winter season

  

season, any of four divisions of the year according to consistent annual changes in the weather. The seasons—winter, spring, summer, and autumn—are commonly regarded in the Northern Hemisphere as beginning respectively on the winter solstice, December 21 or 22; on the vernal equinox, March 20 or 21; on the summer solstice, June 21 or 22; and on the autumnal equinox, September 22 or 23 (at the equinoxes, the days and nights are equal in length; at the winter solstice the day is the year’s shortest, and at the summer solstice it is the year’s longest). In the Southern Hemisphere, summer and winter are reversed, as are spring and fall.

What causes the seasons?

In many parts of the world, weather cycles through the four seasons like clockwork: spring, summer, autumn, and winter.

See all videos for this article

Understand how the tilted rotation of Earth's axis gives more prominent full moon in winter than in summer

Learn why a full moon is more prominent in winter than in summer.

See all videos for this article

Outside the tropics and the polar regions, the essential characteristic of the annual cycle is a temperature oscillation between a single maximum and a single minimum. This oscillation results from the annual variation in the angle at which the Sun’s rays reach Earth’s surface and from the annual variation in the duration of sunlight on Earth’s surface each day. As Earth moves in its orbit around the Sun, its axis maintains a nearly constant orientation in space, inclined about 66°33′ to the orbital plane. During the six-month half of each orbit when the North Pole is inclined toward the Sun, a point in the Northern Hemisphere receives the Sun’s rays at an angle closer to 90° than does a point in the Southern Hemisphere; this causes greater heating and more hours of daylight in the Northern Hemisphere than in the Southern Hemisphere. During the other six months, these conditions are reversed.