The climate of Rajasthan state has varied contrasts and the presence of Aravallis is the greatest influencing factor. The state can broadly be divided into Arid, Semi-Arid and Sub-Humid Regions, on the basis of rainfall intensities.
The Western Rajasthan i.e. in the arid region consist of the districts of Hanumangarh, Jaisalmer, Barmer, Ganganagar, Churu, Jhunjhunu, Sikar, Nagaur, Jodhpur, Pali and Jalore covering an area of nearly 1,43,842 sq.km. The region is characterized by low and highly variable rainfall years creating inhospitable living condition to both human and livestock population.
An area of 9,290 sq km in extreme western parts of the state has true desert conditions. With an improvement in rainfall pattern from the west towards the east Rajasthan semi-arid conditions are created in an area of about 66,830 sq km in the districts of Alwar, Jaipur, Bharatpur, Ajmer, Tonk, Sawai Madhopur, Bhilwara, Bundi, Kota, Chittorgarh, Udaipur, Sirohi, Dungarpur and parts of Jhalawar and Banswara. The distribution of climatic regions of Rajasthan on the basis of rainfall and temperature variations is divided into the following categories: –
(1) Arid Region: The Arid region includes Jaisalmer district, northern parts of Barmer, western of the Phalodi Tehsil of Jodhpur, western parts of Bikaner and southern parts of Ganganagar district. Climate of the region is very severe and arid. Rainfall less than 10 cm in extreme west parts of regions and rest areas record less than 20 cm rainfall. The average temperature during summer is recorded more than 34o C and during winters it ranges in between 12oC to 16oC
(2) Semi-arid Region: The average temperature during winter season ranges between 10o C and 17o C and the summer season temperature range 32o C to 36o C. As the region has erratic as well as torrential rainfall it brings floods too each time. Rainfall ranges 20 to 40 cm. This region comprises the western parts of Ganganagar, Hanumangarh, Jodhpur and Barmer districts. The winter season is very short and arid in the northern parts of this region.
(3) Sub-humid Region: In the semi arid humid region, rainfall is meager and the amount of rainfall is limited to a few monsoon months only. The rainfall is between 40 to 60 cm and the average temperature during summer season ranges from 28o to 34o C whereas it is recorded 12o C in northern parts and 18o C in the southern parts. Alwar, Jaipur, Dausa and Ajmer, eastern parts of Jhunjhunu, Sikar, Pali and Jalore districts, north-western parts of Tonk, Bhilwara and Sirohi districts are included in this category. This region has steppe type of vegetation.
(4) Humid Region: this region receives winter rainfall associated with cyclones along with monsoon season rainfall which varies from 60 to 80 cm. Deciduous trees dominate the region. Humid region is found at the districts of Bharatpur, Dholpur, Sawai Madhopur, Bundi, Kota, Barmer and Rajsamand and the north-eastern parts of Udaipur.
(5) Very Humid Region: Very Humid Region includes south-east Kota, Baran, Jhalawar, Banswara, south-west Udaipur and adjacent areas of Mt. Abu. Here, the summers are very hot and winters are cold and dry. Rainfall received is between 80 cm to 150 cm, which is mostly during the rainy season. Monsoon savanna type of vegetation is present in the region.
Koppen’s classification for the World Climatic regions is totally based on the vegetation, as the effects of temperature and rainfall are directly evident and visible it. Here, the three categories are associated with Tropical/Mega-thermal climates, Dry (arid and semiarid) climates and Mild Temperate/Meso-thermal climates respectively. The classification of Rajasthan according to Koppen is as follows
(1) Aw or Tropical Humid Region: The southern parts of Dungarpur district and Banswara come under the region. Here, arid tropical grasslands and Savanna like region are found along with deciduous trees of Monsoon type. Winter season is arid and cool whereas summers experience scorching heat. Rainfall also mainly occurs in summer season. The temperature is more than 18o C in the coldest month records.
(2) Bshw Climatic Region: This climatic region is semi-arid, where winters are dry and even in summers there is no sufficient amount of rainfall. Vegetation is of steppe type, characterized with thorny bushes and grasses. This region comprises the districts of Barmer, Jalore, Jodhpur, Nagaur, Churu, Sikar, Jhunjhunu and Hanumangarh.
(3) Bwhw Climatic Region: The region has arid-hot desert climate with very scanty rainfall. On the contrary the process of evaporation is very active. Thus, these areas are known as desert region, limited to western parts of Thar Desert. North-western Jodhpur, Jaisalmer, western Bikaner and western parts of Ganganagar district are included in this category.
(4) Cwg Climatic Region: The south-eastern areas of Aravalli are the part of the region. Rains are limited to few monsoon months only. Seasonal winds do not bring rains to this region during winters.