Season

A season is a division of the year [1] marked by changes in weather, ecology, and amount of daylight. Seasons result from Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. [2] [3] 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 migrate, and plants to be dormant.

Red and green trees in autumn (fall)

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 Earth's axial tilt 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 warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere.

In temperate and subpolar regions, four calendar-based seasons are generally recognized: spring, summer, autumn or fall, and winter. Ecologists often use a six-season model for temperate climate regions: prevernal, vernal, estival, serotinal, autumnal, and hibernal. Many tropical regions have two seasons: the rainy, wet, or monsoon season and the dry season. Some have a third cool, mild, or harmattan season. Seasons often held special significance for agrarian societies, whose lives revolved around planting and harvest times, and the change of seasons was often attended by ritual.

A deciduous tree in winter
The six ecological seasons
The four calendar seasons, depicted in an ancient Roman mosaic from Tunisia.

In some parts of the world, some other "seasons" capture the timing of important ecological events such as hurricane season, tornado season, and wildfire season.[ citation needed] The most historically important of these are the three seasons— flood, growth, and low water—which were previously defined by the former annual flooding of the Nile in Egypt.

A deciduous tree in autumn (fall)

Causes and effects

Illumination of the earth at each change of astronomical season
Fig. 1
This diagram shows how the tilt of the 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 incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle.

Axis tilt

The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.5 degrees. [4] (This tilt is also known as "obliquity of the ecliptic".)

Regardless of the time of year, the northern and southern hemispheres 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 (see Fig. 1) than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 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 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the northern and southern hemispheres. Around the March equinox, the northern hemisphere will be experiencing 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 of the Sun at noon (the culmination of the Sun) during a year. The low angle of Sun during the winter months means that incoming rays of solar radiation are 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.

A deciduous tree in winter

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. [5] In fact, Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (farthest point from the Sun) in July, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the northern hemisphere. [6] In general, the effect of orbital eccentricity on Earth's seasons is a 7% variation in sunlight received.

A tree in summer

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. [6] Any noticeable intensification of the southern hemisphere's winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the southern hemisphere. [7]

Maritime and hemispheric

Seasonal weather fluctuations (changes) also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds.

A deciduous tree in autumn (fall)

In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often causes 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 cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and vice versa.

Tropics

In tropical and subtropical regions there is little annual fluctuation of sunlight. However, there are seasonal shifts of a rainy global-scale low pressure belt called the Intertropical convergence zone. As a result, the amount of precipitation tends to vary more dramatically than the average temperature. When the convergence zone is north of the equator, the tropical areas of the northern hemisphere experience their wet season while the tropics south of the equator have their dry season. This pattern reverses when the convergence zone migrates to a position south of the equator.

Mid-latitude thermal lag

In meteorological terms, the summer solstice and winter solstice (or the maximum and minimum insolation, respectively) do not fall in the middles of summer and winter. The heights of these seasons occur up to seven weeks later because of seasonal lag. Seasons, though, are not always defined in meteorological terms.

In astronomical reckoning by hours of daylight alone, the solstices and equinoxes 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 days considered seasonal midpoints. The length of these seasons is not uniform because of the elliptical orbit of the earth and its different speeds along that orbit. [8]

Other Languages
Afrikaans: Seisoen
العربية: فصول السنة
অসমীয়া: ঋতু
Avañe'ẽ: Arajere
azərbaycanca: İlin fəsilləri
বাংলা: ঋতু
Bân-lâm-gú: Kùi-chiat
беларуская: Пара года
भोजपुरी: सीजन
български: Сезон
བོད་ཡིག: ནམ་དུས།
bosanski: Godišnje doba
brezhoneg: Rannvloaz
čeština: Roční období
chiShona: Mwaka yepaNyika
corsu: Staghjone
Cymraeg: Tymor
dansk: Årstid
Deutsch: Jahreszeit
eesti: Aastaajad
Ελληνικά: Εποχές
Esperanto: Sezono
euskara: Urtaro
فارسی: فصل
français: Saison
Frysk: Jiertiid
furlan: Stagjon
客家語/Hak-kâ-ngî: Kui-chiet
한국어: 계절
हिन्दी: ऋतु
hrvatski: Godišnja doba
Ido: Sezono
Bahasa Indonesia: Musim
interlingua: Station del anno
íslenska: Árstíð
italiano: Stagione
ಕನ್ನಡ: ಋತು
къарачай-малкъар: Джылны чакълары
Kreyòl ayisyen: Sezon
Kurdî: Demsal
Kwanyama: Omafimbo odula
лакку: Шинал чIун
Latina: Tempora anni
latviešu: Gadalaiks
lietuvių: Metų laikai
Limburgs: Sezoen
lingála: Eleko
magyar: Évszak
मैथिली: ऋतु
македонски: Годишно време
Malagasy: Fizaran-taona
മലയാളം: ഋതു
मराठी: ऋतू
Bahasa Melayu: Musim
မြန်မာဘာသာ: ရာသီ
Nederlands: Seizoen
Nēhiyawēwin / ᓀᐦᐃᔭᐍᐏᐣ: Kâ-isiwepahki
नेपाली: ऋतु
नेपाल भाषा: ऋतु
日本語: 季節
Nordfriisk: Juarstidj
norsk: Årstid
norsk nynorsk: Årstid
Nouormand: Saîson
occitan: Sason
ਪੰਜਾਬੀ: ਰੁੱਤ
Pangasinan: Season
پښتو: موسم
polski: Pora roku
português: Estação do ano
Ripoarisch: Joohreszitt
română: Anotimp
Runa Simi: Mit'a
русский: Времена года
саха тыла: Дьыл
संस्कृतम्: ऋतुः
Scots: Saison
Sesotho sa Leboa: Sehla
shqip: Stina
sicilianu: Staciuna
Simple English: Season
slovenčina: Ročné obdobie
slovenščina: Letni časi
Soomaaliga: Afarta xilli
کوردی: وەرز
српски / srpski: Годишње доба
srpskohrvatski / српскохрватски: Godišnje doba
Basa Sunda: Usum
suomi: Vuodenaika
svenska: Årstid
Tagalog: Pana-panahon
татарча/tatarça: Ел фасыллары
తెలుగు: రుతువు
Türkçe: Mevsim
Türkmençe: Pasyl
українська: Пора року
اردو: فصول
ئۇيغۇرچە / Uyghurche: پەسىل
Tiếng Việt: Mùa
walon: Såjhon
文言: 四時
吴语: 季節
粵語:
Zazaki: Mewsım
žemaitėška: Metu laikā
中文: 季节
डोटेली: ऋतु