Season


Season

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A season is a subdivision of the year, marked by changes in weather, ecology, and hours of daylight. Seasons result from the yearly revolution of the Earth around the Sun and thetilt of the Earth's axis relative to the plane of revolution.[1] 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 go into hibernation or to migrate, and plants to be dormant.
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 the tilt of the Earth 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 hottest months in the northern hemisphere and December, January and February are the hottest months in the southern hemisphere.
In temperate and subpolar regions, generally four calendar-based seasons (with their adjectives) are recognized: spring (vernal), summer (estival), autumn (autumnal) and winter(hibernal). However, ecologists mostly use a six season model for temperate climateregions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four.
Hot regions have two or three seasons; the rainy (or wet, or monsoon) season and the dryseason, and in some tropical areas, a cool or mild season.
In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornadoseason or a wildfire season.

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[edit]Causes and effects


Illumination of the earth during various seasons

Fig. 1
This is a diagram of the seasons. Regardless of the time of day (i.e. the Earth's rotation on its axis), theNorth 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 theatmosphere is greater when it falls at a shallow angle.
Main article: Effect of sun angle on climate

[edit]Axis tilt

The seasons result from the Earth's axis being tilted to its orbital plane; it deviatesby an angle of approximately 23.5 degrees. Thus, at any given time during summer or winter, one part of the planet is more directly exposed to the rays of the Sun(see Fig. 1). This exposure alternates as the Earth revolves in its orbit. Therefore, at any given time, regardless of season, the northern and southern hemispheresexperience opposite seasons.
The effect of axis tilt is observable from the change in day length, and altitude of the Sun at noon (the culmination of the Sun), during a year.

[edit]Elliptical Earth orbit

Seasonal weather differences between hemispheres are further caused by theelliptical orbit of Earth. Earth reaches perihelion (the point in its orbit closest to theSun) in January, and it reaches aphelion (farthest point from the Sun) in July. Even though the effect this has on Earth's seasons is minor (there is only about a 7% increase/decrease in sunlight received), it does noticeably soften the northern hemisphere's winters and summers. In the southern hemisphere, the opposite effect is observed.

[edit]Distance from sun

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.[2] 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 fartherfrom the sun. This is because the northern hemisphere has more land than the southern, and land warms more readily than sea.[3] Mars however experiences wide temperature variations and violent dust storms  every year at perihelion.[4]

[edit]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.
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, theSouth 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 theNorthern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

[edit]Tropics

In tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, because the amount of precipitation may vary more dramatically than the average temperature. 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. In other tropical areas a three-way division into hot, rainy, and cool season is used. There is no noticeable change in the amount of sunlight at different time of the year. However, many regions (such as the northern Indian ocean) are subject tomonsoon rain and wind cycles.
A study of temperature records over the past 300 years[5][page needed] shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.

[edit]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, the solstices and equinoxes ought to be the middle of the respective seasons, but, because of thermal lag, 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.[6]

[edit]Reckoning

[edit]Meteorological


Animation of seasonal differences especially snow cover through the year
Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. Using this reckoning, the Roman calendar began the year and the spring season on the first of March, with each season occupying three months. In 1780 the Societas Meteorologica Palatina, an early international organization for meteorology, defined seasons as groupings of three whole months. Ever since, professional meteorologists all over the world have used this definition.[7] Therefore, for 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, spring begins on 1 September, summer on 1 December, autumn on 1 March, and winter on 1 June.
In Sweden and Finland, meteorologists use a definition for the seasons based on the temperature. Spring begins when the daily averaged temperature permanently rises above 0°C, summer begins when the temperature permanently rises above +10°C, summer ends when the temperature permanently falls below +10°C and winter begins when the temperature permanently falls below 0°C. "Permanently" here means that the daily averaged temperature has remained above or below the limit for seven consecutive days. This implies two things: first, the seasons do not begin at fixed dates but must be determined by observation and are known only after the fact; and second, a new season begins at different dates in different parts of the country.
Surface air temperature

Diagram was calculated (Abscisse: 21. of each month)
Calculation based on data published by Jones et al. [8]

The picture shows Figure 7 as published by Jones et al.[8]

[edit]Astronomical

[edit]Timing

The precise timing of the seasons as viewed by astronomers is determined by the exact times of transit of the sun over the tropics of Cancer and Capricorn for the solstices and the times of the sun's transit over the equator for the equinoxes. [9] For 2012 these times are:
EquinoxesSolstices
Mar 20, 05:14 UTCJune 20, 23:09 UTC
Sept 22, 14:49 UTCDec 21, 11:12 UTC
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 perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of Junesolstice on 20 or 21 June, the 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.
Seasons1.svg
From the March equinox it 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.

[edit]"Official" designations

By definition, the meteorological seasons start about three weeks ahead of their astronomical counterparts in both the Northern and Southern hemispheres. The earlier-starting meteorological seasons are customarily observed in the largely maritime climates of the Southern hemisphere. This is because of the shorter temperature lag from the shortest to the coldest day of the year over and near water which leads to an earlier arrival of spring weather. Because of the longer temperature lags during the run-up to spring observed in the predominately continental climates north of the equator, the convention in the Northern hemisphere is to observe the later-arriving astronomical seasons: March Equinox instead of March 1 (Spring), June Solstice instead of June 1 (Summer), September Equinox instead of September 1 (Autumn) and December Solstice instead of December 1 (Winter). These different observances in each hemisphere are informal and have not generally been legally proclaimed by governments north or south of the equator. Instead, anyofficial status of the meteorological seasons in the southern hemisphere comes from the World Meteorological Organization which can only make official designations for the meteorological profession. Similarly, the astronomical seasons that are favored in many northern hemispheric countries because they are later starting derive their status from the International Astronomical Union which can only make official designations for the astronomical profession. In these contexts, official for the start of any season has to be taken as meaningcustomary or symbolic rather than legal.

[edit]Polar day and night

Any point north of the Arctic Circle or south of the Antarctic Circle will have one period in the summer when the sun does not set, and one period in the winter 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 located at 82°30′05″N and 62°20′20″W, on the northern tip of Ellesmere Island,Canada (about 450 nautical miles 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 21 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.[10]
However, end of February is not first light. 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 hemisphere, 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 twilight returns the first faint wash of twilight briefly touches the horizon (for just minutes per day), and then twilight increases in duration with pre-dawn brightness each day until sunrise at end of February and on 6 April the sun remains above the horizon until mid October.

[edit]Ecological

365 days tree.ogg
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; hedgehogs hibernate—winter). So, if we can observe a change in daily floral/animal events, the season is changing. Floral and animal activity varies little in tropical and polar regions, so significant seasonal variations in flora and fauna are mostly restricted to the mid latitudes that have temperate climates.
Six seasons can be distinguished which are not tightly calendar-based like the meteorological and astronomical seasons.[11] Mild temperate regions tend to experience the beginning of the hibernal season up to a month later than cool temperate areas, while the prevernal and vernal seasons begin up to a month earlier. For example, prevernal crocus blooms typically appear as early as February in mild coastal areas of British Columbia, the British Isles, and western and southern 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 or late January (mild temperate), March (cool temperate). Deciduous tree buds begin to swell. Migrating birds fly from winter to summer habitats.
  • Vernal (spring): Begins March (mild temperate), 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. Young birds reach maturity and join other adult birds preparing for fall migration.
  • Autumnal (fall): 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.

[edit]Traditional

Traditional seasons are reckoned by insolation, with summer being the quarter of the year with the greatest insolation and winter the quarter with the least. These seasons begin about four weeks earlier than the meteorological seasons and 7 weeks earlier than the astronomical seasons.
In traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the "midsummer" and "midwinter" respectively.
This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures.[citation needed]In Iran, Afghanistan and some other parts of Middle East the beginning of the astronomical spring is the beginning of the new year which is called Nowruz.
So, according to traditional reckoning, winter begins between 5 November and 10 November, Samhain, 立冬 (lìdōng or rittou); spring between 2 February and 7 February, Imbolc, 立春 (lìchūn or risshun); summer between 4 May and 10 May, Beltane, 立夏 (lìxià or rikka); and autumn between 3 August and 10 August, Lughnasadh, 立秋 (lìqiū or risshū). The middle of each season is considered Mid-winter, between 20 December and 23 December, 冬至 (dōngzhì or touji); Mid-spring, between 19 March and 22 March, 春分 (chūnfēn or shunbun); Mid-summer, between 19 June and 23 June, 夏至 (xiàzhì or geshi); and Mid-autumn, between 21 September and 24 September, 秋分 (qiūfēn or shūbun).

[edit]Australia

Main article: Indigenous Australian seasons
The indigenous people of Australia defined the seasons by what was happening to the plants, animals and weather around them. This led to each separate tribal group having different seasons, some with up to eight seasons each year. However most modern Aboriginal Australians and non-aboriginal Australians observe the international meteorological definition for the seasons: 1 September (Spring), 1 December (Summer), 1 March (Autumn), and 1 June (Winter).

[edit]Celts

The ancient Celts used a solar calendar, with the four Pagan agricultural festivals, Imbolc, Beltaine, Lughnasa and Samhain occurring on the cross-quarter days. Today, those festivals are celebrated on 1 February, May, August and November respectively.

[edit]China

Chinese seasons are traditionally based on 24 periods known as solar terms, and begin at the midpoint of solstices and equinoxes.[12]

[edit]India

In the Hindu calendar, there are six seasons or Ritu: Vasanta (Spring), Greeshma (Summer), Varsha (Monsoon) and Sharad (Autumn), Hemanta (pre-hibernal), Shishira (Winter).
The six seasons are ascribed to two months each of the twelve months in the Hindu calendar. The rough correspondences are:
Hindu seasonStartEndHindu Months
Vasantamid-Marchmid-MayChaitra, Vaishakha
Greeshmamid-Maymid-JulyJyeshtha, Ashadha
Varshamid-Julymid-SeptemberShraavana, Bhadrapada
Sharadmid-Septembermid-NovemberAshwin, Kartika
Hemantamid-Novembermid-JanuaryMaargashirsha, Pausha
Shishiramid-Januarymid-MarchMagh, Phalguna

[edit]See also

[edit]References

  1. ^ Khavrus, V.; Shelevytsky, I. (2010). "Introduction to solar motion geometry on the basis of a simple model" . Physics Education 45 (6): 641. Bibcode 2010PhyEd..45..641K . doi:10.1088/0031-9120/45/6/010 .
  2. ^ "Fundamentals of physical geography", PhysicalGeography.net, Ch. 6: Energy and Matter:(h) Earth-Sun Geometry, [1] 
  3. ^ Phillips, Tony, "The Distant Sun (Strange but True: the Sun is far away on the 4th of July) ," Science@NASA , downloaded 24 June 2006
  4. ^ Christian Ho, Nasser Golshan, and Arvydas Kliore, Radio Wave Propagation Handbook for Communication on and Around Mars , JPL Publication 02-5, pp. 59-60, downloaded 23 June 2006.
  5. ^ David Thomson, Science, April 1995
  6. ^ "Astronomy Answers AstronomyAnswerBook: Seasons ," Astronomical Institute, Utrecht University, downloaded 1 August 2008
  7. ^ (Dutch) Begin van de lente (Start of Spring) , KNMI (Royal Dutch Meteorology Institute), 2009-03-20, retrieved 2009-03-20
  8. ^ a b P. D. Jones et. al.: SURFACE AIR TEMPERATURE AND ITS CHANGES OVER THE PAST 150 YEARS, Figure 7 (Seite 24 von 28 der PDF-Datei) 
  9. ^ quinoxes, Solstices, Perihelion, and Aphelion, 2000-2020 
  10. ^ U.S. Naval Observatory 
  11. ^ Michael Allaby (1999). "A Dictionary of Zoology" . Retrieved 2012-05-30.
  12. ^ Ross, Kelley L. "The Solar Terms and the Chinese 60 Year Calendar Cycle" . friesian.com. Retrieved 2010-12-03.
  • 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.

[edit]External links

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