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* [[Årstid]]
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En '''årstid''' er en opdeling af året baseret på ændringer i [[klima]] og [[økologi]]<ref>{{ddo|årstid}}</ref> samt antallet af timer med [[dagslys]] i en given region. På [[Jorden]] skyldes årstiderne [[Jordens omdrejning]] omkring [[Solen]] samt Jordens [[aksehældning]] relativt til [[ekliptika]].<ref name="SunModel">{{Cite journal|url=http://sites.google.com/site/khavrus/public-activities/SolarEng|last=Khavrus|first=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 }}</ref><ref name="SunModel2">{{Cite journal|url=http://sites.google.com/site/khavrus/public-activities/seasons|last=Khavrus|first=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=apple|first2=I.|issue=6 }}</ref> I tempererede og polarregioner markeres årstiderne ved ændringer i intensiteten af [[sollys]], der når Jordens overflade - hvilket kan få dyr til at gå i [[hi]] eller [[Fugletræk|migrere]] og planter til at slumre. Mange kulturer definerer antallet og udformningen af årstider baseret på regionale variationer, og antallet af årstider har derfor varieret blandt både moderne og historiske kulturer.
Den [[nordlige halvkugle]] oplever mere direkte sollys i maj, juni og juli, da halvkuglen i denne periode har retning mod Solen. Det samme sker på den [[sydlige halvkugle]] i november, december og januar. Jordens aksehældning medfører at Solen står højere på humlen i løbet af sommermånederne, hvilket øger [[solstråling|dens stråling]]. På grund af [[sæsonforsinkelse]] er juni, juli og august årets varmeste måneder på den nordlige halvkugle, mens december, januar og februar er de varmeste måneder på den sydlige halvkugle.
In [[temperate]] and [[Subpolar climate|sub-polar]] regions, four seasons based on the [[Gregorian calendar]] are generally recognized: ''[[Spring (season)|spring]]'', ''[[summer]]'', ''[[autumn]]'' or ''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]]'', or ''[[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]].[[File:MatheranPanoramaPointDrySeason.JPG|thumb|upright=1.2|Tropical dry season in [[Maharashtra]], India]]
[[File:MatheranPanoramaPointMonsoon.JPG|thumb|upright=1.2|Tropical wet season/monsoon in [[Maharashtra]], India]]
Seasons often hold special significance for agrarian societies, whose lives revolve around [[planting]] and [[harvest]] times, and the change of seasons is often attended by [[ritual]]. The definition of seasons is also cultural. In India, from ancient times to the present day, six seasons or [[Ritu (Indian season)|Ritu]] based on south Asian religious or cultural calendars are recognised and identified for purposes such as agriculture and trade.
==Causes and effects==
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