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A season is a division of the year,<ref>Merriam-Webster Dictionary</ref> 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.<ref name="SunModel">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="SunModel2">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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 spring

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 (with their adjectives) are generally recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal). In American English and Canadian English, fall is sometimes used as a synonym for both autumn and autumnal. Ecologists often use a six-season model for temperate climate regions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four.

A tree in winter

Various calendars used in South Asia define six seasons.

The six ecological seasons
The four calendar seasons, depicted in an ancient Roman mosaic from Tunisia.
File:Chariot clock The Seasons.jpg
An Empire style chariot clock depicting an allegory of the four seasons. France, c. 1822.

Hot regions have two or three seasons; the rainy (or wet, or monsoon) season and the dry season, and, in some tropical areas, a cool or mild season.

In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornado season, or a wildfire season.

A tree in autumn (fall)

Season sections
Intro  Causes and effects  [[Season?section={{safesubst:#invoke:anchor|main}}Four-season_calendar_reckoning|{{safesubst:#invoke:anchor|main}}Four-season calendar reckoning]]  South Asian (mid-latitude and tropical) six-season calendars  Polar day and night  Non-calendar-based reckoning  \"Official\" designations   See also   References  External links  

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