Climate of Assam Geography:

   The state of Assam lies in the regime of the monsoon climate of the sub-tropical belt. It is surrounded on three sides by hills and mountains and the Meghalaya Plateau on the southern border, i.e., across the way of the incoming south-west monsoon winds. Such a situation and location play a vital role in causing local weather phenomena, although the state is a part of South-East Asiatic Monsoon lands. The climate of the Brahmaputra and the Barak valleys is normally characterised by orographic lows. During the summer the plains become hot and the air over the hills and mountains remans relatively cool. Thus, local low-pressure systems develop over the valleys and create a humid sub-tropical type of climate.

   The Himalayan range standing on the north and east of the Brahmaputra valley protects the area from the extreme cold winds of the Tibetan region and obstructs the warm moist winds blowing from the south-west in summer. The summer monsoon that enters the state through the southern hills including the Meghalaya Plateau precipitates the Brahmaputra valley. The precipitation decreases on the lee-ward side of the hills, in the eastern part of the hilly areas of Karbi Anglong and N.C. Hills and increases towards the foothills of the Himalayas. The lower Brahmaputra valley receives rainfall derived from the clouds that come across the Meghalaya Plateau. Thus, there is a marked variability in the distribution of rainfall in the state.

    The following are some of the factors determining the climate of Assam:

(i) The physiography of Assam and its bordering regions.

(ii) Impact of the south-west and north-east Monsoon that blows over the state.

(iii) Influence of the Mediterranean Disturbance or Western Disturbance during the late winter

(iv) Local mountain and valley winds.

(v) Presence of numerous water bodies, huge forest cover and local pressure variations, etc.

   In general, the climate of Assam is characterised by relative coolness, high relative humidity and heavy rainfall in the summer season and drought in the winter season. The mean annual maximum temperature, i.e., July-August ranges from 30°C to 33°C, and the minimum temperature, i.e., December-January ranges from 8°C to 15°C in different districts of the state. 230 cm of Assam experiences an average annual rainfall.

 Under varying intensities of the weather elements and resultant weather conditions, Assam normally experiences four climatic seasons 

(i) pre-monsoon, 

(ii) monsoon, 

(ii) retreating monsoon and 

(iv) dry Winter

(i) Pre-Monsoon:

  The pre-monsoon begins in the early part of March and continues up to the end of May. It comes from the north-west and is associated with rainfall (mostly in afternoon hours), thunderstorms, lightning and hailstorms. The pre-monsoon is known as Bordoichila in Assam and Kalbaishakhi in West Bengal.

 (ii) Monsoon:

   The monsoon sets in by the last week of May or in early June and lasts up to September or the first half Of October. It is the rainy season, and the state receives maximum rain in this period during the months of June, July and August and the river water of both Brahmaputra and Barak starts to rise and causes an extensive flood.

(iii) Retreating Monsoon:

      The south-west monsoons withdraw back in between the last part of September and the first part of October and continue up to the middle of November. During this time the number of rainy days goes on decreasing.

(iv) Dry Winter:

        The winter season begins in the middle of November and continues up to the end of February. This Season is characterized by low temperature, morning and evening fogs and very little amount of rainfall (because of the western disturbance). The fog is more widespread in the southern part of Brahmaputra valley and Cachar, because of the sweeping down of the fogs from the north bank to south bank by the mountain winds coming from the Himalayas.

Natural Climate of Assam:

   Every year Assam is subjected to various types of natural hazards. Common among them are fiood, river-bank erosion, drought and earthquakes. The location of the state along with its geological structures, topography, climate, etc., are the prime factors for the occurrence of these calamities. However, some of the anthropogenic activities, such as deforestation, construction of dams and reservoirs, etc., have accelerated the intensity of the hazards.

  The natural calamities that are being faced by Assam are discussed below:

Flood:

   Floods in Assam have been a common hazard since early times, as the region is mostly a flat low-lying plain under tropical monsoon climate. The flood water deposits silt every year on the agricultural field making it more fertile. But the intensity and destruction have increased since the great earthquake of August 15, 1950. This disastrous earthquake disturbed the courses of many rivers like Dibang, Lohit, Brahmaputra, Subansiri, Burhi Diohing, Jia Bharali, etc. The flood occurs regularly in about 14 districts of Brahmaputra valley and all the districts of Barak Valley. Generally, the damages during floods are caused by inundation and associated bank erosion. As much as 3,15,000 hectares of land in Assam are found to be flood-prone. However, heavy monsoonal rain and devastating landslides coupled with casy erodibility of rocks, steep slopes and high seismicity constitute the major natural causes of Assam floods

   Another common hazard in Assam is bank erosion. The Brahmaputra and some of its major tributaries are found to cause immense destruction every year by bank erosion. The Brahmaputra channel is shifting southwards and some of its north-bank tributaries are shifting east and west-ward.

    The devastating flood that occurred in July 2004 in Assam has damaged 2000 bridges and 622 km of National Highways, and it also took 212 human lives and many animals. This flood also damaged 1223851.17 hectares of agricultural land, out of which 6888909 hectares have totally been damaged.

  In the same year, on 8th October, 2004, another sudden flash flood occurred in the south Goalpara plain due to the overflow of the headstreams of Jinari and Krishnai rivers that come down from Garo hills because of cloud burst. The same phenomena was repeated after 10 years, i.e., in 2014, in the same district. Human-induced causes are highly responsible for the occurrence of this flood. For example, vegetative cover destruction in the northem slopes of the Garo hills and construction of railway lines has intensified the disaster.

  The Flood Control Department of Assam has so far constructed 984 km of embankments on the banks of Brahmaputra, 2567 km of embankments on its tributaries, 600 km of the drainage canal, 56 sluice gates and 421 bank protection spurs.

  Following are some of the measures to be taken for controlling flood and mitigating the situation immediately:

(i) Construction of embankments in a planned manner.

(ii) Controlling the major rivers by constructing dams and reservoirs.

(iii) Periodic checking of bank erosion.

(iv) Stopping deforestation and plantation of more trees in the catchment areas.

(v) Construction of raised platform for both men and animals near the settlements of flood-prone areas for taking shelter during a heavy flood.

Earthquake:

    Earthquakes are the common natural calamities after a flood in Assam. As the region falls in the plate boundary region of the Himalayan Mountain formation, thus it is tectonically weak and unstable. The Himalayas are rising every year and this affects in Assam by causing frequent earthquakes. The mechanism of Plate Tectonics is working in this region, which is the main cause of frequent earthquakes in North-East India as a whole.

     The great earthquake of 12th June 1897 with an intensity of 8.7 Richter scale has its epicentre in Meghalaya Plateau, in which the south-west of Guwahati sank down and gave rise to the present Chand Dubi Lake, which was a forested area before the earthquake. The lake still bears the submerged stumps of large trees.

 List of some Devastating Earthquakes of Assam:

Date and Year

10th January, 1869

12th June, 1897

9th September, 1923

3rd July, 1930

14th August, 1932

21st January, 1941

23rd October, 1943

29th July, 1947

15th August, 1950

29th July, 1970

 __ August, 1988.