Tropics

The tropics are the band of Earth around the equator where the Sun can ever be directly overhead, unlike the temperate and polar zones. The width of this region in latitude is double the planet’s axial tilt, which currently sits at about 23.4°. This zone is sometimes called the tropical or torrid zone. Because the Sun stays high in the sky all year, the tropics soak up more solar energy annually than any other part of the planet, making them the hottest places on Earth. Seasonal temperature swings are minimal—"winter" and "summer" barely differ in temperature—so seasons are usually marked by changes in rainfall rather than temperature or daylight.

The tropics host a wide range of local climates, including rainforests, monsoons, savannahs, deserts, and even snow-capped mountains at high altitudes. The word "tropical" can also refer to specific weather patterns, which should not be confused with the geographic region. The boundaries of the tropics are the Tropic of Cancer at 23°26′09.0″ N and the Tropic of Capricorn at 23°26′09.0″ S, matching Earth’s axial tilt. These lines shift slightly over time as the tilt changes, along with the polar circles.

The tropics cover 39.8% of Earth’s surface area and 36% of its landmass. As of 2014, they held 40% of the world’s population, a share projected to reach 50% by 2050. Global warming is pushing tropical weather conditions into the subtropics, bringing more extreme events like heatwaves and intense storms, which could make some tropical areas uninhabitable.

**Etymology** The word "tropic" comes from the Greek *tropē*, meaning "to turn" or "change direction." This referred to the parallels where the Sun appeared highest overhead as Earth tilted: the Tropic of Cancer marked the Sun’s northernmost overhead point during summer in the Northern Hemisphere, named after the constellation Cancer as measured in Mesopotamia around 2500 BC. The Tropic of Capricorn marked the southern counterpart during winter there, named after Capricorn. Due to the precession of the equinoxes, the Sun now stands overhead against Taurus and Sagittarius instead.

**Astronomical definition** The tropics lie between the Tropic of Cancer at 23°26′09.0″ N and the Tropic of Capricorn at 23°26′09.0″ S, latitudes set by Earth’s axial tilt. The Tropic of Cancer is the northernmost point where the Sun can be directly overhead, and the Tropic of Capricorn is the southernmost. This means every spot in the tropics experiences the Sun directly overhead at least once a year. The boundaries are geographic conventions, and while the tilt varies slightly due to the Moon, the difference from the true latitudes is tiny.

**Seasons and climate** Many tropical areas have a dry season and a wet season. The wet season (or rainy season) lasts one or more months when most of the year’s rain falls. These seasons occur across the tropics, subtropics, and even some temperate regions. Under the Köppen climate classification, a tropical wet-season month has average precipitation of 60 mm (2.4 in) or more. Some areas see a mid-season break in rain when the Intertropical Convergence Zone or monsoon trough moves poleward. Typical vegetation ranges from moist seasonal forests to savannahs. When the wet season falls in summer, rain comes mainly in the late afternoon and evening. This season improves air and freshwater quality, boosts plant growth, and leads to late-season crop yields. But floods and rains can overflow rivers, force animals to higher ground, wash away soil nutrients, and increase erosion. Malaria rates rise where the rainy season coincides with high temperatures. Animals adapt to the wetter conditions, and food shortages from the previous dry season persist until crops mature.

Not all areas within the geographical tropics have a tropical climate. Under Köppen, much of the region is classed as dry (arid or semi-arid), including the Sahara, Atacama, and Australian Outback. There are also alpine tundra and snow-capped peaks like Mauna Kea, Mount Kilimanjaro, Puncak Jaya, and parts of the Andes down to northern Chile and Peru.

**Climate change** The tropics are experiencing climate change, just like the rest of the world.