Seasons, the four divisions of the year, called spring, summer, autumn (or fall), and winter. North and south of the tropics, summer is the warmest season, winter the coolest Spring and autumn are transitions between the two extremes. In the tropics, little temperature variation occurs with the seasons.

Like the day and the year, the seasons are based on astronomical occurrences. The seasons, however, are related only to the year itself and not to any other units of time.

In the Northern Hemisphere, spring begins about March 21, summer about June 21, autumn about September 23, and winter about December 22. In the Southern Hemisphere, it is autumn that begins in March and spring that begins in September. Winter in the Southern Hemisphere begins about June 21 and summer about December 22, so Christmas comes in summer.

First day of the astronomical seasons in the Northern Hemisphere
2003 March 20June 21Sept. 23Dec. 22
2004 March 20June 20Sept. 22Dec. 21
2005 March 20June 21Sept. 22Dec. 21
2006 March 20June 21Sept. 22Dec. 21
2007 March 20June 21Sept. 23Dec. 22
2008 March 19June 20Sept. 22Dec. 21
2009 March 20June 20Sept. 22Dec. 21
2010 March 20June 21Sept. 22Dec. 21
2011 March 20June 21Sept. 23Dec. 21
2012 March 19June 20Sept. 22Dec. 21
2013 March 20June 20Sept. 23Dec. 21
2014 March 20June 21Sept. 22Dec. 21
2015 March 20June 21Sept. 23Dec. 21
2016 March 19June 20Sept. 22Dec. 21
2017 March 20June 20Sept. 22Dec. 21

Year after year, the seasons form a never-ending cycle. The sun is farthest south (lowest) in the sky and the period of daylight shortest in the Northern Hemisphere about December 22, the date of the winter solstice. From then on, each day the sun reaches a little higher (farther to the north) in the noon sky and each day the period of daylight is a little longer. About March 21, on the vernal equinox, the sun reaches halfway on its northward journey in the sky; it crosses the Equator. On that day, daylight and darkness are the same length.

The days continue to get longer north of the tropics and the sun farther north in the noon sky until June 21, the date of the northern summer solstice. The sun is then directly overhead at 2327' north of the Equatorthe Tropic of Cancer. After the summer solstice, the sun begins to move southward again. Each day in the area north of the tropics, the period of daylight is a little shorter and the night is a little longer. Each day the sun is a little farther south in the sky at noon. In September comes the autumnal equinox, when day and night are again the same length. The sun continues southward until the winter solstice is reached, when the sun is directly over the Tropic of Capricorn (2327' south of the Equator).

Causes of the Seasons

The earth's axisthe imaginary line around which the earth turns in its daily rotationis not at right angles to the path followed by the earth in its orbit around the sun; instead, it is slightly tilted. The seasons as we know them are caused by this tilt and by the fact that the earth's axis always holds the same orientation in space.

The tilt of the earth's axis is 2327', or almost 231/2. If the axis were not tilted, the sun would always be directly over the Equator and the Northern and Southern hemispheres would both have constant, similar weather the year round.

As the earth travels around the sun, the North Pole is sometimes directed toward the sun and sometimes directed away from it. When the North Pole is directed toward the sun, the sun's rays strike most directly on the Northern Hemisphere. The more directly the rays strike, the closer to vertical they are. Vertical rays of sunlight are more effective in producing light and heat than are slanting rays. There are two reasons: (1) slanting rays must pass through a greater thickness of atmosphere than vertical rays and thus lose more of their heat; and (2) slanting rays are spread out over a greater area than vertical rays and are therefore less concentrated. Thus when the North Pole is directed toward the sun, summer is produced in the Northern Hemisphere; the Southern Hemisphere has winter. When the South Pole is directed toward the sun, the Southern Hemisphere receives the most heat and has summer.

The greater efficiency of the sun's rays is one reason summer is warmer than winter. A second reason is that the days are longer. While the sun is shining, the land and air heat up. When the sun goes down, they begin to lose heat. From the vernal equinox to the autumnal equinox, each day in the Northern Hemisphere is longer than the night and more heat is gained from the sun during the day than is lost at night.

The reverse is true as winter approaches and the northern half of the earth begins to cool. Not only does the sun move lower in the sky and become less effective in providing heat, but it shines for a shorter time each day, and heat is lost for a longer time than it is gained.

The earth's orbit is slightly elliptical, or elongated, so the earth is farther from the sun at some times than at other times. The earth is closest to the sun and moving most rapidly in January. In early July, when the earth is farthest from the sun, it is moving more slowly. The earth takes 186 days to go from the vernal to the autumnal equinox, but only 179 days to return from the autumnal to the vernal equinox.

There is also a small difference in the amount of heat received by the earthless heat in July than in January. The total effect of this variation in time and heat is that the seasons in the Northern Hemisphere are a little milder than those in the Southern Hemisphere. The northern winter is shorter and warmer than the southern winter. The northern summer is longer and cooler than the southern summer.

Seasons and Weather

Although the seasons are based on astronomical events, the weather changes that are experienced during the seasons do not occur with the mathematical precision usual for most astronomical events. The day of the winter solsticeabout December 22has the shortest period of daylight and the least effective solar rays of the whole year in the Northern Hemisphere. It is rarely the coldest day of the year, however; the coldest days usually occur sometime in late January or in February. It takes time for the land and the oceans to cool after summer, and until they do, they warm the air somewhat, thus tempering the winter weather.

Similarly, the warmest weather usually occurs more than a month after the summer solsticeJune 21when the land and the oceans have been warmed to the maximums.

Around the time of the equinoxes, the hemisphere that has been cold begins to warm and the hemisphere that has been warm begins to cool. The atmosphere becomes somewhat unstable and the weather begins to fluctuate with great frequency. Thus during the spring and autumn, rains, high winds, and generally changeable weather conditions are common in many areas. The characteristic conditions of spring tend to move outward from the Equator as the sun does. Spring weather, therefore, may occur in some areas before the official opening of spring and later in other areas. Autumn weather may also begin before or after the autumnal equinox.