The rhythm of seasons and the cold weather season

Detail

1. The rhythm of seasons: the IMD's four seasons

• The India Meteorological Department (IMD) divides the Indian year into four seasons.
• Monsoon climate: a climate in which the wind direction reverses with the season and most rain falls in one wet season.
• In India this reversal of winds produces a clear rhythm of seasons. The cold, dry months come from land-to-sea (offshore) winds. The wet months come from sea-to-land (onshore) winds.
• The Class 11 and Class 9 NCERT books give slightly different month ranges. Learn both.

IMD season	Class 11 months	Class 9 months	Main wind or weather system
Cold weather season	December–February	December–early April	Light offshore winds and western disturbances
Hot weather / pre-monsoon season	March–May	April–June (up to July in the north-west)	Rising heat and a low-pressure area forming over the north-west
Southwest monsoon season	June–September	June–September	Moist south-west winds from the Indian Ocean
Retreating / post-monsoon season	October–November	October–December	The monsoon withdraws; the northeast monsoon brings rain to Tamil Nadu

• Trap: Class 11 ends the post-monsoon season in November. Class 9 ends it in December.
• Himalayan states also have two extra seasons, spring and autumn, between the main seasons.

2. The traditional Indian seasons (six ritus)

• Ritu means season. The traditional calendar divides the year into six ritus of about two months each. Each ritu is named after two lunar months.

Ritu	Lunar months	Approx. Gregorian months	Weather
Vasanta (spring)	Chaitra–Vaisakha	March–April	Pleasant and warming
Grishma (summer)	Jyaistha–Asadha	May–June	Hot
Varsha (rainy)	Sravana–Bhadra	July–August	Monsoon rain
Sharada (autumn)	Asvina–Kartika	September–October	Clear skies after the rains
Hemanta (pre-winter)	Margashirsa–Pausa	November–December	Cool
Shishira (winter)	Magha–Phalguna	January–February	Cold

• They fit north and central India. They do not fit south India, where the yearly temperature range is small and there is no real winter.
• Culture link:
• Festivals: Vasanta Panchami welcomes spring. Sharad Purnima is the full-moon night of autumn.
• Ragas: Megh Malhar and Amritavarshini are linked with rain and the rainy season.

3. The cold weather season: temperature

• Onset: it begins by mid-November in north India.
• Coldest months: December and January.
• Mean daily temperature stays below 21°C over most of north India.
• Night temperatures can fall below freezing (0°C) in Punjab and Rajasthan. This causes frost (ice crystals forming on the ground and on plants when the air near the ground cools below 0°C).

Why the north-west is so cold: three causes

• 1. Continentality (being far from the sea, which keeps coastal temperatures mild)
• Punjab, Haryana and Rajasthan lie far inland, away from the sea.
• Land loses heat quickly on clear winter nights.

• Result: a very large gap between day and night temperatures, and cold nights.

• 2. Snowfall in the nearby Himalaya

• Snow covers the Himalayan slopes.
• Cold air from the snowfields flows down to the plains.

• Result: cold waves (spells of unusually low temperature) over the northern plains.

• 3. Cold winds from Central Asia (around February)

• Cold winds blow in from the Caspian Sea region and Turkmenistan.
• Result: cold waves, frost and fog over north-west India.

4. How IMD defines a cold wave

• Cold wave: a period of air temperature low enough to be dangerous, even deadly, to people who are exposed to it. Wind makes it worse. This is called the wind chill effect (moving air takes heat away from the body faster) [2].
• When IMD considers a cold wave: the minimum temperature falls to 10°C or less in the plains or 0°C or less in hilly regions [2].

Basis	Cold wave	Severe cold wave
Departure of minimum temperature from normal	−4.5°C to −6.4°C	−6.4°C or less
Actual minimum temperature (plains only)	4°C or less	2°C or less
Coastal stations	Departure −4.5°C or less and actual minimum 15°C or less	—

• Declaration rule: the criteria must be met at 2 or more stations in a meteorological sub-division for at least 2 days in a row. IMD declares the cold wave on the second day [2].
• Cold day: the minimum temperature is 10°C or less in the plains (0°C or less in the hills), and the maximum temperature is 4.5°C to 6.4°C below normal. When it is more than 6.4°C below normal, it is a severe cold day [2].
• IMD issues colour-coded cold wave warnings for the next 5 days. They appear in its daily bulletins and on a GIS-based map platform (a digital map system) at mausam.imd.gov.in [2].
• Health risk: hypothermia means the body's core temperature drops below about 34.4°C (94°F). Frostbite means body tissue freezes, often the hands, feet, ears and nose [2].
• Where cold waves strike: mainly north India and parts of central India in winter [2].

5. The Peninsula: no real cold season

• South India has no well-defined cold season. There are two reasons:
• the sea keeps temperatures even all year (the maritime influence)

• the region lies close to the equator, so the sun stays high in the sky all year

• Example: Thiruvananthapuram (Kerala, about 8.5°N). The mean maximum temperature is 29.5°C in June. NCERT prints the January figure as "21°C". This is very likely a misprint for about 31°C. The real point is that January is warmer than June there, because June is cloudy and rainy (NCERT: 21°C).

• Exception: the hills of the Western Ghats stay cool in winter because temperature falls with height.

6. Pressure and winds in winter

How the winter high-pressure area forms

• By 22 December the sun is overhead at the Tropic of Capricorn (23.5°S). This is the winter solstice of the Northern Hemisphere.
• North India gets slanting sun rays, and the land cools.
• Cool air is heavy and sinks, so a weak high-pressure area forms over the northern plains.

Isobars (lines joining places of equal air pressure)

• 1019 mb over north-west India
• 1013 mb in the far south
• Pressure is highest in the north-west and lower in the south.

Winds

• Air moves from high pressure to low pressure, so light offshore winds blow outward from the land at only 3–5 km/h.
• Offshore winds come from dry land, so they carry little moisture. Most of India is dry in winter.
• Direction changes with the landscape:

Region	Wind direction
Ganga valley	Westerly / north-westerly (blowing down the valley)
Ganga–Brahmaputra delta	Northerly
Bay of Bengal	North-easterly (this is the northeast monsoon; covered in the retreating-monsoon part of this topic)

• Exercise point (Class 11): when the sun is over the Tropic of Capricorn, high pressure lies over north-west India.

7. Western disturbances

• Western disturbance (WD): a shallow cyclonic depression. This is a weak low-pressure system with winds circling into it. It forms over the eastern Mediterranean Sea and moves eastward into India in winter.
• Jet stream: a narrow band of very fast winds about 9–13 km up in the atmosphere. The westerly jet stream pushes WDs from west to east. In science papers WDs are called extratropical cyclones (storms formed outside the tropics). They grow from instabilities in the subtropical westerly jet [3].

Path and moisture

• Step 1: the WD forms over the east Mediterranean.
• Step 2: it moves across West Asia, Iran, Afghanistan and Pakistan.
• Step 3: it picks up moisture from the Caspian Sea and the Persian Gulf.
• Step 4: it reaches north-west India. It brings rain on the plains and snow on the Himalaya.
• Research adds a link. When the Arctic Oscillation / North Atlantic Oscillation (see-saw pressure patterns over the Arctic and Atlantic) is in its positive phase, the westerly jet over the Middle East becomes stronger. More moisture then comes from the Mediterranean, Caspian and Arabian Sea, and north India gets more winter precipitation [3].
• WDs bring most of the winter precipitation to High Mountain Asia, the high mountains of Asia including the Himalaya [3].

Where the rain and snow fall

• Plains: light winter rain in Punjab, Haryana, Delhi and western Uttar Pradesh. Delhi gets about 53 mm in winter.
• Mountains: snow on the Himalaya.
• Pattern:
• On the plains, rain decreases from west to east.
• In the mountains, snow decreases from north to south.

Why it matters

• Rabi crops are winter crops sown in October–December and harvested in March–April, such as wheat, mustard and gram.
• The small winter rainfall is very useful for rabi wheat.

• It cuts the need for irrigation in Punjab, Haryana and western Uttar Pradesh.

• Himalayan snow melts in summer and feeds the rivers (Indus, Ganga and their tributaries) in the dry months before the monsoon.

• Cold waves follow WDs. When a WD passes, cold north-westerly air flows in behind it, and the clear skies let the ground lose heat quickly at night.

8. Other winter rainfall in India

Region	Winter rain	Cause
Punjab, Haryana, Delhi, western UP	Light rain (Delhi about 53 mm)	Western disturbances
Himalaya (J&K, Himachal, Uttarakhand)	Snowfall	Western disturbances
Central India and the northern Peninsula	Occasional showers	Western disturbances reaching further south
Arunachal Pradesh and Assam	25–50 mm	Western disturbances reaching north-east India
Tamil Nadu coast	Heavy rain in October–November	Northeast monsoon: north-easterly winds pick up moisture over the Bay of Bengal

Prelims Hooks

• IMD seasons: four. Traditional ritus: six. The traditional system fits north and central India, not the south.
• Ritu–month pairs: Vasanta = Chaitra–Vaisakha (March–April). Varsha = Sravana–Bhadra (July–August). Shishira = Magha–Phalguna (January–February). Hemanta = Margashirsa–Pausa (November–December).
• Trap: the post-monsoon season runs October–November in Class 11 but October–December in Class 9. The cold season ends in February in Class 11 but early April in Class 9.
• Winter pressure: 1019 mb over the north-west and 1013 mb in the far south. Winds are offshore and only 3–5 km/h.
• Winter wind directions: westerly/north-westerly in the Ganga valley, northerly in the Ganga–Brahmaputra delta, north-easterly over the Bay of Bengal.
• Western disturbances: origin = east Mediterranean. Route = West Asia → Iran → Afghanistan → Pakistan → north-west India. Steered by the westerly jet stream. Benefit = rabi wheat and Himalayan snow.
• Pattern of WD precipitation: it decreases west → east on the plains and north → south in the mountains.
• IMD cold wave (plains): minimum temperature 10°C or less and 4.5–6.4°C below normal. Severe: more than 6.4°C below normal, or actual minimum 2°C or less. Hills: 0°C or less [2].
• Coastal cold wave: minimum temperature 4.5°C or more below normal and actual minimum 15°C or less [2].
• Thiruvananthapuram: January is warmer than June, so the Peninsula has no well-defined cold season.

Mains Points

• Why winter is harsh in the north-west but mild in the south (GS-I):
• The north-west has continentality, Himalayan snow cover and cold air coming in from Central Asia.
• The south has the sea's even temperatures and lies near the equator.

• The result is a north–south contrast in temperature range, frost and fog.

• Western disturbances as a resource (GS-I/III):

• Their winter rain supports rabi wheat in Punjab, Haryana and western UP.
• Their Himalayan snow feeds summer river flow.

• 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 rainfall vary from year to year [3].

• Cold waves as a disaster (GS-III):

• They hit north and central India, with wind chill, frost damage to crops, and deaths from hypothermia among the homeless and outdoor workers.

• Policy responses: IMD's objective cold wave and cold day criteria, colour-coded 5-day warnings on a GIS platform, and advisories on clothing and first aid [2].

• The seasons and culture (GS-I): the six ritus are reflected in the agricultural calendar, festivals (Vasanta Panchami, Sharad Purnima) and music (Megh Malhar). They match the climate of north and central India, not the Peninsula.