Seasons

Description

A season is a division of the year[1] marked by changes in weather, ecology and hours of daylight. Seasons result from the yearly orbit of the Earth around the Sun and the tilt of the Earth's rotational axis relative to the plane of the orbit.[2][3] In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.


Red and green trees in autumn (fall)
During May, June, and July, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in November, December, and January. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July, and August are the hottest months in the northern hemisphere and December, January, and February are the hottest months in the southern hemisphere.

In temperate and subpolar regions, four calendar-based seasons are generally recognized: spring, summer, autumn or fall, and winter. Ecologists often use a six-season model for temperate climate regions: prevernal, vernal, estival, serotinal, autumnal, and hibernal. Many tropical regions have two seasons: the rainy, wet, or monsoon season and the dry season. Some have a third cool, mild, or harmattan season. Seasons often held special significance for agrarian societies, whose lives revolved around planting and harvest times, and the change of seasons was often attended by ritual.


A deciduous tree in winter

The six ecological seasons

The four calendar seasons, depicted in an ancient Roman mosaic from Tunisia.
In some parts of the world, some other "seasons" capture the timing of important ecological events such as hurricane season, tornado season, and wildfire season.[citation needed] The most historically important of these are the three seasons—flood, growth, and low water—which were previously defined by the former annual flooding of the Nile in Egypt.