decreases rapidly at elevations above . Most precipitation occurs as snowfall during the late winter and spring months .The Himalayas experience their heaviest snowfall between December and February and at elevations above . Snowfall increases with elevation by up to several dozen millimetres per 100 metre (~2 in; 330 ft) increase. Elevations above never experience rain; all precipitation falls as snow. The India Meteorological Department (IMD) designates four climatological seasons: Winter, occurring from December to February. The year's coldest months are December and January, when temperatures average around in the northwest; temperatures rise as one proceeds towards the equator, peaking around in mainland India's southeast. Summer or pre-monsoon season, lasting from March to May. In western and southern regions, the hottest month is April and the beginning of May and for northern regions of India, May is the hottest Month. In May, Temperatures average around in most of the interior. Monsoon or rainy season, lasting from June to September. The season is dominated by the humid southwest summer monsoon, which slowly sweeps across the country beginning in late May or early June. Monsoon rains begin to recede from North India at the beginning of October. South India typically receives more rainfall. Post-monsoon or autumn season, lasting from October to November. In the northwest of India, October and November are usually cloudless. Tamil Nadu receives most of its annual precipitation in the northeast monsoon season. The Himalayan states, being more temperate, experience an additional season, spring, which coincides with the first weeks of summer in southern India. Traditionally, North Indians note six seasons or Ritu, each about two months long. These are the spring season (), summer (grīṣma), monsoon season (varṣā), autumn (śarada), winter (hemanta), and prevernal season (śiśira). These are based on the astronomical division of the twelve months into six parts. The ancient Hindu calendar also reflects these seasons in its arrangement of months. Winter Once the monsoons subside, average temperatures gradually fall across India. As the Sun's vertical rays move south of the equator, most of the country experiences moderately cool weather. December and January are the coldest months, with the lowest temperatures occurring in the Indian Himalayas. Temperatures are higher in the east and south. In northwestern India region, virtually cloudless conditions prevail in October and November, resulting in wide diurnal temperature swings; as in much of the Deccan Plateau, they register at 16–20 °C (29–36 °F).