Tropical wet climate (Af)

Also called: Equatorial climate, Tropical rainforest climate · Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Tropical wet climate (Af) is the Koeppen climate type found near the equator. In this climate every month averages more than 18°C [2], rain falls in every month with no dry season [2], and even the driest month gets 60 mm or more of rain [2]. It matters because it supports tropical evergreen forest, which has the highest biodiversity of any land biome. Af regions such as the Amazon, Congo and Indonesia also store huge amounts of carbon.

Explanation

Why it is hot and wet all year

• Heat:
• The Sun is high overhead all year.
• Sunlight falls almost straight down, so every month gets a lot of heat.

• No month is cool, so there is no real winter. The coldest month averages 18°C or more, which is the test for Group A climates.

• Rain:

• The ITCZ (Inter Tropical Convergence Zone) stays over this belt for most of the year. It is the low-pressure belt near the equator where the trade winds of the two hemispheres meet and rise.
• The rising air is warm and moist. It cools, its water vapour condenses, and heavy rain falls.
• The ITCZ does not move away for long, so rain falls in every month [2].

How the daily rain forms (convectional rain)

• Morning: the Sun heats the ground strongly.
• Midday: hot, moist air rises and cools. Tall cumulonimbus clouds (tall, dark storm clouds) build up.
• Afternoon: heavy rain falls with thunder. These are afternoon convectional thundershowers.
• Evening: the skies often clear.
• Rain is spread evenly through the year [2].

Temperature pattern

• The daily maximum is about 30°C and the minimum about 20°C.
• So the diurnal range (the day–night difference) is about 10°C.
• The annual range of temperature (warmest month minus coldest month) is under 3°C.
• Result: day and night differ more than the seasons do. Hence the saying "night is the winter of the tropics".

Koeppen's label and the vegetation it supports

• A = tropical humid climate. The coldest month is 18°C or more.
• f = no dry season, from the German feucht, "moist" [2].
• Rule: the driest month gets 60 mm or more [2]. This is the line that separates Af from Am and Aw.
• Vegetation: tropical evergreen forest.
• Water and heat are always available, so trees do not shed all their leaves in one season.
• The forest is dense and layered: tall emergent trees on top, then the main canopy, the under-storey and the forest floor.

Where it occurs

• World: close to the equator, roughly within 5°–10° N and S.
• Amazon basin in South America.
• Western equatorial Africa, mainly the Congo basin.

• East Indies islands: Indonesia and Malaysia.

• India: no area of India is true Af. The Andaman & Nicobar Islands come closest.

• Its place in the sequence: moving away from the equator, the order is Af → Am → Aw → B (dry). Along this line:
• The dry season gets longer.
• Total rain falls.
• The annual temperature range rises.
• Forest thins into grassland.

Don't confuse with

• Am (tropical monsoon): one or more months get under 60 mm [2][3], so it has a short dry season. Af has none. Am's yearly rain is often higher than Af's [4], so equatorial does not mean wettest. In India, Amw covers the west coast south of Goa.
• Aw (tropical wet and dry, savanna): more than two months get under 60 mm [2]. It has a long dry winter and its vegetation is deciduous forest and tree-studded grassland. Af stays wet all year and has evergreen forest.
• Diurnal range vs annual range: in Af the day–night range (about 10°C) is bigger than the yearly range (under 3°C). Do not mix them up when an option asks which range is larger.

Prelims Hooks

• Af test: driest month gets 60 mm or more [2], and every month averages more than 18°C [2].
• The "f" in Af means no dry season (German feucht, "moist") [2].
• "Night is the winter of the tropics" describes Af: diurnal range about 10°C, annual range under 3°C.
• The Congo basin is Af. The Llanos (Venezuela) and the Campos (Brazil) are Aw savanna. This is a common matching trap.
• Af rain is afternoon convectional thundershowers from cumulonimbus clouds. It is not monsoon or orographic rain (rain from wind pushed up a mountain).
• Trap: India has no true Af area. The Andaman & Nicobar Islands come closest, and the Western Ghats coast is Amw.

Mains Points

• Distribution explained by the ITCZ (GS-I): where the ITCZ stays all year, rain falls every month (Af).
• Where it arrives only in summer, the result 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).

• Present the zones as a sequence driven by one cause, not as a list.

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

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

Related concepts

• Tropical humid climates (A)
• Tropical monsoon climate
• Tropical wet and dry climate (Aw)

Read more

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