressures are replaced by high pressures. This heralds the decline in the rainfall and varies between 20cm to 25cm; this is largely concentrated in parts of Assam, Barak valley, Arunachal Pradesh and southern Meghalaya plateau.