Sunny Summer Day

Summer is the warmest of the four seasons in the subtropical, temperate, cold and arctic climate zones. Depending on whether it is currently in the northern or southern hemisphere, it is referred to as northern or southern summer. The northern summer occurs at the same time as the southern winter. The seasons occur because the earth's rotation is not in the plane of its orbit around the sun, but is inclined by 23.4° (see ecliptic skew). As a result, the north and south poles are alternately in the grazing sunlight for half a year, and the zenith position of the sun alternates between the northern and southern tropics in the annual cycle.

Astronomically, summer begins with the summer solstice - the time when the sun is perpendicular above the tropic of our own hemisphere and the days are longest. Summer begins in the northern hemisphere on June 20, 21 or 22, and in the southern hemisphere on December 21 or 22. After that, the days become shorter and the nights longer. Northern summer ends with the autumnal equinox on September 22 or 23, and southern summer on March 20 or 21. Because the Earth's orbit around the Sun is slightly elliptical, i.e., deviates from an exact circular orbit by 1.7 percent, the four seasons are not exactly equal in length.

The northern summer is slightly longer than the southern summer because during the northern summer the Earth is slightly farther from the Sun (aphelion on July 5) and its direction toward the Sun changes slightly more slowly. The phenological start of summer can differ considerably from the astronomical one and is influenced not only by the land-sea distribution (maritime vs. continental climate) but also by initial major weather patterns and many kinds of regional factors. A distinction is also made between early summer, midsummer and late summer. Meteorologically, the months of June, July and August are assigned to northern summer, and December, January and February to southern summer. In many cultures customs and rites are connected with the beginning of summer.