Climate of Rajasthan

Climate is generally defined as the average state of weather. The average condition of climatic elements at a particular place at least for a movement is known as weather while the climate means the average conditions determined by observation made over long period (ten years). The study of the climate includes a number of elements like temperature, rainfall, humidity and the wind velocity.

Rajasthan has a hot, dry, and desert climate. For most part of the year, the temperature is high and the air has low humidity (10per cent -40per cent). The average

Yearly rainfall of the state is 52 cm, most of which falls in the rainy months from July to September. High diurnal and yearly range of temperature of some recording stations like Barmer (48oC), Churu (48oC), Sri Ganga Nagar (49oC) and Sikar (47oC) are much above normal due to high speed dust-laden winds which blow from May through June. In winters, the temperatures may fall to below freezing point at some places like Churu, Bikaner, Pilani, Phalodi and Jaisalmer.

The western Rajasthan lying west and north of the Aravalli ranges is laden with a thick mantle of sound which absorbs solar energy very fast during the day and releases that desert region may be over 45oC but may fall below 30oC at the night.

The climate of Rajasthan is hot and arid. It is due to its location in the north western part of Indian sub-continent where maritime effect do not take place on account of its long distance from the sea. Moreover, Rajasthan is situated, just north of Tropic of Cancer, where the solar rays are perpendicular in the summer season.

Seasons

1. The seasonal rhythm of climate in Rajasthan may be divided into 4 seasons.
2. The cold season (November through mid-March)
3. The summer season (Mid-March through June)
4. The rainy season (July through September)
5. The season of Retreating Monsoon (October through November)

The Cold Season

From early November through Mid-march, the temperature starts falling. The lowest temperature is recorded in January. During winters, the cold, dry trade winds blow over the entire north India. Rajasthan is also affected by them. The day temperature in western Rajasthan is below 25oC while in the eastern parts the temperatures may fall even below 22oC. The nights are very cool with temperatures sharply declining to below 5oC. Thus, Churu (-1.3oC), Pilani (-1oC), Phalodi (-1oC) and Jaisalmer (-1.1oC) are some of the lowest temperature recording stations.

In December and January, anti-cyclonic conditions of high pressure belts get established over the Himalayas. Form these high pressure cells high speed cold winds radiate out in the entire plains of northern India. These cold winds are called "Sheet Lahar" which brings down the night temperatures well below 3oC than the normal. Due to low temperature the relative humidity is often high, causing fog and mist in the entire eastern Rajasthan. The descending fog when mixes with smoke of the cities causes a thick invisible layer of black fog which is called "smog."

The southern Rajasthan (mostly Banswara, Dungarpura, Chittorgarh and Udaipur districts), however, are not much affected by cold winds, as they are away from the Himalayan High Pressure Cell. So, in Southern Rajasthan, the night temperatures are normally above 8 degree C.

Most parts of Rajasthan usually get winter rainfall also which is called Mawath or 'maotha'. This winter rain, which usually falls in December, January and early February, is due to occasional entrance of "Mediterranean Lows" Which enter India via Iraq and Iran. Mawath rains are rare in southern Rajasthan but most of the northern and eastern Rajasthan gets 5 to 10 cm of winter rainfall, which is very useful for the 'Rabi' crops.

The Summer Season

The temperatures start rising from mid-march which is the onset of summer season. The sun marching northwards towards tropic of cancer increases the temperatures due to perpendicular rays of sun which fall upon the entire subcontinent. The summer season continues through April, May and June showing very high temperatures in the western and northern parts of Rajasthan. May is the hottest month when temperatures are very high as shown below:

Throughout the summer season, the west winds blow over the entire region. These winds are very warm and create hot winds called Loo which increase the local temperature by more than 3oC above normal. As the west winds come from the sandy deserts of Thar, Baluchistan and Iran, they bring dust raising winds which for a brief period lower down the temperatures.

Due to high temperatures over entire Rajasthan, the atmospheric pressure is very low. The region becomes a "Low Pressure Area" which acts as a catalyser for the south –west monsoon to move northward.

By mid-June, the south-west monsoon reaches the coastal plains of peninsular India, which also affects the temperature conditions of Rajasthan and so the temperatures start decreasing. The maximum temperatures fall down to below 45oC. Thus, the highest temperatures in Jhalawar (36oC), Bharatpur (40oC), Banswara (40oC), Dholpur (41oC), Jawai Dam (35oC) are well below the blistering heat of may .Occasional high atmospheric circulation from Arabian sea may bring thundershowers and hailstone rains, punctuated by sudden rise in temperature and sultry heat.

The summer season is marked by dust-storms which are more frequent in the west. Thus, Jaisalmer and Barmer may have dust storms for 31 days, Sri Ganga Nagar for 27 days, Bikaner for 18 days, Jodhpur for 8 days and Jaipur for 3 days. The intensity of these dust-storms and the speeds of the winds depend largely upon the intensity of low pressure created in specific areas.

The Rainy Season

This is the period of South-west Monsoon which starts from early July (sometimes from the end June) and lasts up to September. Both the branches of the summer monsoon bring rain to Rajasthan. The Arabian Sea branch, which comes first, brings most of the rain in southern Rajasthan and so by the end of June or early July, the southern districts like Banswara, Dungarpur, Udaipur, Chittorgarh, Jhalawar, Kota and Baran get the first showers. The Arabian Sea branch blows parallel to the southern slopes of the Aravallies and does not cross to the west and so quite often, the northern slopes do not get any rain. The Arabian and so the southern districts to Jhalawar (915mm), Banswara (904 mm), Baran (82 mm) and Sawai Madhopur (829 mm) get the highest rainfall.

Thus, we find that out of the 32 districts of Rajasthan, only 18 districts get rains which is equal to or more than that of Rajasthan average (533.0 mm). It is only in the southern and the eastern districts that rainfall is ore while in the western and northern districts the rainfall is far less.

The 300 mm isohyet divides the state into two parts. The northern and western parts consisting of the entire district of Sri Ganga Nagar, Hanumangarh, Churu, Bikaner, Jaisalmer, Barmer, Jalore and Sirohi and part of Jhunjhunu, Sikar and Nagore districts get less than 300 mm of rainfall per year and hence these districts have arid to semi arid climate. The eastern and southern part of the state get more than 300 mm of yearly rains and have therefore steppes climate in which small shrubs, stunted trees and seasonal grasses grow. It is only on the Aravalli Hills and the southern part of the state that tropical deciduous trees are found in the form of open forests.

Besides the district-wise average normal rainfall, heavy rainfall has been recorded at a few places. This may be on account of topographical variations which have high rains and so the orographic rains occur.

S No.	Station	District	Normal Rainfall (in mm)
1.	Mont Abu	Sirohi	1646
2.	Manohar Thana	Jhalawar	1010
3.	Dunpur	Banswara	956
4.	Chhabra	Baran	932
5.	Chhipa Barod	Baran	928
6.	Jhalawar	Jhalawar	915
7.	Bikani	Jhalawar	908
8.	Atru	Baran	902
9.	Khanpur	Jhalawar	899
10.	Arnawar	Jhalawar	899

Characteristics of Rainfall in Rajasthan

1. Rainfall is erratic, and sometimes entire fall of the whole year may fall in a day and cause floods. Like on19th July 1981, Jaipur had 190 mm of rain in day, which caused deluge of large areas, breach of roads, bridges and railroads and collapse of several houses. Such high rainfalls have been noticed in Nagore (year 1974), Jodhpur (1982) and Kota (1986) also. For the rest of the year it may be dry.
2. Rainfall is not timely. The first shower of Monsoon may fall in the end of June or early July, followed by a long period of drought and then again some heavy fall at the time of harvesting. This phenomenon often destroys the crops as is being seen in this year.
3. In western Rajasthan, clear cloudless skies allow the vertical, scorching rays of the sun to fall upon the sandy ground without any vegetal barrier. So the air is almost dry with very little humidity which makes the rainfall impossibility.
4. The upper atmosphere (above 1 to 2 Kms height) has for the most time has dry currents of air, which absorbs moisture from the moisture bearing winds of the south-west monsoon. And so inspite of cloud cover in the lower sky, there is no rain.
5. The Aravalli Mountains stretching from south-west to north-east, allow having an average elevation of only 600 metres which is not a barrier for the monsoon to traverse west, yet they run parallel to the prevailing south-west winds of the Arabian Sea branch and do not permit the monsoon winds to bring rain in the western parts.
6. About 2/3rd of Rajasthan spreading mostly in the Thar Desert of the west and north the Aravllis have very scanty vegetation which does not create humidity by evapo-transpiration of plants. So the air is almost dry and does not attract rain.
7. The Bay of Bengal monsoon has to traverse a long distance of over a thousand Kms to reach Rajasthan and by the time these moist air currents reach Rajasthan, they are dry and bring little rain.
8. The number of rainy days is far less in the west and north. There is a gradual decrease the number of rainy days from east to west. While Jhalawar district get rains for 42 days in a year, Jaipur has only 26 days, Pali 23 days, Jodhpur 16 days, Jaisalmer only seven days. This reduce the average moisture content of the air and for most part of the year, the air is dry.

The Season of Retreating Monsoon

The south-west monsoon starts retreating for early. Firstly, it withdraws from the western Rajasthan, and then gradually from the central, eastern and southern part of the state. By the end of September, the monsoon winds are withdrawn from the entire Rajasthan. So in the month of September and October, the temperature start rising again and the air is dry. Although the maximum temperatures in there two months range from 35degree to 38 degree Celsius, yet they cause intense sultry heat which help in ripening of the Kharif crops. This is called October Heat.