Here I will discuss the climate of north-eastern region. Before discussing the climate, let’s begin with the location of the north-eastern region. Because location of any particular region is one of the major important climatic determinants.
Location of North-East India
North East India is located in the northern hemisphere and it is the north-eastern part of India. The entire region looks like a reverse triangle with its apex at the southernmost tip of Mizoram, but there is no distinct southernmost boundary of the region.
The altitude of the region is varies from almost sea-level to over 7,000 meter above mean sea level. This region is located between 20॰ N latitude and 29॰30١ N latitude and 89॰46١ E longitude and 97॰30١ E longitude. The northern and eastern boundaries of this region are natural barriers represented by the high Himalayan Mountains and Patkai Hill ranges respectively . These natural barriers have a major impact on the climate of this region, and also have political importance.
Climatic characteristics of this region
From the location of these region we can see that this region is located between the latitudes 22॰N and 29.5॰ N and the Tropic of Cancer passes through Tripura and Mizoram which is southern part of this region. Therefore this region falls under the tropical monsoon climate. On the other hand this region is encircled on three sides by high mountain ranges and precipitous plateau Meghalaya and the course of southwest monsoon winds have provided its climate somewhat different from that of the other parts of India. The south west monsoon is the main source of rain in this region.
Factor influencing the climate of this region
1. The situation and alignment of the hills , plateaus and mountains. The Himalayas in the north, the Patkai and other hills and mountains in the east and the Meghalaya plateau in the middle have affected the general tropical warm climate of the region. The Himalayan mountain ranges , the Patkai and the high hill ranges along Manipur and Mizoram borders with Myanmar prevent the rain bearing monsoon winds from escaping from this region. Further, this ranges do not allow the dry and cold winds of central Asia to enter the north eastern region. On the other hand the Meghalaya plateau standing athwart the course of the southwest monsoon winds make them rise orographically, causing the heaviest rainfall in the world in its southern margin.
2. The seasonal change in the pressure condition over the Bay of Bengal on one hand and over the north western landmass of India on the other. This seasonal change of pressure condition occurs due to different seasons (winter, summer etc.) and the northward migration of the sun. The high pressure and the low pressure creates pressure gradients, such a condition results in the occurrence of thunder showers, occasional rains and squalls.
3. The tropical oceanic air mass (south-west monsoon) that blows over the region.
4. Occasional visit of the westerly (Mediterranean) lows in winter. In the winter season, this wind reaches the upper Indus valley and the Upper Gangetic plain, and sometimes travels as far east as the Brahmaputra valley, causing overcast sky, drizzle and rain.
5. Presence of local mountains and valley winds. Dust storm, occurrence of haze, mist and fog occurs due to the presence of local winds.
6. Presence of numerous vast water bodies and extensive forests and development of local cyclones. Occurrence of storms is also common in the plains of Assam, which is not very far from Bay of Bengal. Most of these storms have their origin in the tropical cyclones of the Bay of Bengal.
Seasons of North-East India
On the basis of temperature, pressure , humidity and their spatial distribution over the land mass, the weather condition of North-East India in a year can be categorized into four seasons. Those are described below:
1. Winter season
The temperature begins to fall over North-East India with the southward migration of the sun after 23rd September and winter sets in towards the later part of November. It continues up to the end of February.
December is the driest month of the year. During this period a local low pressure sets in on the Brahmaputra valley. This along with the Mediterranean lows can bring about cloudy and drizzly weather. The cold north-east trade wind occasionally blows over Arunachal Pradesh, Nagaland, Manipur, Meghalaya and Mizoram hills bringing down the temperature over the region. Thick fog is an important weather feature of the winter season over the region.
The rain falls normally during this period, while the north-eastern part of the region i.e. the Arunachal foothills and the upper Brahmaputra plain receive an average rainfall of 10 cm. On the other hand the rest of the North East India receives 5 cm rainfall during this period.
The weather during this period is influenced by the high pressure system of central Asia, the subtropical jet stream and the high pressure center over upper Myanmar.
2. Pre-monsoon season
With the end of February, temperature begins to rise in the region. March, April and May become sufficiently hot and rains do not come in their full form. We can say, the pre-monsoon period is a transitional zone between the dry and cool winter and warm rainy season. During this period the diurnal range of temperature is very high, while the late night is pretty cool but the afternoon is very high. The important characteristics of this season are the rapidly increasing temperature, disappearance of fog and frequent occurrence of hail-storms and thunder-shower. Bordoichila is a well known storm in Assam during this season.
3. Monsoon season
Monsoon season prevails over North East India during the months of June, July, August and September. Due to the northward migration of the Sun the Indian landmass gets heated and a low pressure system occurs. This low pressure system prevails over North-East India also. The monsoon winds enter North East India through two routes. One is from the south, from the Bay of Bengal and the other one is from the south west, from Arabian sea.
The low pressure system over this region attracts the wind from the relatively high pressure belt of the southern seas. The south-west monsoon winds from both the Arabian sea and the Bay of Bengal strike Meghalaya first and then moves further north to strike against the Himalayan foothills. As these winds are within less than 5 km from the surface, they strike against the southern hill slopes and rise up. In this process the rising air masses loss heat at a wet adiabatic rate. The moisture content of the air mass condense and rainfall occurs in the windward side of the hills, especially in the Meghalaya Plateau, Himalayan foothills and the foothills of the eastern ranges. The monsoon season is characterized by heavy rainfall, high relative humidity and light surface wind.
4. The season of Retreating Monsoon
This season occurs in the later part of September. During this time the sun crosses the equator. In this time the temperature begins to decrease, the low pressure system developed over the region loosens its grip and the south-west monsoon winds cease to be attracted. The temperature comes down to 20⁰C – 25⁰C in the plains and to 15⁰C – 20⁰C over the hills. The rainfall decreases and stands at about 12cm-15cm in the region except Meghalaya, where it is more. This is a short season and it covers only the months of October and November.
Climatic region of North-East India
From the above discussion about different seasons, it becomes clear that the climate of North-East India is different from that of other parts of India. The climatic of this region can be divided into the following climatic regions:
Region of Cold, Humid, High altitude climate
This type of climate is found in the eastern hills in the areas above 2000 m and the mid-northern part of Arunachal Pradesh. The temperature is generally low over here. In the winter season, temperature reaches below 0⁰C and in the summer season, temperature does not exceed 20⁰C. Average annual precipitation is above 150 cm. Precipitation is high and sometimes snowfall occurs here. The entire region is mountainous and thus agriculture activities are limited here. Some amount of maize, millets, vegetables, fruits like lemon, apple, pineapple, pear etc. are grown on the hill slope by shifting method.
Region of Humid Subtropical Monsoon Climate
Arunachal foothills, the Brahmaputra valley, Nagaland, eastern part of North Cachar Hills, Manipur (including the valley) and Mizoram falls under this climatic region. The summer is sufficiently hot and humid (18⁰C to 25⁰C) and the winter is longer and drier than in the plains. Shifting cultivation, terrace cultivation, and common sedentary cultivation are practiced here. Maize, millets, pineapple, hill rice, wet rice, vegetables etc. are grown here. This region is covered with dense vegetation and supports many animals including elephants.
Region of hot humid monsoon climate
The plain areas and the northern foothills of Meghalaya come under this region. The temperature over this region is generally high, in the summer maximum temperature reaches 36⁰C while in the winter season temperature reaches hardly 10⁰C. The average annual rainfall of this region is above 150 cm and some places experience 300 cm. Storm and thunderstorm are common weather phenomena during pre-monsson period in this region. This region is very important for agricultural activities because it consists of a plain. Rice, wheat, pulses, tea, sugarcane, jute, mustard, vegetables and common tropical fruits are grown here.
Region of rainy, cool monsoon climate
This type of climate prevails over the higher southern half of Meghalaya to the south of a line joining Darugiri, Nongkhlaw, Umsning and Hamren of Karbi Anglong. Here the temperature reaches 20⁰C in the summer and in the winter season the lowest temperature reaches 0⁰C. The rainfall has been occurring for about 8 months, the amount of average annual rainfall is over 250 cm. The world’s rainiest place Mawsynram falls under this region. Some of the common crops that are grown here are- hill rice, maize, orange, pear, vegetables, hill rice etc. The area is covered with natural pine trees.
Region of alpine climate
The high Himalayan region and the northern part of Arunachal Pradesh comes under this category. This region has a cold climate throughout the year. The temperature remains below 0⁰C for a long time in winter. Cold mountainous wind, which often turns into blizzards, is a common phenomenon here. In the winter season, precipitation occurs in the form of snowfall, while rainfall is only occasional during summer. The alpine grasses, short bushes, junipers and stunts conifers are the main natural vegetation of this region. Barley and oat are grown in the sheltered valleys. Yak and mules are domesticated animals of this region.