The North Indian region experiences a well defined hot weather season between cool and mainly dry winter and the wet monsoon. With the northward march of the sun towards the Tropic of Cancer after vernal equinox the temperature begins to rise and the circumpolar whirl weakens.
“Thermal heating over north-western Pakistan-India gradually establishes a thermal ‘low’ at the surface but while the jet stream remains south of the Himalaya, it maintains its dynamic anticyclone aloft over Afghanistan and the plateau borderland of Pakistan. This ‘lid’ of subsiding warming dry air prevents the surface thermal ‘low’ from having sufficient effect as a lifting agent to carry air aloft and so to bring about precipitation” (Johnson, 1969, p. 17).
Temperature-this is a period of continuous and rapid rise of temperature in India. By April the Peninsular regions south of the Satpura Range heat up with mean maximum temperature of 40°C. In May the mean maximum temperature reaches 42°C in Rajasthan, west U.P., Delhi, southern Punjab and Haryana. Temperature exceeding 54°C is recorded at Sri Ganganagar (Rajasthan). At some places, particularly in north-western India, day temperatures may be as high as 45°C or 47°C. The mean daily minimum temperature during May -also remains quite high and rarely goes below 26°C. The southern parts of India do not experience any hot weather season as such. The temperature is also not very high in eastern parts of the country and in the hilly regions the weather is cool and invigorating.
During April 30°C isotherm of averge temperature encloses a vast area of the country between 10°N and 26° N latitudes (except the west coast and the north-east). By May it covers a small area of northern and western Rajasthan, Haryana, Punjab, Delhi, western U.P. and Coromandel Coast where the temperature is more than 30°C. The diurnal range of temperature ranges between 5°C and 6°C in coastal areas but reaches 20°C in interior parts of the country and in the north-west.