Why India has a monsoon

Free lesson · Meteorology · DGCA CPL and ATPL

One country, four seasons. Why the winds over India turn round every year, and why that brings the monsoon.

Why does India have such different weather from season to season?

One country, four seasons

Five cards. Tap Next.

  1. The Himalayas guard the north The Himalayas stop the bitter winds that pour south out of Central Asia in winter. That is why most of India stays warmer than other places at the same latitude. The same wall later traps the monsoon and turns it west along the plains.
  2. The Thar desert heats up In summer the dry Thar desert and north-west India heat faster than anywhere else. Hot air rises, pressure falls, and a deep heat low forms. That low is the engine that later sucks the monsoon in from the sea.
  3. Four seasons IMD uses four seasons: winter (December to February in the north-west, January and February elsewhere), pre-monsoon or summer (March to May), the south-west monsoon (June to September), and post-monsoon (October and November). Monsoon is the worst time to fly in India: day after day of rain and low cloud.
  4. Monsoon means 'season' 'Monsoon' comes from the Arabic 'mausim', a season. It is the seasonal reversal of the winds and the rain that comes with it. About 75% of India's yearly rain falls in just four months, June to September.
  5. From 13 cm to 1187 cm West Rajasthan gets less than 13 cm of rain in a year, on about 20 rainy days. Mawsynram in Meghalaya gets about 1187 cm, and nearby Cherrapunji about 1178 cm: the wettest places on Earth, with rain on about 180 days a year in the north-east.

Memory hook: Winter: WD, fog, STJ. Summer: heat, andhi, Kal Baisakhi. Monsoon: AMT, break, TEJ. Post-monsoon: cyclones, NE monsoon.

Takeaway: India's weather is run by two giants, the Himalayas and the Thar desert, and by the sun's march between the tropics. Four seasons follow: winter, pre-monsoon, SW monsoon and post-monsoon.

Why does India get a monsoon at all?

Why the winds reverse

The teacher's Earth-and-sun drawing, then two real maps. Tap Next.

  1. Winter: the sun is far south On 22 December the sun stands overhead the Tropic of Capricorn, 23.5°S. India gets its weakest sunshine of the year.
  2. 21 March: over the equator The sun crosses the equator on 21 March (the spring equinox) and keeps marching north.
  3. 21 June: over the Tropic of Cancer On 21 June it is overhead 23.5°N, a line that crosses India. It never goes further north. Then it heads south again: over the equator on 23 September, back to 23.5°S by 22 December. Two dates the teacher says to remember: 21 March (equator) and 21 June (Tropic of Cancer).
  4. The hottest belt moves too The region heated most, the thermal equator, follows the sun north and south. Pressure, wind, cloud and rain all swing with it: that swing makes the seasons. Land heats and cools much faster than sea, so the swing is strongest where a big continent faces a warm ocean: Africa, South Asia and northern Australia.
  5. January: high over cold land In winter the land is colder than the sea, so pressure is high over north India. Air flows off the land: north-westerly over the north, turned by the Coriolis force to north-easterly over the peninsula. Dry and cold.
  6. July: low over hot land In summer the land is far hotter than the sea. Pressure falls over north-west India, and moist winds pour in off the Arabian Sea and the Bay of Bengal: the south-west monsoon.
  7. That reversal is the monsoon "Monsoon" comes from the Arabic "mausim", a season. It means the seasonal reversal of the winds, and the rain that comes with it. It is the one feature that controls India's climate. About 75% of India's annual rain falls in June to September.

Memory hook: Sun north → land hot → low → sea winds in. Sun south → land cold → high → land winds out.

Takeaway: The overhead sun swings between 23.5°S (22 Dec) and 23.5°N (21 Jun), dragging the hottest belt with it. In January the land is colder than the sea: high pressure, winds off the land. In July it is hotter: low pressure, moist winds off the sea. That seasonal reversal of wind and rain is the monsoon.