The Ganga basin forms an extensive bowl of warm air, especially during the day-time. The mean maximum daily temperature even in the coldest month (January) does not fall below 21°C, except in the higher hills, whereas the air temperature starts rapidly rising all over Ganga basin from March onwards, beginning a hot season that prevails from April to June. Usually, May is the hottest month in most part of the basin, except in lower Bengal. In the Gangetic plains, the daily mean maximum temperature in May shoots up as high as 40°C in westwards of Gaya and 42.3°C in Kota in the Central Indian upland region. The minimum temperature is usually higher than 8°C mainly because of the lower plains of the basin. Low temperatures in the basin are often associated with the intrusion of cold air from across the Gangetic plain in the months of December and January.
The air temperature starts falling with the onset of the monsoon from June onwards, making the weather more humid and equable. The diurnal range between the daily mean minimum and the daily mean maximum temperature reduces progressively as the monsoon advances. Eventually, the lowest diurnal range of temperature occurs at the peak of the monsoon, which is usually in August, though sometimes in July. Due to its proximity to the coast, Kolkata stands as an exception, with its highest diurnal range occurring during the coldest month (January). On account of high population density and a heavy concentration of industrial units in the Metropolitan Districts, the effect of temperature is very pronounced, with frequent episodes of smog in the winter evenings followed by mist in the colder morning hours. As per IMD grid data analyzed under this project, the average annual mean temperature of Ganga basin of 35 years (1969-2004) is 24.82°C. The average annual maximum temperature of Ganga basin of 35 years (1969-2004) is 31.22°C and annual maximum temperature was noted on 1987 is 32.05°C. The average annual minimum temperature of Ganga basin of 35 years (1969-2004) is 18.44°C and annual minimum temperature was noted on 1971 is 17.68°C. The temperature profile of the basin is given below:
Temperature profile in the basin (36 Years Average for the period 1969-2004)
(Source: 1. India-WRIS 2. NGBRA report on Environmental and Social Management Framework)