Humid subtropical climate with dry winter (Cwa)

Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Cwa is a warm temperate (Group C) climate with a dry winter (w) and a hot summer (a): the coldest month averages between −3°C and 18°C, and the warmest month is above 22°C [2]. It is found just poleward of the Tropic of Cancer, mainly in the north Indian plains and the interior plains of south China.

It matters because it is the climate of the Ganga plain, one of the most crowded farm regions on Earth. The monsoon controls its seasons, and its cool, dry winter is the one feature that separates it from the tropical savanna (Aw).

Explanation

Reading the code: C + w + a

Letter	Position	Meaning
C	1st	Coldest month between −3°C and 18°C
w	2nd	Dry season in winter
a	3rd	Hot summer: warmest month above 22°C [2]

• Why it is Group C:
• If the coldest month were above 18°C, the climate would be tropical (Group A).

• If the coldest month were below −3°C, it would be a cold, snowy-forest climate (Group D).

• Extra rule: most C subtypes also need at least four months above 10°C [2].

How the seasons work (cause → process → result)

• Summer: hot and rainy
• The land heats up strongly, and the monsoon brings moist winds in from the sea.
• This moist air gives heavy summer rain.

• Result: a hot, wet summer.

• Winter: cool and dry

• The continent cools down, and the winds now blow out from the land.
• Air coming from land carries little moisture, so there is little rain.
• Result: a cool, dry winter, with the coldest month below 18°C.

Why the east side of continents gets Cwa

• On east coasts in subtropical latitudes, warm ocean currents heat the air, and onshore summer winds (winds blowing from sea to land) carry it inland. Normally this gives rain all year → Cfa.
• Where a monsoon dominates (South and East Asia): the winter wind blows out from the cold continent and is dry. So rain falls only in summer → Cwa.
• On west coasts the pattern is reversed: there, the subtropical high (a belt of sinking, dry air at about 30° latitude) sits overhead in summer, so the dry season comes in summer → Cs (Mediterranean).

India's version: Cwg

• On the India climate map in NCERT's India: Physical Environment, the Ganga plain is shown as Cwg.
• "g" = Ganges type:
• The hottest month comes before the summer solstice and the monsoon rains (May–June in the Ganga plain).
• After that, monsoon clouds and rain cool the air, so July is cooler than May–June.

Where it occurs

• Belt: just poleward of the Tropic of Cancer, in the monsoon lands of Asia.
• World:
• North Indian plains (Ganga plain), India.

• Interior plains of south China.

• India (Cwg zone):

• Ganga plain.
• Eastern Rajasthan.
• Northern Madhya Pradesh.
• Much of north-east India.

Don't confuse with

• Aw (tropical savanna): it has the same rain pattern (wet summer, dry winter). But in Aw the coldest month stays above 18°C. Cwa winters are cooler, below 18°C. NCERT calls Cwa winters "warm", which is wrong; a warm winter would make it Aw.
• Cfa (humid subtropical): this is also an east-coast climate with hot summers. But Cfa has no dry season (f), so rain falls all year. Cwa has a dry winter (w).
• Cs (Mediterranean): here the seasons are the other way round. Cs has a dry summer and rainy winter, on west coasts at 30°–40°. Cwa has a rainy summer and dry winter.
• Cwa vs Cwg: both are dry-winter C climates. The "g" (Ganges type) only adds that the hottest month comes before the monsoon and the solstice (May–June).

Prelims Hooks

• Cwa = C + w + a: coldest month −3°C to 18°C, dry winter, warmest month above 22°C [2].
• Location match: Cwa = north Indian plains (Ganga plain) + interior plains of south China, just poleward of the Tropic of Cancer.
• Trap: "Cwa resembles Aw and has warm winters." This is wrong. Cwa winters are cooler (coldest month below 18°C).
• India map: Ganga plain = Cwg. "g" means the hottest month (May–June) comes before the monsoon.
• Cwg zone in India: Ganga plain, eastern Rajasthan, northern Madhya Pradesh, much of north-east India.
• Second-letter key: w = dry winter (Cwa); s = dry summer (Cs); f = no dry season (Cfa, Cfb).

Mains Points

• Monsoon control and east–west contrast (GS-I):
• On east coasts, warm currents and onshore winds give rain all year (Cfa).
• Where the monsoon reverses the winds, winter turns dry, giving Cwa.

• On west coasts at similar latitudes, the summer subtropical high gives dry summers (Cs). So the rain seasons are opposite on the two sides of a continent.

• Farming and population (GS-I/GS-III):

• Hot, rainy summers and cool, dry winters support rice and wheat farming. This feeds very dense populations from the Ganga plain to the plains of south China.
• In the Cwg Ganga plain, this seasonal pattern sets the kharif–rabi cropping cycle (kharif = summer monsoon crops; rabi = winter crops).

• Winter rain from western disturbances (storms that come from the west in winter) helps the rabi wheat crop.

• Risk (GS-III, disasters): almost all of Cwa's rain comes from the monsoon. So north India is open to drought when the monsoon fails and to floods when it is very heavy.

Related concepts

• Warm temperate climates (C)
• Mediterranean climate (Cs)
• Humid subtropical climate (Cfa)
• Marine west coast climate (Cfb)

Read more

• Group C: warm temperate climates (Cwa, Cs, Cfa, Cfb)