Tropics

The tropics are the regions of Earth surrounding the Equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's axial tilt; the width of the tropics (in latitude) is twice the tilt. The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone).

Due to the overhead sun, the tropics receive the most solar energy over the course of the year, and consequently have the highest temperatures on the planet. Even when not directly overhead, the sun is still close to overhead throughout the year, therefore the tropics also have the lowest seasonal variation on the planet; "winter" and "summer" lose their contrast. Instead, seasons are more commonly divided by precipitation variations than by temperature variations.

The tropics maintain wide diversity of local climates, such as rain forests, monsoons, savannahs, deserts, and high altitude snow-capped mountains. The word "tropical" can specifically refer to certain kinds of weather, rather than to the geographic region; these usages ought not be confused.

The Earth's axial tilt is currently around 23.4°, and therefore so are the latitudes of the tropical circles, marking the boundary of the tropics: specifically, ±23°26′09.9″ (or 23.43607°). The northern one is called the Tropic of Cancer, and the southern is the Tropic of Capricorn. As the Earth's axial tilt changes^[a], so too do the tropical and polar circles.

The tropics constitute 39.8% of Earth's surface area^[1] and contain 36% of Earth's landmass.^[2] As of 2014^[update], the region was home also to 40% of the world's population, and this figure was then projected to reach 50% by 2050. Because of global warming, the weather conditions of the tropics are expanding with areas in the subtropics,^[3] having more extreme weather events such as heatwaves and more intense storms.^[4][3] These changes in weather conditions may make certain parts of the tropics uninhabitable.^[5]

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