In the tropics, rain sorts the climates because temperature barely changes
A tropical climate never has a month averaging below 18 °C, so seasons are defined not by cold but by rain. Köppen's system splits the tropics into three kinds using a single number: how much rain falls in the driest month. That one figure separates dripping rainforest from monsoon forest and open savanna.
Tropical climates form group A, the first of Köppen's five major climate groups, and Köppen's scheme is the most widely used classification. Temperatures stay hot and humid all year with a tiny annual range and intense sunshine, so most places have just two seasons, wet and dry. The zone lies mostly within 23.5 degrees of the equator, taking in Central America, north-central South America, central Africa, southern Asia, northern Australia and Pacific islands.
The rainforest type, Af, needs at least 60 millimetres even in its driest month, and the f comes from feucht, German for moist. It usually sits within 5 to 10 degrees of the equator, with average temperatures of 21 to 30 °C and rainfall that can top 100 inches a year, as in the upper Amazon, the northern Congo basin and the East Indies. So many trees grow that their transpiration adds humidity back to the air, and plants stack into vertical layers to compete for light.
The monsoon type, Am, has a short dry spell, almost always in winter, and a threshold for the driest month that depends on total annual rain. It is common in South and Southeast Asia between about 10 degrees north and the Tropic of Cancer, and also occurs in West Africa and South America. Averages hover near 27 °C with an annual range of only about 3.6 °C, and the year divides into a cool dry season, a hot dry spring and the summer monsoon. Its forests have a dense shrub floor, an understory near 15 metres and a canopy of 25 to 40 metres, with teak and sandalwood among the typical trees.
Savanna, Aw or As, lies mostly between 10 and 25 degrees latitude at the tropics' outer edge, in central Africa, parts of South America and northern and eastern Australia. Temperatures run 20 to 30 °C and yearly rain 700 to 1,000 millimetres, with dry months often getting far less than 60. Grasslands dotted with woodland dominate, and grassland biomes cover almost 20 percent of Earth's surface. Tropical plants supply valuable crops including coffee, cocoa and oil palm.
Source: Tropical climate
