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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==
{{main|Effect of sun angle on climate}}
===Axial tilt===
[[File:Seasonearth.png|thumb|right|upright=1.25|Illumination of Earth at each change of astronomical season]]
[[File:seasons.svg|frame|right|This diagram shows how the tilt of [[Earth's axis]] aligns with incoming [[sunlight]] around the [[winter solstice]] of the [[Northern Hemisphere]]. Regardless of the time of day (i.e. the [[Earth's rotation]] on its axis), the [[North Pole]] will be dark and the [[South Pole]] will be illuminated; see also [[arctic winter]]. In addition to the density of [[angle of incidence (optics)|incident]] light, the [[dissipation]] of light in the [[Earth's atmosphere|atmosphere]] is greater when it falls at a shallow angle.]]
The seasons result from the Earth's [[axis of rotation]] being [[axial tilt#Earth|tilted]] with respect to its [[orbital plane (astronomy)|orbital plane]] by an angle of approximately 23.4 [[degree (angle)|degrees]].<ref>{{cite web|last=Cain|first=Fraiser|title=Tilt of the Earth|url=http://www.universetoday.com/26778/tilt-of-the-earth/|access-date=2 May 2014}}</ref> (This tilt is also known as "obliquity of the [[ecliptic]]".)
Regardless of the time of year, the [[Northern Hemisphere|northern]] and [[southern hemisphere]]s always experience opposite seasons. This is because during [[summer]] or [[winter]], one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March{{nbsp}}20 to around September{{nbsp}}22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June{{nbsp}}21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December{{nbsp}}21. The two instants when the Sun is directly overhead at the [[Equator]] are the [[equinox]]es. Also at that moment, both the [[North Pole]] and the [[South Pole]] of the Earth are just on the [[terminator (solar)|terminator]], and hence day and night are equally divided between the two hemispheres. Around the [[March equinox]], the Northern Hemisphere will be experiencing [[Spring (season)|spring]] as the hours of [[daylight]] increase, and the Southern Hemisphere is experiencing [[autumn]] as daylight hours shorten.
The effect of axial tilt is observable as the change in [[day length]] and [[altitude (astronomy)|altitude]] of the Sun at solar [[noon]] (the Sun's [[culmination]]) during the [[year]]. The low angle of Sun during the winter months means that incoming rays of solar radiation are [[effect of sun angle on climate|spread over a larger area]] of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres.
<gallery class="center">
File:Earth-lighting-summer-solstice EN - corrected.png|Illumination of Earth by Sun at the northern solstice.
File:Earth-lighting-winter-solstice EN.png|Illumination of Earth by Sun at the southern solstice.
File:North season.jpg|Diagram of the Earth's seasons as seen from the north. Far right: southern solstice
File:Earth seen from the sun.ogv|Animation of Earth as seen daily from the Sun looking at [[UTC+02:00]], showing the solstice and changing seasons.
File:ReflectedSolarRadiation Solstices.jpg|Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m<sup>2</sup>).
</gallery>
===Elliptical Earth orbit===
Compared to axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth's distance to the Sun because of its [[elliptical orbit]].<ref>"Fundamentals of physical geography", ''PhysicalGeography.net'', Ch. 6: Energy and Matter:(h) Earth-Sun Geometry, [http://www.physicalgeography.net/fundamentals/6h.html]</ref> In fact, Earth reaches [[perihelion]] (the point in its orbit closest to the Sun) in January, and it reaches [[aphelion]] (the point farthest from the Sun) in July, so the slight contribution of [[orbital eccentricity]] opposes the temperature trends of the seasons in the Northern Hemisphere.<ref name="Science@NASA">{{cite web |url=http://science.nasa.gov/science-news/science-at-nasa/2002/02jul_aphelion |title=The Distant Sun |last=Phillips |first=Tony |date=July 4, 2003 |website=[[NASA]] |access-date=April 21, 2021}}</ref> In general, the effect of orbital eccentricity on Earth's seasons is a 7% variation in sunlight received.
Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; research shows that the Earth as a whole is actually slightly warmer when ''farther'' from the sun. This is because the Northern Hemisphere has more land than the Southern, and land warms more readily than sea.<ref name="Science@NASA"/>
Any noticeable intensification of southern winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the Southern Hemisphere.<ref>{{cite web|last=Phillips|first=Tony|title=Earth at Perihelion|url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast04jan_1/|work=Science News|publisher=NASA|access-date=14 May 2013}}</ref>
===Maritime and hemispheric===
Seasonal weather fluctuations (changes) also depend on factors such as proximity to [[ocean]]s or other large bodies of water, [[ocean current|currents]] in those oceans, [[El Niño-Southern Oscillation|El Niño]]/ENSO and other oceanic cycles, and prevailing [[wind]]s.
In the [[temperate]] and [[polar regions of Earth|polar regions]], seasons are marked by changes in the amount of [[sunlight]], which in turn often causes [[biological life cycle|cycles]] of dormancy in plants and [[hibernation]] in animals. These effects vary with [[latitude]] and with proximity to bodies of water. For example, the [[South Pole]] is in the middle of the continent of [[Antarctica]] and therefore a considerable distance from the moderating influence of the southern oceans. The [[North Pole]] is in the [[Arctic Ocean]], and thus its temperature extremes are buffered by the water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.
The seasonal cycle in the polar and temperate zones of one hemisphere is opposite to that of the other. When it is summer in the Northern Hemisphere, it is winter in the Southern, and vice versa.
===Tropics===
[[Image:BlueMarble monthlies animation.gif|thumb|right|Animation of seasonal differences especially [[snow]] cover through the year]]
The [[tropics|tropical]] and [[subtropical]] regions see little annual fluctuation of sunlight. However, seasonal shifts occur along a rainy, low-pressure belt called the [[Intertropical Convergence Zone]] (ICZ). As a result, the amount of [[precipitation (meteorology)|precipitation]] tends to vary more dramatically than the average temperature. When the Zone is north of the Equator, the northern tropics experience their wet season while the southern tropics have their dry season. This pattern reverses when the Zone migrates to a position south of the Equator.
===Mid-latitude thermal lag===
In meteorological terms, the [[solstice]]s (the maximum and minimum [[insolation]]) do not fall in the middles of summer and winter. The heights of these seasons occur up to 7 weeks later because of [[seasonal lag]]. Seasons, though, are not always defined in meteorological terms.
In [[astronomy|astronomical]] reckoning by hours of [[daylight]] alone, the solstices and [[equinox]]es are in the ''middle'' of the respective seasons. Because of seasonal lag due to [[thermal absorption]] and release by the oceans, regions with a [[continental climate]], which predominate in the [[Northern Hemisphere]], often consider these four dates to be the ''start'' of the seasons as in the diagram, with the [[cross-quarter day]]s considered seasonal midpoints. The length of these seasons is not uniform because of Earth's [[elliptical orbit]] and its [[Kepler's laws of planetary motion|different speeds along that orbit]].<ref>"[http://www.astro.uu.nl/~strous/AA/en/antwoorden/seizoenen.html#v466 Astronomy Answers AstronomyAnswerBook: Seasons]," Astronomical Institute, Utrecht University, downloaded 1 August 2008</ref>
=={{anchor|Reckoning}}Four-season reckoning==
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Most calendar-based methods use a four-season model to identify the warmest and coldest seasons, which are separated by two intermediate seasons. Calendar-based reckoning defines the seasons in relative rather than absolute terms. Accordingly, if floral activity is regularly observed during the coolest quarter of the year in a particular area, it is still considered winter despite the traditional association of flowers with spring and summer. Additionally, the seasons are considered to change on the same dates everywhere that uses a particular calendar method regardless of variations in climate from one area to another.
===''Official'' seasons===
As noted, a variety of dates and even exact times are used in different countries or regions to mark changes of the calendar seasons. These observances are often declared "official" within their respective areas by the local or national media, even when the weather or climate is contradictory.<ref name="official spring">{{cite news |url=http://www.cbc.ca/news/canada/canadians-brace-for-a-cold-spring-start-coast-to-coast-1.1363821 |title=Canadians brace for a cold spring start |author=CBC News Canada |year=2013 |access-date=2014-10-01 |publisher=[[CBC News]]}}</ref> However they are mainly a matter of custom only, and have not generally been proclaimed by governments north or south of the equator for civil purposes.<ref name="official seasons">{{cite web |url=http://www.npl.co.uk/science-technology/time-frequency/time/faqs/when-do-the-four-seasons-officially-begin-(faq-time) |title=FAQ-Time |author=NPL |year=2007 |access-date=2014-10-01 |publisher=NPL}}</ref><ref>[http://www.weathersa.co.za/learning/weather-questions/82-how-are-the-dates-of-the-four-seasons-worked-out How are the dates of the four seasons worked out.]</ref> As a result of [[European colonization]], the four-season European model is customarily used almost throughout the world, although the seasons are reversed between the northern and southern hemispheres.
===Meteorological===
{{further|Meteorological winter|Meteorological spring|Meteorological summer|Meteorological fall}}
[[File:Bäume Jahreszeit 2013.jpg|thumb|upright=1.2|Four temperate and subpolar seasons: (above) Winter, Spring, (below) Summer, Autumn/Fall]]
Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. In 1780 the Societas Meteorologica Palatina (which became defunct in 1795), an early international organization for meteorology, defined seasons as groupings of three whole months as identified by the Gregorian calendar. Ever since, professional meteorologists all over the world have used this definition.<ref>{{citation|title=Begin van de lente (Start of Spring)|publisher=KNMI (Royal Dutch Meteorology Institute)|date=2009-03-20|url=http://www.knmi.nl/cms/content/22141/begin_van_de_lente|format=Dutch|access-date=2009-03-20|url-status=dead|archive-url=https://web.archive.org/web/20090327085605/http://www.knmi.nl/cms/content/22141/begin_van_de_lente|archive-date=2009-03-27}}</ref>
Therefore, for temperate areas in the northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December. For the southern hemisphere temperate zone, spring begins on 1 September, summer on 1 December, autumn on 1 March, and winter on 1 June.<ref name="Australian weather and the seasons">{{Cite web |url=http://australia.gov.au/about-australia/australian-story/austn-weather-and-the-seasons |title=Australian weather and the seasons |access-date=2012-10-22 |archive-url=https://web.archive.org/web/20121021091448/http://australia.gov.au/about-australia/australian-story/austn-weather-and-the-seasons |archive-date=2012-10-21 |url-status=dead }}</ref><ref>{{cite web|url=http://www.wildland.com/destinations/south-america/argentina/seasonsclimate.aspx|title=Details - Argentina - Seasons & Climate|website=www.wildland.com|access-date=27 April 2018}}</ref> In Australasia the meteorological terms for seasons apply to the temperate zone that occupies all of [[New Zealand]], [[New South Wales]], [[Victoria (Australia)|Victoria]], [[Tasmania]], the south-eastern corner of [[South Australia]] and the south-west of Western Australia, and the south east [[Queensland]] areas south of [[Brisbane]].
{| class="wikitable"
|+Meteorological temperate seasons
|-
! Northern hemisphere !! Southern hemisphere !! Start date !! End date
|-
| [[Winter]] || [[Summer]] || 1 December || 28 February (29th if leap year)
|-
| [[Spring (season)|Spring]] || [[Autumn]] || 1 March || 31 May
|-
| [[Summer]] || [[Winter]] || 1 June || 31 August
|-
| [[Autumn]] || [[Spring (season)|Spring]] || 1 September || 30 November
|}
In Sweden and Finland, meteorologists and news outlets use the concept of [[thermal season]]s, which are defined based on mean daily temperatures.<ref>[https://web.archive.org/web/20060903170055/https://www.ilmatieteenlaitos.fi/weather/climate_2.html The onsets of the thermal seasons], Finnish Meteorological Institute</ref> The beginning of spring is defined as when the mean daily temperature permanently rises above 0 °C. The beginning of summer is defined as when the temperature permanently rises above +10 °C, autumn as when the temperature permanently falls below +10 °C, and winter as when the temperature permanently falls below 0 °C. In Finland, "permanently" is defined as when the mean daily averaged temperature remains above or below the defined limit for seven consecutive days. (In Sweden the number of days ranges from 5 to 7 depending on the season.) This implies two things:
* the seasons do not begin on fixed dates and must be determined by observation and are known only after the fact
* the seasons begin on different dates in different parts of the country.
{| class="wikitable" style="margin:auto; width:800px;"
|-
! colspan="2" | [[Surface air temperature]]
|-
|<!--col1--> [[File:Surface air temperature global.jpg|400px|right]]
|<!--col2--> [[File:Jones et al. Surface air temperature.jpg|400px|right]]
|-
|<!--col1-->[[Diagram]] was calculated ([[abscisse]]: the 21st of each month).<br />Calculation based on [[data]] published by Jones et al.<ref name="SAT">P. D. Jones et al.: [http://seaice.apl.washington.edu/Papers/JonesEtal99-SAT150.pdf Surface Air Temperature and its Changes Over the Past 150 Years, Figure 7 (Seite 24 von 28 der PDF-Datei)] {{webarchive|url=https://web.archive.org/web/20100716200522/http://seaice.apl.washington.edu/Papers/JonesEtal99-SAT150.pdf |date=2010-07-16 }}</ref>
|<!--col1-->The picture shows ''Figure 7'' as published by Jones et al.<ref name="SAT"/>
|}
The [[India Meteorological Department]] (IMD) designates four climatological seasons:<ref>{{Cite web|url=http://imd.gov.in/section/nhac/wxfaq.pdf|title=FAQ|website=India Meteorological Department}}</ref>
* '''Winter''', occurring from December to February. The year's coldest months are December and January, when temperatures average around {{Convert|10|–|15|C|F}} in the northwest; temperatures rise as one proceeds towards the equator, peaking around {{Convert|20|–|25|C|F}} in mainland India's southeast.
* '''Summer''' or '''pre-monsoon''' season, lasting from March to May. In western and southern regions, the hottest month is April; for northern regions of India, May is the hottest month. Temperatures average around {{Convert|32|–|40|C|F}} in most of the interior.
* '''Monsoon''' or '''rainy''' season, lasting from June to September. The season is dominated by the humid southwest summer monsoon, which slowly sweeps across the country beginning in late May or early June. Monsoon rains begin to recede from North India at the beginning of October. South India typically receives more rainfall.
* '''Post-monsoon''' or '''autumn''' season, lasting from October to November. In the northwest of India, October and November are usually cloudless. Tamil Nadu receives most of its annual precipitation in the northeast monsoon season.
===Astronomical===
{{solstice-equinox}}
Astronomical timing as the basis for designating the temperate seasons dates back at least to the [[Julian calendar]] used by the ancient Romans.{{cn|date=April 2021}} It continues to be used worldwide, although some countries like Australia, New Zealand,<ref>{{cite news |last1=Deguara |first1=Brittney |title=When does winter officially start in New Zealand? |url=https://www.stuff.co.nz/national/113038178/when-does-winter-officially-start-in-new-zealand |access-date=4 October 2020 |work=Stuff |date=27 May 2019 |language=en}}</ref> Pakistan and Russia prefer to use meteorological reckoning. The precise timing of the seasons is determined by the exact times of the sun reaching the tropics of Cancer and Capricorn for the [[solstice]]s and the times of the sun's transit over the equator for the [[equinox]]es, or a traditional date close to these times.<ref>{{cite web|url=http://aa.usno.navy.mil/data/docs/EarthSeasons.php|title=Earth's Seasons|publisher=The United States Naval Observatory (USNO)|date=September 21, 2015|website=Astronomical Applications Department|access-date=June 23, 2017}}</ref>
The following diagram shows the relation between the line of solstice and the line of [[apsides]] of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the [[apsis#Earth's perihelion and aphelion|perihelion]] (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 19, 20 or 21 March, the point of June solstice on 20 or 21 June, the [[apsis#Earth's perihelion and aphelion|aphelion]] (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.
[[File:Seasons1.svg|thumb|center|middle|upright=3|alt=Illustration of seasonal distances from Earth to the Sun|Note: Distances are exaggerated and not to scale]]
These "astronomical" seasons are not of equal length, because of the [[ellipse|elliptical nature]] of the orbit of the Earth, as discovered by [[Johannes Kepler]]. From the March equinox it currently takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. Thus the time from the March equinox to the September equinox is 7.56 days longer than from the September equinox to the March equinox.
====Variation due to calendar misalignment====
The times of the equinoxes and solstices are not fixed with respect to the modern Gregorian calendar, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to keep the March equinox no later than 21 March as accurately as is practical. ''Also see: [[Gregorian calendar#Calendar seasonal error|Gregorian calendar seasonal error]].''
The calendar equinox (used in the calculation of Easter) is 21 March, the same date as in the Easter tables current at the time of the Council of Nicaea in AD 325. The calendar is therefore framed to prevent the astronomical equinox wandering onto 22 March. From Nicaea to the date of the reform, the years 500, 600, 700, 900, 1000, 1100, 1300, 1400 and 1500, which would not have been leap years in the Gregorian calendar, amount to nine days, but astronomers directed that ten days be removed.
Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23 and December 21; the four-year average slowly shifts to earlier times as the century progresses. This shift is a full day in about 128 years (compensated mainly by the century "leap year" rules of the Gregorian calendar) and as 2000 was a leap year the current shift has been progressing since the beginning of the last century, when equinoxes and solstices were relatively late. This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates.
Note that all the times are given in [[Coordinated Universal Time|UTC]] (roughly speaking, the time at [[Greenwich]], ignoring [[British Summer Time]]). People living farther to the east (Asia and Australia), whose local times are in advance, will see the astronomical seasons apparently start later; for example, in [[Tonga]] (UTC+13), an equinox occurred on September 24, 1999, a date which will not crop up again until 2103. On the other hand, people living far to the west (America) whose clocks run behind UTC may experience an equinox as early as March 19.
==
Over thousands of years, the Earth's [[axial tilt]] and orbital eccentricity vary (see [[Milankovitch cycles]]). The equinoxes and solstices move westward relative to the stars while the perihelion and aphelion move eastward. Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change — the average temperature difference between summer and winter in location — will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees.
Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
===
[[File:Phases of the Sun (NHemi).png|thumb|right|The annual cycle of insolation (Sun energy, shown in blue) with key points for seasons (middle), [[quarter days]] (top) and [[cross-quarter days]] (bottom) along with months (lower) and Zodiac houses (upper). The cycle of temperature (shown in pink) is delayed by [[seasonal lag]].]]
Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. It was the method for reckoning seasons in medieval Europe, especially by the [[Celts]], and is still ceremonially observed in [[Republic of Ireland|Ireland]] and some East Asian countries. Summer is defined as the quarter of the year with the greatest insolation and winter as the quarter with the least.
The solar seasons change at the cross-quarter days, which are about 3–4 weeks earlier than the meteorological seasons and 6–7 weeks earlier than seasons starting at equinoxes and solstices. Thus, the day of greatest insolation is designated "midsummer" as noted in [[William Shakespeare]]'s play ''[[A Midsummer Night's Dream]]'', which is set on the summer solstice. On the [[Celtic calendar]], the start of the seasons corresponds to four [[Pagan]] agricultural festivals - the traditional first day of winter is 1 November ([[Samhain]], the Celtic origin of [[Halloween]]); spring starts 1 February ([[Imbolc]], the Celtic origin of [[Groundhog Day]]); summer begins 1 May ([[Beltane]], the Celtic origin of [[May Day]]); the first day of autumn is 1 August (Celtic [[Lughnasadh]]).
{| class="wikitable"
|+Irish seasons
|-
! Season !! Start date !! End date
|-
| [[Winter]] || 1 November ([[All Saints' Day]]) || 31 January
|-
| [[Spring (season)|Spring]] || 1 February ([[St. Brigid's Day]]) || 30 April
|-
| [[Summer]] || 1 May ([[May Day]]) || 31 July
|-
| [[Autumn]] || 1 August ([[Lughnasadh]]) || 31 October ([[Hallowe'en]])
|}
===
{{anchor|China|Japan|East Asia}}<!--linked-->
The [[Chinese calendar|traditional calendar in China]] has 4 seasons based on 24 periods known as [[solar terms]].<ref>{{cite web|url=http://www.friesian.com/chinacal.htm|title=The Solar Terms and the Chinese 60 Year Calendar Cycle|publisher=friesian.com|access-date=2010-12-03|last=Ross|first=Kelley L}}</ref> The four seasons ''chūn'' ({{lang|zh|{{linktext|春}}}}), ''xià'' ({{lang|zh|{{linktext|夏}}}}), ''qiū'' ({{lang|zh|{{linktext|秋}}}}), and ''dōng'' ({{lang|zh|{{linktext|冬}}}})—universally translated as "spring", "summer", "autumn", and "winter"—each center around the respective solstice or equinox. Astronomically, the seasons are said to begin on Lichun ({{lang|zh|{{linktext|立春}}}}, "the start of spring") on about 4 February, Lixia ({{lang|zh|{{linktext|立夏}}}}) on about 6 May, Liqiu ({{lang|zh|{{linktext|立秋}}}}) on about 8 August, and Lidong ({{lang|zh|{{linktext|立冬}}}}) on about 7 November. These dates were not part of the traditional lunar calendar, however, and moveable holidays such as [[Chinese New Year]] and the [[Mid-Autumn Festival]] are more closely associated with the seasons. It forms the basis of other such systems in [[East Asia]]n [[lunisolar calendar]]s.
==Six season reckoning==
Some calendars in south Asia use a six-season method where the number of seasons between summer and winter can number from one to three. The dates are fixed at even intervals of months.
In the [[Hindu calendar]] of tropical and subtropical India, there are six seasons or [[Ritu (Indian season)|Ritu]] that are calendar-based in the sense of having fixed dates: [[Vasanta (Ritu)|Vasanta]] (spring), Greeshma (summer), [[Ritu (Indian season)|Varsha]] ([[monsoon]]), [[Sharad]] (autumn), Hemanta (early winter), and Shishira (prevernal or late winter). The six seasons are ascribed to two months each of the twelve months in the Hindu calendar. The rough correspondences are:
{| class="wikitable"
|-
! Hindu season
! Start
! End
! Hindu Months
! Mapping to English Names
|-
| [[Vasanta (Ritu)|Vasanta]]
| Mid-March
| Mid-May
| [[Chaitra]], [[Vaishakha]]
| spring
|-
| Greeshma
| Mid-May
| Mid-July
| [[Jyeshtha]], [[Ashadha]]
| summer
|-
| Varsha
| Mid-July
| Mid-September
| [[Shraavana]], [[Bhadrapada]]
| monsoon
|-
| Sharad
| Mid-September
| Mid-November
| [[Ashvin]], [[Kartika (month)|Kartika]]
| autumn
|-
| Hemanta
| Mid-November
| Mid-January
| [[Maargashirsha]], [[Pushya]]
| early winter
|-
| Shishira
| Mid-January
| Mid-March
| [[Maagha|Magh]], [[Phalguna]]
| prevernal or late winter
|}
The [[Bengali Calendar]] is similar but differs in start and end times. It has the following seasons or ritu:
{| class="wikitable
|-
! Bengali season
! Start
! End
! Bengali Months
! Mapping to English Names
|-
| Bosonto বসন্ত (ঋতু) (Spring)
| Mid-February
| Mid-April
| [[Falgun]], [[Choitro]]
| [[Spring (Season)|Spring]]
|-
| Grishmo (গ্রীষ্ম) (Summer)
| Mid-April
| Mid-June
| [[Boishakh]], [[Joishtho]]
| [[Summer]]
|-
| Borsha (বর্ষা) (Monsoon)
| Mid-June
| Mid-August
| [[Asharh]], [[Srabon]]
| [[Monsoon]]
|-
| Shorot (শরৎ) (Autumn/ Fall)
| Mid-August
| Mid-October
| [[Bhadro]], [[Ashwin]]
| [[Autumn]]
|-
| Hemonto (হেমন্ত) (Frost/ Late Autumn)
| Mid-October
| Mid-December
| [[Kartika (month)|Kartik]], [[Ogrohayon]]
| [[Dry season|Late Autumn]]
|-
| Sit (শীত) (Winter)
| Mid-December
| Mid-February
| [[Poush]], [[Magh (Bengali calendar)|Magh]]
| [[Winter]]
|}
The [[Tamil calendar]] follows a similar pattern of six seasons
{| class="wikitable"
|-
!Tamil season
!Gregorian Months
!Tamil Months
|-
|MuthuVenil (Summer)
|April 15 to June 14
|Chithirai and Vaikasi
|-
|Kaar (Monsoon)
|June 15 to August 14
|Aani and Aadi
|-
|Kulir (Autumn)
|August 15 to October 14
|Avani and Purattasi
|-
|MunPani (Winter)
|October 15 to December 14
|Aipasi and Karthikai
|-
|PinPani (Prevernal)
|December 15 to February 14
|Margazhi and Thai
|-
|IlaVenil (Spring)
|February 15 to April 14
|Maasi and Panguni
|}
==Non-calendar-based reckoning==
[[File:Jahreszeiten Jahresringe.jpg|thumb|The six modern mid-latitude ecological seasons.<br> From bottom, clockwise:<br> prevernal, vernal, estival, serotinal, autumnal, hibernal]]
[[File:365 days tree.ogv|thumb|Seasonal changes regarding a tree over a year]]
Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; [[hedgehog]]s hibernate—winter). So, if we can observe a change in daily floral/animal events, the season is changing. In this sense, ecological seasons are defined in absolute terms, unlike calendar-based methods in which the seasons are relative. If specific conditions associated with a particular ecological season don't normally occur in a particular region, then that area cannot be said to experience that season on a regular basis.
In Great Britain, the onset of spring used to be defined{{When|date=February 2021}} as when the maximum daily temperature reached 50°F (10°C) in a defined sequence of days. This almost always occurred in March. However, with [[global warming]] this temperature is now not uncommon in the winter.{{citation needed|date=September 2020}}
===Modern mid-latitude ecological===
Six ecological seasons can be distinguished which do not have fixed calendar-based dates like the meteorological and astronomical seasons.<ref>{{cite web | author = Michael Allaby | title = A Dictionary of Zoology | year = 1999 | url = http://www.encyclopedia.com/doc/1O8-prevernal.html | access-date= 2012-05-30}}</ref> [[Oceanic climate|Oceanic]] regions tend to experience the beginning of the hibernal season up to a month later than [[Köppen climate classification#Group D: Continental/microthermal climates|continental climates]]. Conversely, prevernal and vernal seasons begin up to a month earlier near oceanic and coastal areas. For example, prevernal [[crocus]] blooms typically appear as early as February in coastal areas of [[British Columbia]], the [[British Isles]], but generally don't appear until March or April in locations like the [[Midwest]] USA or parts of eastern [[Europe]]. The actual dates for each season vary by climate region and can shift from one year to the next. Average dates listed here are for mild and cool temperate climate zones in the Northern Hemisphere:
* '''Prevernal''' (early or pre-spring): Begins February (mild temperate), to March (cool temperate). Deciduous tree buds begin to swell. Some types of migrating birds fly from winter to summer habitats.
* '''Vernal''' (spring): Begins mid March (mild temperate), to late April (cool temperate). Tree buds burst into leaves. Birds establish territories and begin mating and nesting.
* '''Estival''' (high summer): Begins June in most temperate climates. Trees in full leaf. Birds hatch and raise offspring.
* '''Serotinal''' (late summer): Generally begins mid to late August. Deciduous leaves begin to change color in higher latitude locations (above 45 north). Young birds reach maturity and join other adult birds preparing for autumn migration. The traditional "harvest season" begins by early September.
* '''Autumnal''' (autumn): Generally begins mid to late September. Tree leaves in full color then turn brown and fall to the ground. Birds migrate back to wintering areas.
* '''Hibernal''' (winter): Begins December (mild temperate), November (cool temperate). Deciduous trees are bare and fallen leaves begin to decay. Migrating birds settled in winter habitats.
===Indigenous ecological===
{{See also|Indigenous Australian seasons}}
Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined the seasons ecologically by observing the activity of the plants, animals and weather around them. Each separate tribal group traditionally observes different seasons determined according to local criteria that can vary from the hibernation of polar bears on the arctic tundras to the [[growing season]]s of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year,<ref name="Australian weather and the seasons"/> as do the Sami people in Scandinavia. Many indigenous people who no longer live directly off the land in traditional often nomadic styles, now observe modern methods of seasonal reckoning according to what is customary in their particular country or region.
The North American [[Cree]] and possibly other [[Algonquian languages|Algonquian]] speaking peoples used or still use a 6-season system. The extra two seasons denoting the freezing and breaking up of the ice on rivers and lakes.<ref>{{cite book|author=Rose Roberts|title=Radical Human Ecology: Intercultural and Indigenous Approaches|url=https://books.google.com/books?id=zgLtCwAAQBAJ&pg=PT350|year=2016|publisher=Routledge|isbn=978-1-317-07191-4|page=350}}</ref>
{| class="wikitable"
|-
!Cree season
!Approximate months
!English translation
|-
|Pipon
|Jan/Feb
|Winter
|-
|Sekwun
|Mar/Apr
|Break-up
|-
|Mithoskumin
|May/Jun
|Spring
|-
|Nepin
|Jul/Aug
|Summer
|-
|Tukwakin
|Sep/Oct
|Autumn
|-
|Mikiskaw
|Nov/Dec
|Freeze-up
|}
The [[Noongar]] people of South-West Western Australia recognise maar-keyen bonar,<ref>{{cite web|url=https://courses.edx.org/courses/course-v1:CurtinX+CAS1x+2T2018/8eac4c85693d418e9edc2a39081723de/ |title=Noongar Glossary |publisher=Courses.edx.org |access-date=2019-03-21}}</ref> or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors<ref>{{cite news|url=http://www.abc.net.au/news/2018-03-16/perth-noongar-have-six-seasons-autumn-second-summer/9550678 |title=Perth's Noongar seasons explain why autumn feels like a second summer |publisher=Abc.net.au |date=2018-03-16 |access-date=2019-03-21}}</ref> such as changing winds, flowering plants, temperature and migration patterns and lasts approximately two standard calendar months. The seasons also correlate to aspects of the human condition, intrinsically linking the lives of the people to the world that surrounds them and also dictating their movements, as with each season, various parts of country would be visited which were particularly abundant or safe from the elements.<ref>{{cite web|url=https://www.noongarculture.org.au/food/ |title=Kaartdijin Noongar |work=Noongarculture.org.au |access-date=2019-03-21}}</ref>
{| class="wikitable"
|-
!Noongar season
!Approximate Months
!Cultural Parallel
|-
|Birak (First Summer)
|December to January
|Season of the Young
|-
|Bunuru (Second Summer)
|February to March
|Season of Adolescence
|-
|Djeran (Autumn)
|April to May
|Season of Adulthood
|-
|Makuru (The First Rains)
|June to July
|Fertility Season
|-
|Djilba (The Second Rains)
|August to September
|Season of Conception
|-
|Kambarang (Wildflower Season)
|October to November
|Season of Birth
|}
===Tropical===
[[File:Diamond Head Hike - Wet & Dry Seasons.jpg|thumb|upright=1.6|Wet and dry seasons]]
====Two seasons====
In the tropics, where seasonal dates also vary, it is more common to speak of the [[wet season|rainy]] (or wet, or [[monsoon]]) season versus the [[dry season]]. For example, in [[Nicaragua]] the dry season (November to April) is called "summer" and the rainy season (May to October) is called "winter", even though it is located in the northern hemisphere. There is no noticeable change in the amount of sunlight at different times of the year. However, many regions (such as the northern [[Indian ocean]]) are subject to [[monsoon]] [[rain]] and wind cycles.
Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season. Thus, the tropics are characterized by numerous "mini-seasons" within the larger seasonal blocks of time.
In the tropical parts of Australia in the northern parts of [[Queensland]], [[Western Australia]], and the [[Northern Territory]], wet and dry seasons are observed in addition to or in place of temperate season names.<ref>{{cite web|url=http://www.australia.gov.au/about-australia/australian-story/austn-weather-and-the-seasons|title=Australian weather and the seasons|publisher=Australian Government|access-date=2016-02-26|url-status=dead|archive-url=https://web.archive.org/web/20151104195910/http://www.australia.gov.au/about-australia/australian-story/austn-weather-and-the-seasons|archive-date=2015-11-04}}</ref>
{| class="wikitable"
|+Meteorological Tropical seasons
|-
! Northern Hemisphere !! Southern Hemisphere !! Start date !! End date
|-
| [[Dry season]] || [[Wet season]] || 1 November || 30 April
|-
| [[Wet season]] || [[Dry season]] || 1 May || 31 October
|}
====Three seasons====
The most historically important of these are the three 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]]. In some tropical areas a three-way division into hot, rainy, and cool season is used. In Thailand three seasons are recognised <ref>[http://www.royin.go.th/?knowledges=%E0%B8%84%E0%B8%B3%E0%B9%80%E0%B8%A3%E0%B8%B5%E0%B8%A2%E0%B8%81%E0%B8%8A%E0%B8%B7%E0%B9%88%E0%B8%AD%E0%B8%A4%E0%B8%94%E0%B8%B9%E0%B8%95%E0%B9%88%E0%B8%B2%E0%B8%87-%E0%B9%86 Royal Thai Institute]</ref>
{| class="wikitable"
|-
!Thai season
!Months
|-
|Ruedu nao (cold season)
|mid October to mid February
|-
|Ruedu ron (hot season)
|mid February to mid May
|-
|Ruedu fon (rainy season)
|mid May to mid October
|}
===Polar===
Any point north of the [[Arctic Circle]] or south of the [[Antarctic Circle]] will have one period in the summer called "polar day" when the sun does not set, and one period in the winter called 'polar night' when the sun does not rise. At progressively higher latitudes, the maximum periods of "[[midnight sun]]" and "[[polar night]]" are progressively longer.
For example, at the military and weather station [[Alert, Nunavut|Alert]] located at 82°30′05″N and 62°20′20″W, on the northern tip of [[Ellesmere Island]], [[Canada]] (about 450 [[nautical mile]]s or 830 km from the North Pole), the sun begins to peek above the horizon for minutes per day at the end of February and each day it climbs higher and stays up longer; by 21 March, the sun is up for over 12 hours. On 6 April the sun rises at 0522 [[UTC]] and remains above the horizon until it sets below the horizon again on 6 September at 0335 UTC. By October 13 the sun is above the horizon for only 1 hour 30 minutes, and on October 14 it does not rise above the horizon at all and remains below the horizon until it rises again on 27 February.<ref>{{cite web|url=http://aa.usno.navy.mil/|title=U.S. Naval Observatory|access-date=27 April 2018}}
</ref>
First light comes in late January because the sky has [[twilight]], being a glow on the horizon, for increasing hours each day, for more than a month before the sun first appears with its disc above the horizon. From mid-November to mid-January, there is no twilight.
In the weeks surrounding 21 June, in the northern polar region, the sun is at its highest elevation, appearing to circle the sky there without going below the horizon. Eventually, it does go below the horizon, for progressively longer periods each day until around the middle of October, when it disappears for the last time until the following February. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, from mid-November to mid-January, there is no twilight and it is continuously dark. In mid January the first faint wash of twilight briefly touches the horizon (for just minutes per day), and then twilight increases in duration with increasing brightness each day until sunrise at end of February, then on 6 April the sun remains above the horizon until mid October.
===Military campaigning seasons===
Seasonal weather and climate conditions can become important in the context of military operations. Seasonal reckoning in the military of any country or region tends to be very fluid and based mainly on short to medium term weather conditions that are independent of the calendar.
For navies, the presence of accessible ports and bases can allow naval operations during certain (variable) seasons of the year. The availability of [[ice-free harbor | ice-free]] or [[warm-water port]]s can make navies much more effective. Thus [[Russia]], historically navally constrained when confined to using [[Arkhangelsk]] (before the 18th century) and even [[Kronstadt]], has particular interests in maintaining and in preserving access to [[Baltiysk]], [[Vladivostok]], and [[Sevastopol]].<ref>
{{cite book
| editor1-last = Stokke
| editor1-first = Olav Schram
| editor2-last = Tunander
| editor2-first = Ola
| editor2-link = Ola Tunander
| editor3-last = Nansen
| editor3-first = Fridtjof
| editor3-link = Fridtjof Nansen
| title = The Barents region: cooperation in Arctic Europe
| url = https://books.google.com/books?id=KZ9mAAAAMAAJ
| series = International Peace Research Institute, Oslo Prio Series
| volume = 10
| publisher = SAGE
| date = 1994
| page = 98
| edition = reprint
| access-date = 21 August 2019
| quote = Witte wanted the main base of the Russian Navy to be situated at a location where waters were ice-free the whole year round [...].
| isbn = 9780803978973
}}
</ref>
Storm seasons or polar winter-weather conditions can inhibit surface warships at sea.
Pre-modern armies, especially in Europe, tended to campaign in the summer months - peasant [[conscription | conscripts]] tended to melt away at [[harvest time]], nor did it make economic sense in an agricultural society to neglect the [[sowing]] season.<ref>
{{cite book
| chapter = Games and war in Ancient Greece: Discussion
| editor1-last = Cornell
| editor1-first = Tim J.
| editor1-link = Tim Cornell
| editor2-last = Allen
| editor2-first = Thomas B.
| title = War and Games
| url = https://books.google.com/books?id=nQAXzSJsqKcC
| series = Studies on the nature of war
| volume = 3
| location = Woodbridge, Suffolk
| publisher = Boydell Press
| date = 2002
| page = 34
| isbn = 9780851158709
| access-date = 21 August 2019
| quote = The warriors are farmers and the farmers are warriors, so you have to time your war in relation to the harvest and to other agricultural events. [...] Spring and autumn marked the beginning and end of the campaign season, and the rest of the year was closed for war.
}}
</ref>
Any modern [[Manoeuvre warfare | war of manouevre]] profits from firm ground - summer can provide dry conditions suitable for marching and transport, frozen snow in winter can also offer a reliable surface for a period, but [[rasputitsa | spring thaws or autumn rains]] can inhibit campaigning. Rainy-season floods may make rivers temporarily impassable, and winter snow tends to block mountain passes. Taliban offensives are usually confined to the [[Afghan Fighting Season]].
==See also==
* [[Horae]], Greek goddesses of seasons
* [[Indian summer]]
* [[Persephone]], Greek mythological figure associated with the rebirth of vegetation in the spring
* [[Sun path]]
* [[Vertumnus]], Roman god of the seasons
==Notes==
{{reflist|group=note}}
==References==
{{
* Maris, Mihaela, St. Luchian School, Bacau, Romania, ''Seasonal Variations of the Bird Species'', ref. ecological seasons pp. 195–196 incl. and pp. 207–209 incl.
==External links==
{{external links|date=December 2015}}
{{Commons category|Seasons|seasons}}
{{Wikiquote|Seasons|seasons}}
{{NIE Poster|Seasons}}
* [http://www.badastronomy.com/bad/misc/badseasons.html When do the Seasons Begin?] (from the [[Bad Astronomer]])
* [https://www.bbc.co.uk/dna/h2g2/A526673 Why the Earth has seasons] article on [[h2g2]].
* [https://web.archive.org/web/20061203055805/http://www.deh.gov.au/parks/kakadu/artculture/seasons.html Aboriginal seasons of Kakadu]
* [http://www.bom.gov.au/iwk/climate_culture/Indig_seasons.shtml Indigenous seasons (Australian Bureau of Meteorology)]
* [https://web.archive.org/web/20120302222040/http://www.stirling.au.com/educ/traditional_culture.pdf Mt Stirling Seasons]
* [http://www.abc.net.au/science/features/indigenous/ The Lost Seasons]
* [http://www.herringisland.org/seasons.htm Melbourne's six seasons]
* [http://outreach.as.utexas.edu/marykay/highschool/EarthSeason.doc Tutorial on Earth/Sun Relations and Seasons]
* [http://www.sunpreview.com Sunpreview Season Forecast Project]
* [http://svs.gsfc.nasa.gov/vis/a000000/a003200/a003272/ Satellite photo demonstrating seasons changes in 2004 on NASA website]
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