Winter is the coldest season of the year in polar and temperate zones (winter does not occur in the tropical zone). It occurs after autumn and before spring in each year. Winter is caused by the axis of the Earth in that hemisphere being oriented away from the Sun. Different cultures define different dates as the start of winter, and some use a definition based on weather. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures. The moment of winter solstice is when the sun's elevation with respect to the North or South Pole is at its most negative value (that is, the sun is at its farthest below the horizon as measured from the pole). The day on which this occurs has the shortest day and the longest night, with a daylength increasing and nightlength decreasing as the season processes after the solstice. The earliest sunset and latest sunrise dates outside the polar regions differ from the date of the winter solstice, however, and these depend on latitude, due to the variation in the solar day throughout the year caused by the Earth's elliptical orbit (see earliest and latest sunrise and sunset).
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