Climate is the statistical average of prevailing meteorological conditions. It is always regional and applies over a given period of time. The most common climate variables are temperature, precipitation and wind pressure. On Earth, the climate depends on a region’s latitude, altitude, topography, vegetation and proximity to the oceans. Extraterrestrial forces that influence climate include the orbit of the Earth and planets around the Sun, and variations in the Sun’s intensity.
A tropical climate zone is one where temperatures remain constant throughout the year and rarely fall below 65 degrees. Precipitation defines the seasons as either rainy or dry. Tropical climate zones are found on the land and oceans between the Tropics of Cancer and Capricorn. Trade winds carry seasonal rains -- monsoons -- to the east coasts of continents and irrigate lush vegetation in rain forests. Drier grasslands, such as steppes and savannas, occur on the northern and southern limits of this zone.
Arid subtropical zones lie to the north and south of the tropical zone. Their main characteristics are sparse rainfall of only 10 inches per year and daily temperature extremes. Temperatures vary between freezing at nighttime and up to 110 degrees during the day. Subtropical regions include the Sahara Desert, southern Arabia, and parts of Jordan, Iraq, Iran, northwest India and Australia. A Mediterranean climate is a subdivision of this zone with rainfall of up to 35 inches annually. This zone lies around the Mediterranean Sea coast in Europe, the Pacific Northwest and most of California in the United States, Central Chile and western Australia.
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Temperate zones lie between 40 and 60 degrees latitude on either side of the equator. Temperature variations between the seasons are moderate -- annual temperatures average 60 degrees -- and precipitation is constant throughout the year. This zone is the most comfortable for humans and houses 60 percent of the world’s population. Temperate regions include northwest and northeast America, southern Chile and Argentina, most of Europe and parts of eastern Asia. Extremes of temperature that reach 100 degrees in the summer and -20 degrees in the winter occur in the continental subzones of central Eurasia and central North America.
Temperatures in polar regions remain below 50 degrees throughout the year and fall to 30 degrees below zero in the winter. Such areas include northern Canada, Greenland and northern Eurasia. In the subzone of Antarctica, inner Greenland and the Arctic Sea, temperatures remain below freezing all year round and can fall to 58 degrees below zero at their coldest.