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A '''season''' is a division of the year<ref>{{cite web|url=http://www.merriam-webster.com/dictionary/season|title=Definition of SEASON|website=www.merriam-webster.com|access-date=27 April 2018}}</ref> based on changes in [[weather]], [[ecology]], and the number of [[daylight]] hours in a given region. On [[Earth]], seasons are the result of the [[axial parallelism]] of Earth's [[axial tilt|tilted orbit]] around the [[Sun]].<ref name="SunModel">{{Cite journal|url=http://sites.google.com/site/khavrus/public-activities/SolarEng|last1=Khavrus|first1=V.|title=Introduction to solar motion geometry on the basis of a simple model|year=2010|journal=Physics Education|volume=45|doi=10.1088/0031-9120/45/6/010|pages=641–653|last2=Shelevytsky|first2=I.|issue=6|bibcode=2010PhyEd..45..641K|s2cid=120966256 |access-date=2011-05-13|archive-date=2016-09-16|archive-url=https://web.archive.org/web/20160916180315/https://sites.google.com/site/khavrus/public-activities/SolarEng|url-status=dead}}</ref><ref name="SunModel2">{{Cite journal|url=http://sites.google.com/site/khavrus/public-activities/seasons|last1=Khavrus|first1=V.|title=Geometry and the physics of seasons|year=2012|journal=Physics Education|volume=47|doi=10.1088/0031-9120/47/6/680|pages=680–692|last2=Shelevytsky|first2=I.|issue=6 |s2cid=121230141 }}</ref><ref name="Lerner and Lerner 2003">{{cite book | last=Lerner | first=K. Lee | last2=Lerner | first2=Brenda Wilmoth | title=World of earth science | publisher=Thomson-Gale | publication-place=Farmington Hills, MI | date=2003 | isbn=0-7876-9332-4 | oclc=60695883 | page=487|quote=Although these distances seem counterintuitive to residents of the Northern Hemisphere who experience summer in July and winter in January — the seasons are not nearly as greatly affected by distance as they are by changes in solar illumination caused by the fact that Earth’s polar axis is inclined 23.5 degrees from the perpendicular to the ecliptic (the plane of the solar system through or near which most of the planet’s orbits travel) and because the Earth exhibits parallelism (currently toward Polaris, the North Star) as it revolves about the Sun.}}</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 undergo [[hibernation]] or to [[Migration (ecology)|migrate]], and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical cultures whose number of seasons varies.
The [[Northern Hemisphere]] experiences most direct sunlight during May, June, and July (thus the traditional celebration of [[Midsummer]] in June), as the hemisphere faces the Sun. For the [[Southern Hemisphere]] it is instead in November, December, and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer [[month]]s, which increases the [[solar flux]]. However, due to [[seasonal lag]], June, July, and August are the warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere.
In [[temperate]] and [[Subpolar climate|sub-polar]] regions, four seasons based on the [[Gregorian calendar]] are generally recognized: ''[[Spring (season)|spring]]'', ''[[summer]]'', ''[[autumn]]'' (''fall''), and ''[[winter]]''. Ecologists often use a six-season model for temperate [[climate]] regions which are not tied to any fixed calendar dates: ''prevernal'', ''vernal'', ''estival'', ''serotinal'', ''autumnal'', and ''hibernal''. Many tropical regions have two seasons: the ''[[wet season|rainy]]''/''[[wet season|wet]]''/''[[monsoon]] season'' and the ''[[dry season]]''. Some have a third ''cool'', ''mild'', or ''[[harmattan]] season''. "Seasons" can also be dictated by the timing of important ecological events such as ''[[Atlantic hurricane season|hurricane season]]'', ''[[tornado season]]'', and ''[[wildfire]] season''.{{citation needed|reason=This content does not appear anywhere in the article body.|date=June 2016}} Some examples of historical importance are the ancient Egyptian seasons—''[[Akhet (season)|flood]]'', ''[[Season of the Emergence|growth]]'', and ''[[Shemu|low water]]''—which were previously defined by the [[flooding of the Nile|former annual flooding]] of the [[Nile]] in [[Egypt]].
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