The climate of India comprises a wide range of weather conditions across a vast geographic scale and varied topography, making generalisations difficult. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid desert in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The nation has four seasons: winter (January–February), summer (March–May), a monsoon (rainy) season (June–September), and a post-monsoon period (October–December).
India's unique geography and geology strongly influence its climate; this is particularly true of the Himalayas in the north and the Thar Desert in the northwest. The Himalayas act as a barrier to the frigid katabatic winds flowing down from Central Asia. Thus, North India is kept warm or only mildly cooled during winter; in summer, the same phenomenon makes India relatively hot. Although the Tropic of Cancer—the boundary between the tropics and subtropics—passes through the middle of India, the whole country is considered to be tropical.
Summer lasts between March and June in most parts of India. Temperatures exceed 40 °C (104 °F) during the day. The coastal regions exceed 30 °C (86 °F) coupled with high levels of humidity. In the Thar desert area temperatures can exceed 45 °C (113 °F).
Summer is followed by the southwest monsoon rains that provide most of India with its rainfall. The rain-bearing clouds are attracted to the low-pressure system created by the Thar Desert. The official date for the arrival of the monsoon is 1 June, when the monsoon crosses the Kerala coast. The southwest monsoon splits into two arms, the Bay of Bengal arm and the Arabian Sea arm. The Bay of Bengal arm moves northwards crossing northeast India in early June. It then progresses eastwards, crossing Delhi by June 29. The Arabian Sea arm moves north-wards and deposits much of its rain on the windward side of Western Ghats. By early July, most of India receives rain from the monsoons.
The monsoons start retreating by August from northern India and by October from Kerala. This short period after the retreat is known as the retreat of the monsoons and is characterised by still weather. By November, winter starts setting in the northern areas.
Winters start in November in northern India and late December in southern India. Winters in peninsula India see mild to warm days and cool nights. Further north the temperature is cooler. Temperatures in some parts of the Indian plains sometimes fall below freezing. Most of northern India is plagued by fog during this season.
The highest temperature recoded in India was 50.6 °C (123.08 °F) in Alwar in 1955. The lowest was −45 °C (−49 °F) in Kashmir. Recent claims of temperatures touching 55 °C (131 °F) in Orissa have been met with some scepticism by the Indian Meteorological Department, largely on the method of recording of such data.