Tropical Humid Climates (Group A)

At a Glance

• Group A Coolest month 18 degrees Celsius or above
• Af Rainforest; rain all year
• Am Monsoon; short dry season
• Aw Savanna; dry winter
• Controlling belt Intertropical Convergence Zone
• Exam link UPSC Prelims, Geography: El Niño and Climate Classification

Where Does It Fit?

This is the fifth topic of the category on El Niño and climate classification. The previous topic described the Köppen scheme, and this one describes its first group, the tropical humid climates. The next topic covers the dry and warm temperate climates. Previous topic: Köppen Scheme of Climate Classification. Next topic: Dry and Warm Temperate Climates (Groups B and C).

What Is the Tropical Humid Climate?

In short: Group A is hot all year, with the rainfall controlled by the Intertropical Convergence Zone.

The tropical humid climates are found within about 15 to 25 degrees of the equator. The Sun is high throughout the year, so that every month has an average temperature of at least 18 degrees Celsius, and the annual range is small. The rainfall depends on the movement of the Intertropical Convergence Zone, ITCZ, the belt of rising air where the trade winds meet, which swings north and south with the Sun, as in General Circulation of the Atmosphere. Near the equator it is overhead nearly all year; farther away it brings a wet season and a dry season. The group has three types.

What Are the Features of the Tropical Rainforest Climate?

• Weather: hot, humid and the same every day: the daily range of temperature, about 8 to 10 degrees, is larger than the annual range. Rain falls in the afternoon from convectional clouds, often with thunder, as in Precipitation and Types of Rainfall.
• Vegetation: tall evergreen trees in layers, with a canopy that blocks sunlight, a thin understorey, lianas and epiphytes. The Amazon forests are called selvas. It has the highest biodiversity of any biome.
• Soils: heavy rain leaches nutrients, making acidic, red laterite soils that are poor in humus, as in Major Soil Groups of India. The fertility is in the living biomass, so that when the forest is cleared, farming fails in a few years, which is why shifting cultivation is common.
• People: sparse populations such as the Pygmies of the Congo and the tribes of the Amazon, with plantation crops such as rubber, oil palm, cocoa and bananas.
• Threats: deforestation for cattle, soya, palm oil and mining, which reduces the rainfall that the forest itself recycles. Scientists warn that parts of the Amazon may approach a tipping point to savanna if clearing and warming continue.

What Are the Monsoon and Savanna Climates?

The tropical monsoon climate is created by the seasonal reversal of winds. In summer, moist winds from the ocean bring torrential rain, and in winter, dry land winds bring a short dry season. In India it is found along the Malabar coast and in the Western Ghats, with evergreen and semi-evergreen forests. The monsoon system is explained in Mechanism of the Indian Monsoon. It has dense populations, because of the combination of heat and plentiful water: South Asia and Southeast Asia grow rice, jute, tea, sugarcane and spices.

The tropical savanna climate lies on the poleward side of the rainforest, where the ITCZ brings rain only in summer and the subtropical high brings drought in winter. The grass grows tall in the wet season, and turns brown and often burns in the dry season. Fire, grazing and drought keep trees scattered. The African savanna supports the largest herds of grazing animals, such as zebra, wildebeest and elephants, and predators such as lions. People herd cattle, as the Masai do, and grow millet, sorghum, cotton and groundnut. Savannas are under pressure from overgrazing, farming and drought, which drive desertification at their dry edges.

As an analytical point, the three climates form a gradient controlled by one thing, the length of the dry season, and since the ITCZ is shifting in a warming world, the boundaries between them may move.

Why Is This Topic Important?

• Core group Starting point of world climates
• Ecosystems Rainforest and savanna conservation
• India Monsoon and savanna regions

What Came Next?

The next topic describes the dry and warm temperate climates. Next topic: Dry and Warm Temperate Climates (Groups B and C).

Key People and Terms

• ITCZ Equatorial belt of rising air
• Selvas Amazon rainforests
• Laterite Leached red tropical soil
• Savanna Tropical grassland with scattered trees
• Llanos Savanna of the Orinoco in Venezuela
• Campos Savanna of Brazil

Exam Corner

Mains Angle

Questions ask for features and problems. These points add depth.

• Gradient Length of dry season
• Soil Leached and fragile
• Threat Deforestation and tipping points