A tropical climate is one where every month of the year has an average temperature of at least 18°C (64°F), paired with significant rainfall. These climates occupy a belt stretching roughly 15 to 25 degrees north and south of the equator, covering parts of Central and South America, sub-Saharan Africa, South and Southeast Asia, and northern Australia.
What Makes a Climate “Tropical”
The defining feature is warmth that never lets up. Unlike temperate regions where winter brings a dramatic temperature drop, tropical areas stay consistently warm year-round. The widely used Köppen classification system sets the bar at 18°C (64°F) as the minimum average temperature for every single month. If even one month dips below that threshold, the climate falls into a different category.
This persistent warmth exists because of how the sun hits the Earth near the equator. Sunlight arrives at a near-vertical angle, delivering intense energy to the surface throughout the year. The result is minimal seasonal temperature variation. In many tropical cities, the difference between the warmest and coolest month is only a few degrees, something that would be unrecognizable to someone living in, say, Chicago or London.
Why Tropical Regions Get So Much Rain
The same intense solar heating that keeps temperatures high also drives massive rainfall. Near the equator, hot air rises rapidly, creating a persistent band of low atmospheric pressure. As warm, moist air climbs, it cools and releases its moisture as heavy rain. This rising air is part of a large-scale atmospheric circulation pattern called the Hadley cell, where air flows toward the equator at the surface, rises, then moves poleward in the upper atmosphere before sinking again in the subtropics.
The zone where air converges and rises shifts north and south with the seasons, dragging rain belts with it. This is why some tropical areas have a pronounced wet and dry season rather than year-round downpours. The closer you are to the equator, the more consistently you receive rainfall. Move toward the edges of the tropics, and dry seasons become longer and more intense.
Three Types of Tropical Climate
Not all tropical climates look or feel the same. The Köppen system breaks them into three subtypes based on how rainfall is distributed throughout the year.
- Tropical rainforest (Af): Rain falls heavily every month, with at least 60 mm (about 2.4 inches) in even the driest month. Daily temperatures typically range from 20°C to 25°C (68°F to 77°F). Think of the Amazon Basin, the Congo, or Borneo. These regions feel perpetually humid, with lush, dense forest canopy.
- Tropical monsoon (Am): Most of the year is wet, but one or two months see rainfall drop below 60 mm. The dry spell is brief enough that the landscape stays green. Parts of coastal India, West Africa, and the Philippines experience this pattern, where seasonal wind shifts bring torrential monsoon rains followed by a short dry period.
- Tropical savanna (Aw): This is the most seasonal of the three. The dry season is longer and more pronounced, and total annual rainfall is lower, typically between 50 and 175 cm (20 to 69 inches). Precipitation comes mostly from afternoon thunderstorms during the wet season. Grasslands with scattered trees dominate the landscape. Large parts of East Africa, northern Australia, and central Brazil fall into this category.
Biodiversity in the Tropics
Tropical climates support a staggering concentration of life. Tropical forests alone harbor 62% of all terrestrial vertebrate species on Earth, more than twice the number found in any other land-based biome. That figure, published in the journal Frontiers in Ecology and the Environment, only accounts for vertebrates. When invertebrates and plants are included, the true proportion is likely even higher, though comprehensive data for those groups remains limited.
This richness comes from a combination of factors: consistent warmth allows year-round growth and reproduction, abundant rainfall supports dense vegetation, and the sheer age of tropical ecosystems has given species millions of years to diversify. A single hectare of tropical rainforest can contain more tree species than exist in all of northern Europe.
Crops That Depend on Tropical Conditions
Many of the world’s most economically important crops can only grow in tropical climates. Coffee, cocoa, tea, and sugar cane dominate tropical cash cropping by land use. Staple foods like bananas, plantains, and breadfruit are essential dietary components for hundreds of millions of people in tropical economies. Other crops that thrive in these conditions include pineapple, mango, papaya, taro, and macadamia nuts.
The combination of warmth, rainfall, and long growing seasons makes tropical agriculture highly productive, but also vulnerable. Many of these crops have narrow temperature tolerances, and the same moisture that fuels growth also promotes pests and fungal diseases. Tropical farming has its own rhythms, often timed to wet and dry seasons rather than the spring-to-fall cycle familiar in temperate regions.
The Tropical Belt Is Expanding
The boundaries of the tropics are not fixed. Multiple lines of evidence from satellite data, weather balloon measurements, and atmospheric models show that the tropical belt has been widening since at least the late 1950s. One study tracking ozone gradients between the tropics and subtropics found the expansion rate in the Northern Hemisphere was roughly 1.1 degrees of latitude per decade between 1979 and 1991. Overall, the distance between the northern and southern boundaries of the main tropical circulation pattern has grown by approximately 2 to 4.5 degrees since 1979.
This expansion pushes subtropical dry zones further from the equator, bringing drier conditions to regions that previously had more moderate rainfall. Areas on the margins of the tropics, including parts of the Mediterranean, southern Australia, and the American Southwest, are already experiencing this shift. The observed widening has outpaced what climate models predicted, suggesting the process may be accelerating faster than scientists initially expected.
For the roughly 40% of the world’s population living in or near the tropics, these shifts matter. Changing rainfall patterns affect agriculture, water supply, and the spread of heat-sensitive diseases. Understanding what a tropical climate is also means recognizing that its boundaries are a moving target.