Climate of Delhi

Summer begins in early April and continues till the middle of June, with the heat peaking in late May and early June. It is characterized by extreme heat, low humidity, very hot winds and thunderstorms. Delhi's proximity to the Thar Desert results in hot, dry continental winds, called loo, at times blowing all across from the West Asian mainland, making the days feel hotter. These winds, blowing over from vast land stretches, are very hot and dry. Since the Western Disturbance depression moves eastward (and is the reason for cyclonic occurrences in Eastern Coastal areas) by this time of the year, there is no moisture-laden wind to increase humidity. The air therefore remains dry or very dry during day. For most of its summer season, Delhi has a semi-arid climate.[1] Coming from Spring, the city witnesses a spurt in day temperature around early April, whereas nights still remain pleasant. By the latter part of April or during early May, maximum temperatures exceed 40 °C (104 °F) while the ambience remains very dry. Night temperatures cross the 20 °C (68 °F) mark towards the latter part of April. May is Delhi's hottest month during which temperatures may reach 45 °C (113 °F) or higher.[14] This month is characterized by frequent thunderstorms.[15] Dust storms are another feature of Delhi's summer,[16][17] and can be severe and destructive when accompanied by strong winds, particularly under cumulonimbus formation.[18] These are caused due to fine dust brought along by the hot winds arriving from the desert. They make the surroundings appear pale yellow, bring temperatures slightly down and are usually followed by thunderstorms. Post mid-June, temperatures start falling slowly, while humidity shows a gradual rise. A visual characteristic of summer in Delhi is the summer bloom, particularly the blooming Bougainvillea, Amaltas, Gulmohar, Shireesh and Jacaranda trees, which look spectacular when fully flowering during peak summer in May.[19]

Monsoon winds arrive in Delhi by either the end of June or the first week of July.[11] The arrival of moisture laden South-Western winds, traveling from the Arabian Sea marks the onset of the humid season in Delhi. This season is marked by high levels of humidity and high heat. Day temperatures drop below 40 °C (104 °F) as humidity suddenly soars.[20] July is marked by high heat and relatively less precipitation (as compared to August). This transition from scorching to sweltering heat between June and July makes the latter feel very uncomfortable. August is Delhi's wettest month. The heat is considerably reduced and it is relatively cooler for most part of the month. There is dense cloud formation in the sky and at least a week of distinct, heavy rainfall.[21] By September, the amount and frequency of precipitation drops, though humidity remains high.[22] Towards the end of September, moisture content in the air begins to fall and monsoon ends by early October. In India, the Rainy season is referred to as "Rituraani", meaning the Queen of Seasons.

The end of monsoon marks the arrival of a transition season. Autumn arrives by early or mid October, and is marked by very dry ambiance, warm days and pleasant nights. Maximum temperatures drop below 30 °C (86 °F) by late October and there is a gradual fall in average temperature. Minimum temperature drops below 20 °C (68 °F).[23] During Autumn, the wind direction begins changing from South-Westerly to North-Westerly. In recent decades, Delhi has seen a hazardous increase in air pollution levels and haze in the months of October and November caused by stubble burning by farmers in the Indian states lying north of Delhi for fresh sowing at this time of the year, which is exacerbated by the usually almost still air around this time that causes the pollutants to hang in Delhi's air for many days. Around late autumn, the variation between morning and afternoon temperatures in a day becomes considerable, and can often be more than 20 °C (68 °F), with minimum dropping to under 10 °C (50 °F) and maximum still hovering slightly under 30 °C (86 °F). This season ends by early December.

Winter arrives in Delhi by early December. Minimum temperatures gradually enter single digits by this time of the year, while days are cool. Though usually not cold initially, December suddenly becomes cold in the latter half, as chilly north-western winds from the Himalayas begin sweeping the Northern Plains. These cold waves are caused by a depression created by Western Disturbance, which bring cloud cover and winter rains to the Plains, and add to snowfall in the North-Western Indian Subcontinent. By early January, when winter peaks in Delhi, the minimum temperatures plunge to the vicinity of 0 °C (32 °F),[24][25][26] though very rarely entering the negative scale. Maximum temperatures, too may drop down into single digits[27][28] and always stay under 15 °C (59 °F). When the minimum temperature ventures very close to the 0 °C (32 °F) mark, Delhi witnesses frost.[29] Snow is a practical impossibility for Delhi (and the rest of Northern Plains) due to very dry nature of its winter- the coldest conditions happen under clear skies when icy winds rush in from the Himalayas, and a cloud cover (which is necessary for causing snowfall) rather warms the city by trapping heat, thereby junking any possibility of snow. Delhi's winter is marked by very dense fog and haze, which dramatically reduce visibility[30] and makes days colder by cutting off sunlight. In the opposite scenario, cold north-westerly winds from the upper reaches of Himalayas blowing across the city makes the days feel colder, despite any sunshine and the nights further cold.[31] Post mid-January, average temperatures begin to rise very gradually, though the rise is almost contained by the cold north-western winds which result due to very heavy snowfall that occurs in the Himalayas during this part of the month.[32] It may rain towards the end of January and the precipitation is usually accompanied by hail,[33] resulting in slight increase in minimum temperatures due to cloud cover. Maximum temperatures again cross 20 °C (68 °F) and days become pleasant. By mid-February or somewhat beyond, minimum temperatures cross the 10 °C (50 °F) mark and days start getting warmer gradually, marking the end of winter. Delhi can sometimes have a prolonged season of chill, extending into March.[34][35][36]

Around the middle of February, Delhi's climate sees another transition, this time from Winter to Summer. The transition weather is known as Spring and is characterized by warm days, cool nights, dry ambiance and lively natural surroundings.[37] The weather is pleasant and there is warm, brilliant sunshine during the day. For all its characteristics, in India it is referred to as "Rituraaj", meaning the King of Seasons.[38] Spring rains[39] are a characteristic of this season. These rains may be accompanied by hail and can be heavy. Average temperatures show a slow, gradual rise as the wind direction shift from North-West to South-West, thereby getting warmer. Around late spring/ early summer, the variation between morning and afternoon temperatures in a day becomes considerable, and can often be more than 20 °C (68 °F), with maximum rising to mid 30s °C (93–97 °F) and minimum in the mid 10s °C (57–61 °F). Spring ends by the latter half of March and the day temperatures exceed 30 °C (86 °F) by then,[40] marking the onset of the next summer.