The average weather conditions of a sizeable area of the earth's surface over a period of time usually spread over a span of at least 30 years.

The regional variation in temperature is described below:
(i)While in the summer the mercury occasionally touches 55°C in the western Rajasthan, it drops down to as low as minus 45°C in winter around Leh.
(ii)Churu in Rajasthan may record a temperature of 50°C or more on a June day while the mercury hardly touches 19°C in Tawang, Arunachal Pradesh on the same day.
(iii)On a December night, temperature in Drass , Jammu and Kashmir may drop down to minus 45°C while Thiruvananthapuram or Chennai on the same night records 20°C or 22°C.
(iv)In Kerala and in the Andaman Islands, the difference between day and night temperatures may be hardly seven or eight degree Celsius.
(v)But in the Thar desert, if the day temperature is around 50°C, at night, it may drop down considerably upto 15°-20°C.

The Himalayan Mountains :

Land and Water :
(i)India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north.
(ii)As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent.
(iii)Difference in air pressure causes reversal in the direction of monsoon winds.