Tropical humid climates (A)

Also called: Group A climates · Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Tropical humid climates (Group A) are the Koeppen climates in which the coldest month averages 18°C or more, so no month is cool and there is no real winter. Every month averages more than 18°C in all three types, Af, Am and Aw [2]. They cover the hot, wet belt between the tropics, including most of India. Examiners often test the 18°C rule, the 60 mm rule and India's climate map.

Explanation

Why Group A is hot and wet

• Location: between the Tropic of Cancer (23½°N) and the Tropic of Capricorn (23½°S).
• Heat comes from a high Sun:
• The Sun is high overhead all year.

• Sunlight falls almost straight down, so every month gets a lot of heat.

• Rain comes from the ITCZ:

• The ITCZ (Inter Tropical Convergence Zone) is the low-pressure belt near the equator. The trade winds of the two hemispheres meet there and rise.
• The rising air is warm and moist. It cools as it rises.

• Its water vapour condenses (turns into water drops), and heavy rain falls.

• Result:

• Rainfall is high.

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

• Test to remember: Group A depends on the coldest month (≥ 18°C), not the hottest month.

The three types: split by the 60 mm line

• Koeppen splits Group A by how much rain falls in the driest month, using a 60 mm line [2].
• Koeppen's letters: f = no dry season (German feucht, "moist"); m = monsoon; w = dry winter; s = dry summer [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

How each type works

• Af (tropical wet):
• Rain falls in every month. Rain is spread evenly through the year [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 and thunder come in the afternoon. Skies often clear by evening.
• The day maximum is about 30°C and the night minimum about 20°C. So the diurnal range (day–night difference) is about 10°C, while the annual range is under 3°C.
• Day and night differ more than the seasons do. Hence the saying "night is the winter of the tropics".

• Vegetation: tropical evergreen forest. Water and heat are always available, so trees do not shed all their leaves in one season. The canopy is dense and layered (emergent trees, main canopy, under-storey, forest floor). It has the highest biodiversity of any land biome.

• Am (tropical monsoon):

• Heavy summer rain and a short dry winter. The monthly mean temperature is above 18°C in every month [3].
• 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 Aw [3].
• The hottest month comes just before the rains begin [2]. Clouds and rain then cool the land, so in India May is usually hotter than July.

• How the monsoon brings rain:

  • In summer, land heats faster than sea. Low pressure forms over the land.
  • Moist sea winds 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.

• Aw (tropical wet and dry / savanna):

• Rain is less than in Af and Am, and it changes a lot from year to year.
• 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 winter is dry:
  • 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.
  • The result is a dry winter, which is the "w" in Aw.
• 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 (trees shed leaves in the dry season to save water) and tree-studded grassland, which is tall grass with scattered trees such as acacia and baobab.

The sequence away from the equator

• Af → Am → Aw → B (dry). As you move along this sequence:
• The dry season gets longer.
• Total rain falls.
• The annual temperature range rises.
• Forest thins into grassland.

Where it occurs

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

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

• Af (India): no area. The Andaman and Nicobar Islands come closest.

• Am (world):
• The Indian subcontinent.
• North-eastern South America.
• Northern Australia.

• Most Am areas lie in southern and south-eastern Asia [5].

• Am (India):

• Amw (monsoon type with a short dry winter) covers the west coast south of Goa, which is the Konkan–Malabar coast of coastal Karnataka and Kerala.

• It also covers parts of north-east India.

• Aw (world):

• It lies poleward of the Af belt (towards the poles) in both hemispheres, roughly between 5° and 20° latitude [4].
• NCERT examples: Brazil north and south of the Amazon forest, Bolivia and Paraguay (South America), Sudan, and the region south of Central Africa.
• Other examples: the Llanos of Venezuela (Orinoco basin grasslands), the Campos of Brazil and northern Australia.

• On the western side of continents it borders dry climates (B). On the eastern side it borders Cf or Cw (warm temperate) climates.

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

• As (India): the Coromandel coast of Tamil Nadu. It gets its main rain in winter from the north-east monsoon.

Don't confuse with

• Aw vs As: both have more than two dry months. Aw has a dry winter. As has a dry summer [2]. In India, As is the Coromandel coast and Aw is the Peninsular plateau.
• Af vs Am: Af has no month under 60 mm. Am has at least one month under 60 mm [2]. But Am's yearly total is often higher than Af's [5], so the equatorial type is not always the wettest.
• Diurnal range vs annual range (Af): the day–night range (~10°C) is bigger than the annual range (<3°C). That is why "night is the winter of the tropics" describes Af.
• "Tree-shredded" vs tree-studded grassland: NCERT prints "tree-shredded". This is a misprint for tree-studded grassland, which is the savanna of Aw.

Prelims Hooks

• The Group A test is coldest month ≥ 18°C, not the hottest month. Every month is above 18°C in Af, Am and Aw [2].
• 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.
• 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. No part of mainland India is Af.
• Llanos (Venezuela) and Campos (Brazil) = Aw savanna. Congo basin = Af.
• Aw borders B climates on the western side of continents and Cf/Cw on the eastern side.

Mains Points

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

• Present the zones as one sequence (Af → Am → Aw → B), not as a list.

• India's climatic mix and savanna problems (GS-I/GS-III): the Western Ghats separate 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).

• Kerala and coastal Karnataka grow plantation crops and keep evergreen forests.
• The Deccan relies on rain-fed crops and suffers repeated drought. Unreliable Aw rain brings crop failure, fodder shortage and dry-season fires.

• Overgrazing and clearing can turn damaged savanna into desert, as in Sudan and the Sahel. Responses include watershed management, drought-tolerant crops and dryland farming in peninsular India.

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

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

Related concepts

• Tropical wet climate (Af)
• Tropical monsoon climate
• Tropical wet and dry climate (Aw)

Read more

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