Mirror of Assam - Climate
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North -East India is located between the latitudes 22o N and 29.5o N The tropic of cancer passes along its southern part through Tripura and Mizoram. Therefore, it has essentially tropical climate. At the same time, being within the monsoon belt of South and South-East Asia, the region is under the tropical monsoon climate. But its location and topography, encircled on three sides by high mountain ranges and the presence of a precipitous plateau (Meghalaya) athwart the course of the incoming south west monsoon wines, have rendered its climate somewhat different from that of the other parts of India. The factors influencing the region’s climate may be listed as follows:

Factors Influencing the Climate of North-East India:

1. The situation and alignment of the hills, Plateaus and mountains in the region.

2. The seasonal change in the pressure condition over the Bay of Bengal on the one hand and over the north-western landmass of India on the other.

3. The tropical oceanic (south-west monsoon) air masses that blow over this region.

4. Occasional visit of the westerly (Mediterranean) lows in winter.

5. Presence of local mountain and valley winds.

6. Presence of numerous vast water bodies and extensive forests and development of local cyclones.

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. (1) Many of these hills and mountains are high enough (sometimes between 1 km to 5 km a.s.l) rendering the climate cool and bracing. Besides, (2) the Himalayan mountain chain the Patkai and the high hills ranges along Manipur and Mizoram borders with Myanmar prevent the rain bearing monsoon winds from escaping from this region Further, these ranges do not allow the dry and cold winds of central Asia to enter the North East region. (3) 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. (4) The Himalayan and eastern hill ranges also cause orographic rise of the monsoon winds with consequent heavy rainfall in North East India. (5) Another very important effect of the encircling hills and mountains on the climate of the region is that in summer while the plains become hot, the air over the hills and mountain remain relatively cool. Thus local low pressure systems are built up over the valleys . These low pressure systems over the Brahmaputra and Barak valleys obviously modify the climate in this region.

The climate of the region can be understood clearly with the help of the following description :

In winter an extensive high pressure system prevails over central Asia and the northern part of India. There then exists a low pressure over the seas and the ocean south of India. As winter passes, the landmass becomes warm quickly and by about late March and April, a low pressure is built up over northern India,. The air mass over the ocean being heated up more slowly, there prevails a relative high pressure over the seas and the ocean. With the northward migration of the sun , the land mass of India becomes very hot in April and May and the pressure gradient between north India and the southern seas becomes very steep. A low pressure system thus envelops the lower altitudes of north East Indian region. The winds then begin to blow from the south and south-west of the region bringing about a significant change in the weather condition of this region. Such a condition results in the occurrence of thunder showers squalls and occasional rains. This is one of the reasons why temperature is never very high in the north eastern region.

The Mediterranean lows, which reach the upper Indus valley and the upper Gangetic plain in winter , sometimes travel as far east of Brahmapurta valley, causing overcast sky , drizzle and rain.

The important local phenomena that affect the climate of the region are (1) the mountain and valley winds, (2) the dust storms, (3) the haze, mist and fog and (4) the cyclones. The valley winds begin to blow from the plains lower down to hills and mountains from about 10 o’clock in the forenoon to about 10 o’ clock in the evening. Conversely, the mountain wind blows down the slope to the plains from about midnight to about 9 o’ clock in the morning. The cool valley winds check the rise of temperature over the hills in summer during day time, but in winter they further lowers the temperature over the hills. On the other hand, the cool mountain winds drop the night temperature of the plains both in summer and winter. The effects of the mountain winds are, however, limited to the foothill plains alone.

The occurrence of dust storms is another important feature of the spring weather of the western half of the Brahmaputra valley. The local winds, caused due to the heating of the ground in February and March, resulting in the ascent of the ground -level air, sweep the ground at a moderate to high speed, spread sand dust all over the lower level of the atmosphere. Under such a situation the humidity decreases and visibility is reduced.

The presence of haze, mist and fog is still another characteristic of the region. As the region experiences heavy rainfall in the monsoon period, its atmosphere remains impregnated with water vapour while its rivers, streams, lakes, swamps and other low lying areas remain full of water. In October, when temperature falls in general. the saturated atmosphere give out haze and mist, especially at late night and early morning. With the passage of time, as the temperature falls, dense fogs appear in December and January, which hang over ground till late forenoon.

It should be noted that the occurrence of storms is also common in North East India, especially in the two plains of Assam. Most of these storms have their origin in the tropical cyclones of the Bay of Bengal Being not very far from the tip of the Bay, the plains experience cyclonic storms that reach them after blowing over the Bangladesh plain. Some of the storms, especially the ones experienced in spring are due to local thermodynamic conditions.

Seasons : 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 :

Winter Season : With the southward migration of the sun after September 23, temperature to fall over North-East India and winter sets in towards the later part of November. It continues upto the end of February. The weather during this period is influenced by the high pressure system of central Asia, the sub tropical jet stream and the high pressure centre over upper Myanmar. The temperature falls down over the high hills and mountains often to 0 Celsius. In fact the high Himalayan areas of Arunachal experiences a temperature below 0o C. especially at night. The hills of Nagaland, Manipur and Mizoram experience a minimum temperature of 4o C. Meghalaya, similarly has a very low temperature, Shillong experiences a minimum of 1o C . Tripura , because of the low altitude of its hills and its southerly location , experiences a minimum of 12o C. The Brahmaputra and Barak Plains show an average temperature of 13 o C. Within the Brahmaputra valley itself, the eastern upper valley experiences a lower minimum of 7 o -8 o C while in the western part it is about 10 o- 12 o C.

December is the driest month of the year. During this period a local low pressure sets in the Brahmapurta valley. This along with the Mediterranean lows, can bring about cloudy and drizzly weather. Besides, the cold north east trade winds occasionally blow over eastern Arunachal, Nagaland, Manipur , Meghalaya and Mizoram hills, bringing down the temperature of these areas. An important feature of the weather of this period is the presence of thick fog are all over the region. The hills have thick fog in the evenings, while the plains have it early in the mornings. In the Brahmaputra Valley fogs are more common in the south bank than on the north bank. This is became the mountain winds cumin down the Himalayan slope drive the fog to the south bank. In the Barak Valley also fogs are more common in the southern part than in the northern part. The total number of foggy days in the two valleys varies between 60 to 100.

The rainfall is normally scanty 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, the rest of North East India receives 5 cm. during this period. The normally clear and sunny sky, cool and gentle breeze and morning fog together make the winter weather pleasant.

Pre- Monsoon Season :

With the end of February, temperature begins to rise in the region. March, April and May become sufficiently hot and the rains are yet to come in their full form. Thus the pre-monsoon is a transitional season between the dry, cool winter and warm, rainy monsoon seasons. The important characteristics of this season are the rapidly increasing temperature, disappearance of fog and not infrequent occurrence of thunder -showers and hail storms.

With the northward migration of the sun, the temperature increases rapidly in the region especially, over the Plains. Thus the pressure decreases rapidly and the gradient of pressure between the hills and plains becomes steep. This, not only facilitates incoming of the Mediterranean lows from the west and tropical cyclones from the south, but also gives rise to local cyclonic conditions. All these do bring storms and rains. The average temperature in this season rises to 250 C over the plains and 200 C over the hills. The amount of rainfall varies from 15.6 cm in western Assam to 23.13 cm in eastern Assam. Over the hills the rainfall is less, but in the Barak Valley it becomes as high as 32. 5 cm.

It has been found that Guwahati and its surrounding area experience on the average 8 stormy days and 13, 6 cm of rainfall in the month of April. Of late the amount of rainfall in April has been found to be increasing . For example, the Borjhar meteorological centre recorded a total of 16.3 cm in April, 1988, 18.5 cm in April, 1989 and 34.8 cm in April, 1990.

The diurnal range of temperature during this period is very high While the late night are pretty cool, the afternoons are very hot. However with the passage of time, the temperature range decreases, both days and nights becoming hot and the frequency of rainy days increases. The rains normally occur in the afternoons. Sometimes hail storms come along with such rains. Since these storms appear to come from the west in the eastern part of India, they are called "Nor’ wester". Locally they are known as ‘Bardoichila ; in Assam and ‘Kal Baishakhi" in West Bengal. These storms are said to be the result of the joint action of the Mediterranean lows progressing from the west and tropical cyclones from the south. The eastward progressing Mediterranean lows in eastern India attract the moisture laden cyclonic current from the Bay of Bengal. When these two currents meet, full -blown cyclones occur carrying hail storms and thunder showers. Sometimes these storms move at a velocity of 50-65 km. It is these cyclonic storms that cause immense destruction in coastal Bangladesh, West Bengal and Orissa. In north -east India they occur with a slightly lesser intensity. However, quite a few cyclonic storms pass over the plains of north-east India during this period.

It has been noticed that with the advance of the season the number of rainy also increases. For example, the average number of rainy days increases from six in March to 12 in April and further to 14 in May. At the beginning of this season i. e. in March, there occur dust-storms in the western part of the Brahmaputra Valley. After dry winter when the low-level sweeping winds blow over hot ground, dust is blown every afternoon bringing down visibility and making the atmosphere uncomfortable

Monsoon Season :

The season of monsoon prevails over north East India during the months of June, July, August and September. With the northward migration of the sun the landmass gets heated and a low pressure system is established firmly over India.

The temperature over north east India is quite high and reaches a maximum of 350 C over the plains and 250 C over the hills.

The low pressure system, that builds up in the north India plain, prevail over North East India also. If the polar or sub-tropical jet streams withdraw from over India to central Asia the moisture laden south west monsoon winds from the southern seas rush in. This is because the low pressure over north India attracts the winds from the relatively high pressure belt over the southern seas. The south -west monsoon winds from both the Arabian Sea and the Bay of Bengal strike Meghalaya first and then movers 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 hills slopes and rise up. In this process the rising air masses lose heat at wet adiabatic rate. The moisture contained is condensed 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 south -west monsoon winds, laden with moisture, reach north east India towards the middle of June or the beginning of the Indian month of Ashara. However , the whole of the region is prevailed upon by the monsoon winds by about June 20 and rains begin.

The monsoon winds enter North East through two routes

One from the south from Bay of Bengal and the other from the south-west from the Arabian Sea. The Bay of Bengal current being obstructed by Arakan Yoma moves northward and causes rainfall in Tripura , Mizoram and Barak plain . And then it strikes against the southern face of the Meghalaya Plateau and then rises causing orographic rainfall at the southern part of the Plateau. That is why this part (Cherrapunji Mowsynram region) receives one of the highest average annual rainfalls in the world. The actual reasons for which this region receives such a high rainfall are as follows : A part of the moisture bearing air current from the Bay of Bengal and a part of another similar current from the Arabian sea meet over Sylhet Plain south of Meghalaya and are faced with the with the steep slope of the Meghalaya Plateau. There are many deep steep gorges on this slope. The moisture laden air masses enter into these narrow gorges and get compressed. But as soon as they rise up the valley slopes they expand and get cooled and condensed leading to the formation of clouds and occurrence of rainfall. Mowsynram (250 19’N and 910 36 E receives an average annual rainfall of 1240 cm and Cherrapunji (250 17’N and 91044’E) 1080 cm . After having ascended over the Meghalaya, the air masses move further north and east The east flowing air masses cause rainfall in Barak Valley, Manipur and Nagaland. The north flowing masses descend along the northern slope of Maghalaya, spread over the Brahmaputra Valley and strike against the foothills of Arunachal. The descendence on the northern slopes creates a rain shadow zone in the leeward side of Meghalaya over the Lanks -Lumding region, where average annual rainfall is only 106 cm. This is the driest part in north east India. In this connection it may be noted that even Shillong which lies 45 km north of Mawsynram receives only 215 cm of average annual rainfall.

The second current of air mass from the Arabian Sea enters North East India over the relatively low Garo Hills and North Bengal Plain and moves up the Brahmaputra Valley. It also strikes against the Arunachal foothills and causes heavy rainfall. A local low pressure system developed over the upper Brahmaputra Valley also attracts it resulting in the occurrence of a higher amount of rainfall there and in its surrounding foothills. This part, therefore, has an average annual rainfall of above 300 cm.

The season of south east monsoon is thus, characterized by heavy rainfall, high relative humidity and light surface wind. Although the temperature varies between 20 0 C in the plains and between 180 C and 30 0 C over the hills the weather becomes uncomfortable because of high humidity . As the relative humidity goes on increasing with the increasing rainfall, the month of August becomes most sultry and uncomfortable. The number of rainy days is about 18 in each of June, July and August and about 14 in September. About 80% of the total average annual rainfall in the region comes during this season. The amount of rainfall received in this season is about 180 cm in Assam, 185 cm in the lower parts of Arunachal, 180 cm in Meghalaya, 170cm in each of Nagaland and Manipur and 130 cm in each of Mizoram and Tripura. The occurrence of short rainless, sunny and hot periods in between the spells of rainfall is also a common feature of the season.

The Season of Retreating Monsoon :

Towards the later part of September the sun crosses the equator. The temperature begins to decrease, the low pressure system developed over northern India loosens its grip and the south west monsoon winds ceases to be attracted. The sky clears out with only the isolated patches of cirrus and cumulus cloud wandering aimlessly high up. As the relative humidity remains very high , even a slight fall of temperature is enough to make haze and mist appear in the distant horizons, especially in the mornings and evenings. The temperature comes down to 200C -250 C in the plains and to 150 C-200C over the hills. The diurnal range of it varies between 2.40 C and 5.60 C , the more being over the hills . The rainfall decreases and stands at about 12cm - 15 cm in the region, except in the southern margin of Meghalaya, where it is more. A few of the tropical cyclones that develop in the Andaman Sea, Gulf of Thailand and South China Sea sometimes affect Bangladesh and North -East India during this season. However, the number of rainy days decreases to 6- 7 in October and 2-3 in November. The wind direction is from the north and north-east. This is a short season covering only the months of October and November. However, it is bracing.

Climatic Sub-Regions :

The description above shows that the climate of North East India is to some extent different from that of other part of India. If the detailed climatic conditions of the different parts of North East India are observed carefully, one find some variations, and based on these, the region can be divided into the following climatic sub- regions:

1. Region of Cold, Humid High Altitude Climate : This type of climate prevails over the northern part of Arunachal and the eastern hills in the areas above about 2000 m. The temperature is generally low over here the lowest, touching below 00 C, in winter and highest not exceeding 20 0 C even in the hottest month. Precipitation is high, coming sometimes in the from of snowfall. Average annual precipitation is above 150 cm. Since this includes mountainous areas, agricultural facilities are grown on the hill slope either by shifting method and / or by terrace method. Citrus like apple, pear, lemon, etc. are also grown here

2. Region of Humid Sub-Tropical Monsoon Climate : This type of climate prevails over Arunachal foothills bordering the Brahmaputra Valley, Nagaland, eastern part of North Cachar Hills, Manipur (including the Valley) and Mizoram. The temperature in this region is higher than in the previous region, but lower than in the plains, average annual being 100 C -200 C the summer is sufficiently hot (180C- 250 C ) and humid , while the winter is longer and drier than in the plains. Hill rice, wet rice (over the limited valley bottoms and irrigated terraces), maize, millets, vegetables, pine apples, etc. are grown in this region. Shifting cultivation, terrace cultivation and common sedentary cultivation (over the valley bottoms ) are the traditional methods of agriculture found here. The region is covered with dense vegetation and support many wild animals, including elephants.

3. Region of Hot, Humid Monsoon Climate : This type of climate prevails over all the plain areas and the northern foothills of Meghalaya. The temperature over here is generally high summer maximum reaching 360 C and winter minimum hardly going down to 100 C. Rainfall is high, the average annual amount being above 150 cm and reaching 300 cm in many places. The relative humidity is high (above 60%) for about 9 months of the year. Storms and thunder showers are common in the pre-monsoon period. However, This region is agriculturally very important because it consists of plains. Rice, wheat, jute tea, pulses. mustard and vegetables, apart from common tropical fruits, are grown here.

4. Region of Rainy, Cool Monsoon Climate :This type of climate prevails over the higher southern half of Meghalaya to the of south of a line joining Darugiri, Nongkhlaw, Umsning and Hamren of Karbi Anglong. This area is characterised by low temperature (lower than 200 C even in summer and 00C in winter ) throughout the year and with rainfall occurring for about 8 months. The amount of average annual rainfall is over 250 cm. It is this area that supports the rainiest places like Mawsynram. Common crops grown are hill rice, rice (on valley bottoms) , orange, pear, apricot, pine apple, etc. The area has natural pine forests.


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