Tropics

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The tropics is a region of the Earth by the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23° 26′ 16″ (or 23.4378°) N and the Tropic of Capricorn in the southern hemisphere at 23° 26′ 16″ (or 23.4378°) S. The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone).

The tropics include all the areas on the Earth where the Sun reaches a point directly overhead and a point directly underneath at least once during the solar year. Outside the tropics, the Sun never reaches a point directly overhead or a point directly underneath at any time during the year.

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Seasons and climate

"Tropical" is sometimes used in a general sense for a tropical climate to mean warm to hot and moist year-round, often with the sense of lush vegetation.

The seasons in the tropics are dominated by the movement of the tropical rain belt (or ITCZ the intertropical convergence zone) which moves from the northern to the southern tropics and back over the course of a year, resulting in a dry season and a wet season rather than the various temperatures and day lengths indicative of the spring, summer, autumn and winter pattern found in areas outside tropics.

However, the starting dates of the seasons are related to the tropics, despite the fact that these dates only apply in the temperate and polar regions with only the winter solstice date applying in the tropics because the summer solstice occurs when the Sun is at the zenith, which occurs at different dates for different latitudes. Spring begins when the Sun is directly over the Equator (vernal equinox). Summer begins when the Sun is directly over the Tropic of Cancer in the north or when the Sun is directly over the Tropic of Capricorn in the south (summer solstice). Autumn begins when the Sun is again directly over the Equator (autumnal equinox). Winter begins when the Sun is directly over the Tropic of Capricorn in the north or when the Sun is directly over the Tropic of Cancer in the south (winter solstice).

Regions within the tropics may well not have a tropical climate. There are alpine tundra and snow-capped peaks, including Mauna Kea, Mount Kilimanjaro, and the Andes as far south as the northernmost parts of Chile and Argentina. Under the Köppen climate classification, much of the area within the geographical tropics is classed not as "tropical" but as "dry" (arid or semi-arid) including the Sahara Desert and Australian Outback.

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