There is a gradual rise in temperature from mid-February onwards and it starts increasing rapidly by March end and continues through May to mid-June. Unless the rapidly increasing temperatures are checked by the incursion of humid easterlies an average temperature of about 40"C prevails in most of the plain region. The hot and dry westerlies locally termed as ‘loo’, and blowing from Punjab-Haryana tract further aggravate the condition in want of sufficient relative humidity. Places like Gonda, Meerut and Bareilly experience comparatively lower temperatures because of their nearness to the Himalaya. The ‘loo’, relative humidity and the occurrence of heat waves sometimes accentuate the temperature conditions. The peak temperature reaches in Agra & Gonda around 49°C. Pre-monsoon showers are sufficiently meager (1% - 5%) compared to the annual amount of rainfall.
In Uttar Pradesh as a whole the mean annual temperature ranges between 25°C to 28°C. The temperature does not show appreciable change south of the 25°C isotherm (passing through Aligarh, Bahraich and Gorakhpur) except of Jhansi.
By mid-June the relative humidity starts increasing and it reaches a maximum of about 70% in July-August. About 85.90% of the annual rainfall of the plain tract is received from summer monsoons during the four months (Chaumasa) of June to September. The amount of rainfall, however, decreases from west to east and north to south. The trend of the isohyets is observed from north-west to south-east with angular formations. During these four months with high humidity and high temperature the weather becomes sultry aid oppressive and more so in the eastern and southern sections of the state. Occasionally, when Bay monsoon currents weaken-the same are revitalized by the Arabian currents. The agricultural operations, now to a larger extent determined by canal waters, are still dependent on the onset, withdrawal, breaks and nature of showers.
By the month of October the summer rainfall actively comes to an end and a sudden fall in atmospheric temperature is recorded. During the transitional months of October and November the relative humidity is at the lowest ebb. Soon after, the winter sets in with a further fall in temperature. By the end of November the temperature comes down to 20°C and further falls to 15°C in December and January. The blowing of westerlies, onset of western depressions, accompanied drizzle form rainfall and resultant cold wave bring appreciable falls in winter temperatures. Some places record below freezing point temperatures. About 5 % of the annual precipitation of Uttar Pradesh is received during winters. Thus, the monotony of the winter season is broken by western disturbances. Though the amount of winter rainfall is quite meager but with a lesser rate of evapo-transpiration because of low temperature and dampness of the atmosphere even this amount is quite effective and beneficial to ‘rabi’ crops.