Season
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A season is a division of the year[1] 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 tilted orbit around the Sun.[2][3][4] 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. 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
In
.Seasons often hold special significance for agrarian societies, whose lives revolve around
Causes and effects
Axial parallelism
The Earth's orbit exhibits approximate axial parallelism, maintaining its direction toward Polaris (the "North Star") year-round. This is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right.[5][6][7][8] Minor variation in the direction of the axis, known as axial precession, takes place over the course of 26,000 years, and therefore is not noticeable to modern human civilization.
Axial tilt
The seasons result from the Earth's
Regardless of the time of year, the
The effect of axial tilt is observable as the change in
-
Illumination of Earth by Sun at the northern solstice.
-
Illumination of Earth by Sun at the southern solstice.
-
Illumination of Earth at each change of astronomical season
-
Animation of Earth as seen daily from the Sun looking at UTC+02:00, showing the solstice and changing seasons.
-
Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m2).
Elliptical Earth orbit
Compared to axial parallelism and axial tilt, other factors contribute little to seasonal temperature changes.
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.[11] 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.[12]
Maritime and hemispheric
Seasonal weather fluctuations (changes) also depend on factors such as proximity to
In the
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
The
Mid-latitude thermal lag
In meteorological terms, the
In
Four-season reckoning
Most calendar-based partitions use a four-season model to demarcate the warmest and coldest seasons, which are further separated by two intermediate seasons. Calendar-based reckoning defines the seasons in relative rather than absolute terms, so the coldest quarter-year is considered winter even if floral activity is regularly observed during it, despite the traditional association of flowers with spring and summer. The major exception is in the tropics where, as already noted, the winter season is not observed.
The four seasons have been in use since at least Roman times, as in
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.[15] These are mainly a matter of custom and not generally proclaimed by governments north or south of the equator for civil purposes.[16][17]
Meteorological
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.[18]
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.
Northern hemisphere | Southern hemisphere | Start date | End date |
---|---|---|---|
Winter | Summer | 1 December | 28 February (29th if leap year) |
Spring | Autumn | 1 March | 31 May |
Summer | Winter | 1 June | 31 August |
Autumn | Spring | 1 September | 30 November |
In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, which are defined based on mean daily temperatures.[21] 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.
Surface air temperature
| |
---|---|
The picture shows Figure 7 as published by Jones et al.[22] |
The India Meteorological Department (IMD) designates four climatological seasons:[23]
- Winter, occurring from December to February. The year's coldest months are December and January, when temperatures average around 10–15 °C (50–59 °F) in the northwest; temperatures rise as one proceeds toward the equator, peaking around 20–25 °C (68–77 °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 32–40 °C (90–104 °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.
In China, a common temperature-based reckoning holds that it is winter for the period when temperatures are below 10°C on average and summer for the period when temperatures are above 22°C on average. This means that areas with relatively extreme climates (such as the
Astronomical
event | equinox | solstice | equinox | solstice | ||||
---|---|---|---|---|---|---|---|---|
month | March[27] | June[28] | September[29] | December[30] | ||||
year | day | time | day | time | day | time | day | time |
2019 | 20 | 21:58 | 21 | 15:54 | 23 | 07:50 | 22 | 04:19 |
2020 | 20 | 03:50 | 20 | 21:43 | 22 | 13:31 | 21 | 10:03 |
2021 | 20 | 09:37 | 21 | 03:32 | 22 | 19:21 | 21 | 15:59 |
2022 | 20 | 15:33 | 21 | 09:14 | 23 | 01:04 | 21 | 21:48 |
2023 | 20 | 21:25 | 21 | 14:58 | 23 | 06:50 | 22 | 03:28 |
2024 | 20 | 03:07 | 20 | 20:51 | 22 | 12:44 | 21 | 09:20 |
2025 | 20 | 09:02 | 21 | 02:42 | 22 | 18:20 | 21 | 15:03 |
2026 | 20 | 14:46 | 21 | 08:25 | 23 | 00:06 | 21 | 20:50 |
2027 | 20 | 20:25 | 21 | 14:11 | 23 | 06:02 | 22 | 02:43 |
2028 | 20 | 02:17 | 20 | 20:02 | 22 | 11:45 | 21 | 08:20 |
2029 | 20 | 08:01 | 21 | 01:48 | 22 | 17:37 | 21 | 14:14 |
Astronomical timing as the basis for designating the temperate seasons dates back at least to the
Nowadays the astronomical timing has winter starting at the winter solstice, spring at the spring equinox, and so on. This is used worldwide, although some countries like Australia, New Zealand,[33] 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 solstices and the times of the sun's transit over the equator for the equinoxes, or a traditional date close to these times.[34]
The following diagram shows the relation between the line of solstice and the line of
These "astronomical" seasons are not of equal length, because of the 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 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 extra days, but astronomers directed that ten days be removed. Because of this, the (
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 a century progresses. This shift is a full day in about 128 years (compensated mainly by the century "leap year" rules of the Gregorian calendar); 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 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.
All the times are given in 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, see the astronomical seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall 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.
Change over time
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.
Solar
Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. This was the case with the seasons described by the Roman scholar
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
Season | Start date | End date |
---|---|---|
Winter | 1 November (All Saints' Day) | 31 January |
Spring | 1 February ( St. Brigid's Day ) |
30 April |
Summer | 1 May (May Day) | 31 July |
Autumn | 1 August (Lughnasadh) | 31 October ( Hallowe'en )
|
Solar terms
The
Six-season reckoning
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 season | Start | End | Hindu months | Mapping to English names |
---|---|---|---|---|
Vasanta
|
Mid-March | Mid-May | Vaishakha
|
spring |
Grīshma | Mid-May | Mid-July | Jyeshtha, Ashadha
|
summer |
Varshā | Mid-July | Mid-September | Bhadrapada
|
monsoon |
Sharat
|
Mid-September | Mid-November | Kartika
|
autumn |
Hemanta | Mid-November | Mid-January | Maargashirsha, Pushya
|
early winter or late autumn |
Shishira
|
Mid-January | Mid-March | Magh, Phalguna
|
prevernal or late winter |
The
Bengali season (ঋতু) | Start | End | Bengali months | Mapping to English names |
---|---|---|---|---|
Greeshmo (গ্রীষ্ম)(summer) | Mid-April | Mid-June | Boishakh, Joishtho | Summer |
Bôrsha (বর্ষা) (monsoon) | Mid-June | Mid-August | Asharh, Srabon | Monsoon |
Shôrôt (শরৎ) (autumn/ fall) | Mid-August | Mid-October | Ashwin
|
Autumn |
Hemônto (হেমন্ত) (frost/ late autumn) | Mid-October | Mid-December | Kartik, Ogrohayon
|
Late Autumn |
Sheet (শীত) (winter) | Mid-December | Mid-February | Poush, Magh | Winter |
Bôsônto বসন্ত(spring) | Mid-February | Mid-April | Falgun, Choitro | Spring
|
The Odia Calendar is similar but differs in start and end times.
Odia Season (ଋତୁ) |
Season | Odia months | Gregorian |
---|---|---|---|
ଗ୍ରୀଷ୍ମ Grīṣmå |
Summer | Båiśākhå–Jyeṣṭhå | April–June |
ବର୍ଷା Bårṣā |
Monsoon | Āṣāṛhå–Śrābåṇ | June–August |
ଶରତ Śåråt |
Autumn | Bhādråb–Āświn | August–October |
ହେମନ୍ତ Hemåntå |
Pre-Winter | Kārtik–Mārgåśir | October–December |
ଶୀତ Śīt |
Winter | Pouṣå–Māghå | December–February |
ବସନ୍ତ Båsåntå |
Spring | Fālgun–Chåitrå | February–April |
The Tamil calendar follows a similar pattern of six seasons
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
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; hedgehogs hibernate—winter). So, if a change in daily floral and animal events can be observed, 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 do not normally occur in a particular region, then that area cannot be said to experience that season on a regular basis.[citation needed]
Modern mid-latitude ecological
Six ecological seasons can be distinguished without fixed calendar-based dates like the meteorological and astronomical seasons.[36] Oceanic regions tend to experience the beginning of the hibernal season up to a month later than 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 Midwestern United States and 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
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 seasons of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year,[19] 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 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.[citation needed]
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,[37] or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors[38] 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.[39]
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
Two seasons
In the tropics, where seasonal dates also vary, it is more common to speak of the 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. Instead, many regions (such as the northern Indian Ocean) have varying 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.[40]
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—
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
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
Pre-modern armies, especially in Europe, tended to campaign in the summer months - peasant
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
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- S2CID 120966256. Archived from the originalon 2016-09-16. Retrieved 2011-05-13.
- S2CID 121230141.
- ^ OCLC 60695883.
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.
- ISBN 978-1-285-96971-8. Retrieved 2022-12-02.
- ^ The Popular Educator. John Cassell. 1856. p. 89. Retrieved 2022-12-02.
- ISBN 978-1-4020-3264-6. Retrieved 2022-12-02.
- ISBN 978-1-4496-5591-4. Retrieved 2022-12-02.
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- ^ "Fundamentals of physical geography", PhysicalGeography.net, Ch. 6: Energy and Matter:(h) Earth-Sun Geometry, [1] Archived 2006-07-06 at the Wayback Machine
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- ^ a b "Australian weather and the seasons". Archived from the original on 2012-10-21. Retrieved 2012-10-22.
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- ^ Équinoxe de printemps entre 1583 et 2999
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- ^ Équinoxe d’automne de 1583 à 2999
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- Bibcode:1992JBAA..102...40M.. Therefore, the average value over the last 2000 years has been 365.24281 days, 0.00719 days less than an average Julian year. This means the solstice was 2000×0.00719=14.38 days later, that is, on December 23 in the middle of the day.A hundred years earlier it would have been on the 24th.
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Stokke, Olav Schram; ISBN 9780803978973. Retrieved 21 August 2019.
Witte wanted the main base of the Russian Navy to be situated at a location where waters were ice-free the whole year round [...].
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ISBN 9780851158709. Retrieved 21 August 2019.
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.
- 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
This article's use of external links may not follow Wikipedia's policies or guidelines. (December 2015) |
- When do the Seasons Begin? (from the Bad Astronomer)
- Why the Earth has seasons article on h2g2.
- Aboriginal seasons of Kakadu
- Indigenous seasons (Australian Bureau of Meteorology)
- Mt Stirling Seasons
- The Lost Seasons
- Melbourne's six seasons
- Tutorial on Earth/Sun Relations and Seasons
- Sunpreview Season Forecast Project
- Satellite photo demonstrating seasons changes in 2004 on NASA website Archived 2015-03-18 at the Wayback Machine