Summer is followed by the southwest monsoon rains that provide most of India 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 north-wards 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.

Geology

India has a varied geology spanning the entire spectrum of the geological time period. India’s geological features are classified based on their era of formation.

The Pre-Cambrian period formations of Cudappah and Vindhyan systems are spread out over the eastern and southern states. A small part of this period is spread over western and central India.

The Paleozoic Era formations from the Cambrian, Ordovician, Silurian and Devonian system are found in the Western Himalaya region in Kashmir and Himachal Pradesh.

The Mesozoic Era Deccan Traps formation is seen over most of the northern Deccan. Geologists believe that the Deccan Traps were the result of sub-aerial volcanic activity. The Trap soil is black in colour and conducive to agriculture. The Carboniferous system, Permian System, Triassic and Jurassic systems are seen in the western Himalayas. The Jurassic system is also seen in Rajasthan.

Tertiary Period imprints are seen in parts of Manipur, Nagaland, parts of Arunachal Pradesh and along the Himalayan belt. The Cretaceous system is seen in central India in the Vindhyas and part of the Indo-Gangetic plains. The Gondowana system is also seen in the Narmada River area in the Vindhyas and Satpuras. The Eocene system is seen in the western Himalayas and Assam. Oligocene formations are seen in Kutch and in Assam.

The Pleistocene system is found over central India. It is rich in minerals such as lignite, iron ore, manganese and aluminium. The Andaman and Nicobar Island groups are thought to have been formed in this era by volcanoes.

The Himalayas are a result of the convergence and deformation of the Indo-Australian and Eurasian Plates. Their continued convergence raises the height of the Himalayas by 1 cm each year.

Natural disasters

India is prone to several natural disasters, responsible for huge losses in life and property. Natural disasters in India include droughts; flash floods, as well as widespread and destructive flooding from monsoonal rains; severe cyclones; and earthquakes.

Floods are the most common natural disaster in India. During the monsoon season, heavy rainfall may cause rivers to distend their banks, often flooding the surrounding areas. The Brahmaputra River is prone to perennial flooding during the monsoon season. Floods are responsible for a number of deaths and property loss in many parts of India. With the exception of a few states, almost all of India is prone to flooding.

Indian agriculture is heavily dependent on the monsoon as a source of water. In some parts of India, the failure of the monsoons results in water deficiency in the region causing extensive crop losses. Drought prone regions include south Maharashtra, north Karnataka, Andhra Pradesh, Orissa, Gujarat and Rajasthan. In the past, failure of monsoons has led to famines causing great damage.

Tectonic plates beneath the earth’s surface are responsible for yearly earthquakes along the Himalayan belt and in northeast India. This region is classified as a Zone V, indicating that it is a very high-risk area. Parts of western India, around the Kutch region in Gujarat and Koyna in Maharashtra, are classified as a Zone IV region (high risk). Other areas have a moderate to low risk chance of an earthquake occurring.

Cyclones are another natural disaster, affecting thousands living in the coastal regions. Cyclones are severe and bring with them heavy rains that cut off supplies and relief to the affected areas. On 2004-12-26, a tsunami caused by the 2004 Indian Ocean earthquake struck the Andaman and Nicobar Islands and India’s east coast resulting in the loss of over ten thousand individuals. Until then India was thought to have negligible activity related to tsunamis, though there is historical anecdotal evidence of its occurrence in the past.