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{{short description|Subdivision of the year based on orbit and axial tilt}}
{{Other uses}}
{{Weather|expanded=Calendar seasons}}
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 |
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]].
In [[temperate]] and [[Subpolar climate|sub-polar]] regions, four seasons based on the [[Gregorian calendar]] are generally recognized: ''[[Spring (season)|spring]]'', ''[[summer]]'', ''[[autumn]]'' (
{{Multiple image
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[[File:North season.jpg|thumb|Axial parallelism is a characteristic of the Earth (and most other orbiting bodies in space) in which the direction of the axis remains parallel to itself throughout its orbit.]]
The Earth's orbit exhibits approximate [[axial parallelism]], maintaining its direction
===Axial tilt===
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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.▼
▲The effect of axial tilt is observable as the change in [[day length]] and the [[altitude (astronomy)|altitude]] of the Sun at solar [[noon]] (the Sun's [[culmination]]) during the [[year]]. The low angle of the 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">
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===Tropics===
[[Image:BlueMarble monthlies animation.gif|thumb|right|Animation of seasonal differences especially [[snow]] cover through the year]]
The [[tropics|tropical]] and (to a lesser degree) [[subtropical]] regions see little annual fluctuation of sunlight and temperature due Earth's moderate 23.4-degree tilt being insufficient to appreciably
===Mid-latitude thermal lag===
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===Official===
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>
===<span class="anchor" id="Meteorological"></span>Meteorological===
{{further|Winter|Spring (season)|Summer|Autumn}}
[[File:Bäume Jahreszeit 2013.jpg|thumb|upright=1.2|Four temperate and subpolar seasons: (above) winter, spring, (below) summer, autumn]]
'''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.<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>
According to this definition, 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]].
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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|archive-url=https://web.archive.org/web/20191231045616/http://imd.gov.in/section/nhac/wxfaq.pdf |archive-date=2019-12-31 |url-status=dead}}</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
* '''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.
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====Variation due to calendar misalignment====
{{See also|Gregorian calendar#Calendar seasonal error}}
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.
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 extra days, but astronomers directed that ten days be removed. Because of this, the ([[Proleptic Gregorian calendar|proleptic]]) Gregorian calendar agrees with the Julian calendar in the [[third century]] of the [[Christian era]], rather than in the fourth.
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{{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 on 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
==Six-season reckoning==
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Some calendars in south Asia use a six-season partition 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 (
{| class="wikitable"
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! Mapping to English names
|-
| [[Vasanta (
| Mid-March
| Mid-May
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| spring
|-
| [[Grishma
| Mid-May
| Mid-July
| [[Jyeshtha (month)|Jyeshtha]], [[Ashadha]]
| summer
|-
| [[Varsha (season)|Varshā]]
| Mid-July
| Mid-September
| [[
| monsoon
|-
| [[
| Mid-September
| Mid-November
| [[Ashvin (month)|Ashvin]], [[Kartika (month)|Kartika]]
| autumn
|-
| [[Hemanta (season)|Hemanta]]
| Mid-November
| Mid-January
| [[
| early winter or late autumn
|-
| [[
| Mid-January
| Mid-March
| [[
| prevernal or late winter
|}
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! Bengali months
! Mapping to English names
|-▼
| Bôsônto বসন্ত(spring)▼
| Mid-February▼
| Mid-April▼
| [[Falgun]], [[Choitro]]▼
| [[Spring (Season)|Spring]]▼
|-
| Greeshmo (গ্রীষ্ম)(summer)
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| [[Poush]], [[Magh (Bengali calendar)|Magh]]
| [[Winter]]
▲|-
▲| Bôsônto বসন্ত(spring)
▲| Mid-February
▲| Mid-April
▲| [[Falgun]], [[Choitro]]
▲| [[Spring (Season)|Spring]]
|}
The [[Odia calendar|Odia Calendar]] is similar but differs in start and end times.
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==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
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
===Modern mid-latitude ecological===
Six ecological seasons can be distinguished without 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 do not appear until March or April in locations like the [[
* '''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.
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====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.
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.
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====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>{{Cite web |url=http://
{| class="wikitable"
|