Group A: tropical humid climates (Af, Am, Aw)

Detail

What makes a climate "Group A"

• Tropical humid climates (A): climates where the coldest month averages 18°C or more. No month is cool, so there is no real winter.

• Every month averages more than 18°C (64°F) in all three types, Af, Am and Aw [2].

• Where: between the Tropic of Cancer (23½°N) and the Tropic of Capricorn (23½°S).

• Why it is hot and wet:
• Heat: the Sun is high overhead all year. Sunlight falls almost straight down, so every month gets a lot of heat.
• Rain: the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet and rise) lifts warm, moist air. Rising air cools, its water vapour condenses, and heavy rain falls.

• Result: rainfall is high and the annual range of temperature (warmest month minus coldest month) is very low. Near the equator it is often under 3°C.

• How Group A is split: by how much rain falls in the driest month, using a 60 mm line [2]:

Type	Name	Dry season	Driest month	Main vegetation
Af	Tropical wet (equatorial rainforest)	None	60 mm or more [2]	Tropical evergreen forest
Am	Tropical monsoon	Short	Under 60 mm, but total rain is very heavy [2][3]	Forest survives
Aw	Tropical wet and dry (savanna)	Long, in winter	More than two months under 60 mm [2]	Deciduous forest, tree-studded grassland

• Koeppen's letters: f = no dry season (German feucht, "moist"); m = monsoon; w = dry winter; s = dry summer [2].

Af — Tropical wet climate

• Where:
• The Amazon basin (South America).
• Western equatorial Africa, mainly the Congo basin.
• The East Indies islands (Indonesia, Malaysia).

• These lie close to the equator, roughly within 5°–10° N and S.

• Rain:

• Rain falls in every month. There is no dry season [2].
• Most of it comes as afternoon convectional thundershowers:
  • The morning Sun heats the ground strongly.
  • Hot, moist air rises and cools. Tall cumulonimbus clouds (tall, dark storm clouds) form.
  • Heavy rain falls with thunder in the afternoon. Skies often clear by evening.

• Threshold: the driest month gets 60 mm or more [2].

• Temperature:

• The daily maximum is about 30°C and the minimum about 20°C, so the diurnal range (day–night difference) is about 10°C.
• The annual range is under 3°C.
• The difference between day and night is bigger than the difference between seasons. Hence the saying "night is the winter of the tropics".

• Rain is spread evenly through the year [2].

• Vegetation: tropical evergreen forest. Trees do not shed all their leaves in one season, because water and heat are always available.

• The canopy is dense and layered (emergent trees, a main canopy, an under-storey and the forest floor).
• It has the highest biodiversity of any land biome (see biodiversity-conservation).

Am — Tropical monsoon climate

• Where:
• The Indian subcontinent.
• North-eastern South America.
• Northern Australia.

• Most of it lies in southern and south-eastern Asia [5].

• Climate:

• Heavy summer rain and a short dry winter.
• Monthly mean temperature is above 18°C in every month [3].
• Threshold: one or more months get under 60 mm [2][3].
• Total rain is still heavy enough for forest to survive. The yearly total is often higher than in Af [5].
• The annual temperature range is smaller than in savanna (Aw) [3].

• The hottest month comes just before the rains begin [2]. Once the clouds and rain arrive, they cool the land. In India, May is usually hotter than July.

• How the monsoon brings the rain:

• In summer the land heats faster than the sea, so low pressure forms over the land.
• Moist winds from the sea are drawn in. In India these are the south-west monsoon winds from the Indian Ocean.
• The winds hit the Western Ghats and are pushed up. The air cools and gives very heavy rain on the side facing the wind (the windward side).

• In winter the winds blow from land to sea, so the season is dry.

• In India:

• Amw (monsoon type with a short dry winter) covers the west coast south of Goa (Konkan–Malabar coast: coastal Karnataka and Kerala).
• It also covers parts of north-east India.
• (NCERT India, Climate chapter: Amw = "west coast of India south of Goa".) Details are in indian-climate-monsoon.

Aw — Tropical wet and dry (savanna) climate

• Where:
• Poleward of the Af belt (towards the poles) in both hemispheres, roughly between 5° and 20° latitude [4].
• On the western side of continents it borders dry climates (B).

• On the eastern side it borders Cf or Cw climates (warm temperate climates; f = no dry season, w = dry winter).

• NCERT examples: Brazil north and south of the Amazon forest, Bolivia, Paraguay (South America), Sudan, and the region south of Central Africa.

• Further examples:
• The Llanos of Venezuela (Orinoco basin grasslands).
• Northern Australia.

• Most of peninsular India. (NCERT India: Aw covers most of the Peninsular plateau, south of the Tropic of Cancer.)

• Rain:

• Less than in Af and Am, and it changes a lot from year to year.
• More than two months get under 60 mm [2].
• Rain falls in the high-sun (summer) season, when the ITCZ moves overhead [4].
• The wet season is short. The dry season is long, with severe drought.

• The dry season grows longer further from the equator [4].

• Why the dry season happens:

• In summer the ITCZ moves towards the region and brings rising air and rain.
• In winter the ITCZ moves back towards the other hemisphere. The subtropical high (a belt of sinking, dry air near 30° latitude) and dry trade winds take over.

• Result: a dry winter, which is the "w" in Aw.

• Aw vs As: both have more than two dry months. Aw has a dry winter. As has a dry summer [2]. (NCERT India: As covers the Coromandel coast of Tamil Nadu, which gets its main rain in winter from the north-east monsoon.)

• Temperature: the diurnal range is largest in the dry season. Clear, dry skies let the ground heat up fast by day and lose heat fast by night.
• Vegetation:
• Deciduous forest, where trees shed their leaves in the dry season to save water.
• Tree-studded grassland (savanna): tall grass with scattered trees such as acacia and baobab. (NCERT prints "tree-shredded"; this is a misprint for "tree-studded".)
• Savanna wildlife and grassland regions are in world-physical-features.

How the three types fit together

• Moving away from the equator: Af → Am → Aw → B (dry).
• The dry season gets longer.
• Total rain falls.
• The annual temperature range rises.
• Forest thins into grassland.

Feature	Af	Am	Aw
Rainy months	All 12	Most	Few (summer only)
Annual temperature range	Lowest (<3°C)	Low	Higher than Am [3]
Main rain-maker	ITCZ convection	Monsoon winds	Seasonal shift of the ITCZ
India	None (Andaman & Nicobar Islands come closest)	Amw: west coast south of Goa, parts of NE	Most of peninsular India

Prelims Hooks

• Group A test: coldest month ≥ 18°C. It is not a test of the hottest month.
• 60 mm rule [2]:
• Af: driest month ≥ 60 mm.
• Am: at least one month < 60 mm, but very wet overall.

• Aw/As: more than two months < 60 mm.

• "Night is the winter of the tropics" describes Af: the diurnal range (~10°C) is bigger than the annual range (<3°C).

• In Am, the hottest month comes just before the monsoon rains [2]. In India this is May, not June or July.
• India map trap: Amw = west coast south of Goa; As = Coromandel coast (Tamil Nadu); Aw = most of the Peninsular plateau.
• Aw vs As: w = dry winter; s = dry summer [2].
• The Llanos (Venezuela) and the Campos (Brazil) are Aw savanna. The Congo basin is Af.
• Aw borders B climates on the western side of continents and Cf/Cw on the eastern side.
• Am's yearly rain is often more than Af's [5]. Do not assume equatorial means wettest.
• NCERT "tree-shredded grassland" means tree-studded grassland (savanna).

Mains Points

• Distribution pattern (GS-I): Group A types follow the seasonal north–south shift of the ITCZ.
• Where the ITCZ stays all year, there is rain every month (Af).
• Where it arrives only in summer, there is a wet season and a long drought (Aw).
• Where seasonal monsoon winds strengthen it, a short dry season comes with very heavy rain (Am).

• Use this to explain the zones as a sequence, not a list.

• India's climatic mix: the Western Ghats split Amw on the windward coast from Aw on the plateau, which lies in the rain-shadow (the dry side of a mountain, away from the wind).

• Effect: Kerala and coastal Karnataka grow plantation crops and keep evergreen forests.

• The Deccan relies on rain-fed crops and suffers repeated drought.

• Savanna problems (GS-III): rain in Aw is unreliable and the drought is long.

• This brings crop failure, fodder shortage and dry-season fires.
• Overgrazing and clearing can turn degraded savanna into desert (as in Sudan and the Sahel).

• Responses: watershed management, drought-tolerant crops and dryland farming in peninsular India.

• Af forests and climate change: Af regions (Amazon, Congo, Indonesia) hold the richest biodiversity and huge carbon stores.

• Deforestation reduces moisture recycling (trees give water back to the air), which can lengthen dry spells.
• Over time this can push Af towards Am or Aw conditions. Link to biodiversity loss and carbon sinks.