Western disturbances

Also called: Western temperate cyclones · Topic: Climate of India and the Monsoon · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 10, Ch 4 "Agriculture"; Class 11, Ch 4 "Climate"

Meaning

Western disturbances (WDs) are shallow low-pressure systems, called cyclonic depressions. They form over the eastern Mediterranean Sea and are pushed east by the westerly jet stream into north-west India during the cold weather season. They bring light winter rain to the plains and snow to the Himalaya.

They are the main source of rain in north India's dry winter. That rain supports rabi wheat, and the Himalayan snow they bring feeds the rivers before the monsoon arrives.

Explanation

What a western disturbance is

• Shallow cyclonic depression: a weak low-pressure area, with winds circling in towards its centre. It is "shallow" because it is weak and not very deep.
• In science papers WDs are called extratropical cyclones (storms that form outside the tropics). They grow from instabilities in the subtropical westerly jet [3].
• Jet stream: a narrow band of very fast winds about 9–13 km up in the atmosphere.
• The westerly jet stream blows from west to east.

• It steers WDs from the Mediterranean towards India.

• Season: the cold weather season. Class 11 gives this as December–February. Class 9 gives it as December–early April. In north India it begins by mid-November.

How it forms and travels (cause → process → result)

• Step 1 (origin): the WD forms over the eastern Mediterranean Sea.
• Step 2 (route): the westerly jet pushes it east across West Asia, Iran, Afghanistan and Pakistan.
• Step 3 (moisture): on the way it picks up moisture from the Caspian Sea and the Persian Gulf.
• Step 4 (result): it reaches north-west India.
• On the plains it brings light rain.

• In the mountains it brings snow to the Himalaya.

• After it passes:

• Cold north-westerly air flows in behind it.
• Clear skies let the ground lose heat quickly at night.
• The result is cold waves (spells of unusually low temperature), frost and fog.

Why WDs stand out in a dry winter

• The winter background:
• By 22 December the sun is overhead at the Tropic of Capricorn. North India gets slanting rays, and the land cools.
• A weak high-pressure area forms over north-west India (1019 mb, against 1013 mb in the far south).

• Light offshore winds (winds blowing from land to sea) move outward at only 3–5 km/h. They come from dry land and carry little moisture.

• Result: most of India is dry in winter. WDs are the main systems that bring rain and snow to the north in this season.

What controls how strong they are

• Arctic Oscillation / North Atlantic Oscillation (AO/NAO): see-saw changes in air pressure over the Arctic and the Atlantic.
• When the AO/NAO is in its positive phase:
• the westerly jet over the Middle East becomes stronger
• more moisture comes from the Mediterranean, Caspian and Arabian Sea

• north India gets more winter precipitation [3]

• So winter rain and snow from WDs change from year to year [3].

Where it occurs

• Origin: the eastern Mediterranean Sea.
• Route: West Asia → Iran → Afghanistan → Pakistan → north-west India.
• Moisture sources: the Caspian Sea and the Persian Gulf.
• Plains (rain): Punjab, Haryana, Delhi and western Uttar Pradesh. Delhi gets about 53 mm of rain in winter.
• Mountains (snow): the Himalaya in Jammu and Kashmir, Himachal Pradesh and Uttarakhand.
• Further south: occasional showers in central India and the northern Peninsula.
• North-east India: Arunachal Pradesh and Assam get 25–50 mm in winter from WDs that reach this far east.
• Wider region: WDs bring most of the winter precipitation to High Mountain Asia, the high mountains of Asia that include the Himalaya [3].
• Spatial pattern:
• On the plains, rain decreases from west to east.
• In the mountains, snow decreases from north to south.

Don't confuse with

• Northeast monsoon: north-easterly winds pick up moisture over the Bay of Bengal and bring heavy rain to the Tamil Nadu coast in October–November. WDs bring light winter rain to the north-west and come from the Mediterranean.
• Tropical cyclones: strong storms that form over warm tropical seas. WDs are extratropical (formed outside the tropics), shallow and weak, and they travel overland from the west.
• Cold winds from Central Asia: these blow in from the Caspian Sea region and Turkmenistan, mainly around February. They bring cold, frost and fog but no rain. WDs are low-pressure systems that bring rain and snow.
• Cold wave: a weather condition (dangerously low temperature), not a weather system. A cold wave often follows a WD when cold north-westerly air flows in behind it.

Prelims Hooks

• Origin–route match: east Mediterranean → West Asia → Iran → Afghanistan → Pakistan → north-west India. They are steered by the westerly jet stream, not by the monsoon winds.
• Moisture sources: the Caspian Sea and the Persian Gulf. Trap: it is not the Bay of Bengal.
• Pattern: WD rain decreases west → east on the plains, and WD snow decreases north → south in the mountains.
• Crop link: WD rain is most useful for rabi wheat in Punjab, Haryana and western UP. It does not help kharif crops.
• Figures: Delhi gets about 53 mm of winter rain. Arunachal Pradesh and Assam get 25–50 mm of winter rain from WDs.
• Scientific name: WDs are extratropical cyclones born from instabilities in the subtropical westerly jet [3].

Mains Points

• WDs as a resource (GS-I/III):
• Winter rain supports rabi wheat and reduces the need for irrigation in Punjab, Haryana and western UP.
• Himalayan snow melts in summer and feeds the Indus, the Ganga and their tributaries in the dry months before the monsoon.

• A weak WD year brings moisture stress for rainfed wheat and less snow. Changes in the westerly jet linked to the AO/NAO make winter rain vary from year to year [3].

• WDs and winter hazards (GS-III):

• After a WD passes, cold north-westerly air and clear night skies bring cold waves, frost and fog to north and central India.
• This causes frost damage to crops, and deaths from hypothermia among the homeless and outdoor workers.

• IMD issues colour-coded cold wave warnings for the next 5 days on a GIS-based map platform [2].

• North–south winter contrast (GS-I):

• The north-west gets WD rain, snow and cold waves, and is also shaped by continentality (being far from the sea).
• The Peninsula has no well-defined cold season, because of the sea's influence and its nearness to the equator.

Related concepts

• Seasons of India
• Traditional Indian seasons
• Cold weather season

Read more

• The rhythm of seasons and the cold weather season