Season

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 the tilt of the Earth's axis relative to the plane of revolution. 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, four calendar-based seasons (with their adjectives) are generally recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal). However, ecologists mostly use a six-season model for temperate climate regions 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 dry season, 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, tornado season, or a wildfire season.

The six ecological seasons

The four calendar seasons, depicted in an ancient Roman mosaic from Tunisia

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.

Main article: Effect of sun angle on climate

Axis tilt

The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by 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 hemispheres experience opposite seasons.

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 is spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Lower intensity light is less able to heat the ground. 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.

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. 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 is opposite the temperature trends of the seasons in the Northern hemisphere. In general, the effect of orbital eccentricity on Earth's seasons is a 7% increase/decrease 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. 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. Mars, however, experiences wide temperature variations and violent dust storms every year at perihelion.

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, 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.

A study of temperature records over the past 300 years shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.

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.

Four-season calendar reckoning

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. Most calendar-based methods use a four season model to identify the warmest and coolest or coldest seasons which are separated by two intermediate seasons.

Modern mid-latitude 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. 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.	The picture shows Figure 7 as published by Jones et al.

Modern mid-latitude astronomical

Modern astronomical timing is the basis for designating the temperate seasons on most modern Gregorian calendars world-wide, although some countries like Australia, New Zealand, and Russia prefer to use meteorological reckoning. 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. For 2013 these times are:

Equinoxes	Solstices
Mar 20, 11:02 UTC	June 21, 05:04 UTC
Sept 22, 20:44 UTC	Dec 21, 17:11 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 June solstice 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.

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.

Traditional solar: Europe and east Asia

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 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 the customary astronomical seasons. 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 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 dayt of autumn is 1 August (Celtic Lughnasadh). The Celtic dates corresponded to four Pagan agricultural festivals.

The traditional calendar seasons in China and some other east Asian countries are variations of the solar method. They are traditionally based on 24 periods known as solar terms, and begin just after the cross-quarter days between the solstices and equinoxes. The traditional first day of the Chinese winter is 10 November (立冬 Chinese lìdōng); spring begins 7 February (立春 lìchūn); summer starts on 10 May, (立夏 lìxià); the first day of autumn is on 10 August (立秋 (lìqiū).

Calendar deviations from four-season reckoning

Although most calendar-based reckonings use a four season model, there are some calendars, especially in south Asia, that use a six season method that identifies more than two intermediate seasons between winter and summer. As with other calendars, the dates are fixed at even intervals of months and are the same everywhere regardless of local variations in climate.

Traditional south Asian (mid-latitude and tropical)

In the Hindu calendar of tropical and subtropical India, there are six seasons or Ritu that are calendar-based in the sense of having fixed dates: Vasanta (spring), Greeshma (summer), Varsha (monsoon), Sharad (autumn), Hemanta (winter), and Shishira (prevernal). The six seasons are ascribed to two months each of the twelve months in the Hindu calendar. The rough correspondences are:

Hindu season	Start	End	Hindu Months	Mapping to English Names
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	Ashwin, Kartika	autumn
Hemanta	mid-November	mid-January	Maargashirsha, Pausha	winter
Shishira	mid-January	mid-March	Magh, Phalguna	prevernal

Bengali Calendar is similar but differs in start and end time. It has the following seasons or ritu:

Bengali season	Start	End	Bengali Months	Mapping to English Names
Grishmo	Mid-April	Mid-June	Boishakh, Joishtho	Summer
Borsha	Mid-June	Mid-August	Asharh, Srabon	Monsoon
Shorot	Mid-August	Mid-October	Bhadro, Ashwin	Autumn
Hemonto	mid-October	mid-December	Kartik, Ogrohayon	Late Autumn
Sit	mid-December	mid-February	Poush, Magh	Winter
Bosonto	mid-February	mid-April	Falgun, Choitro	Spring

Tamils follow a similar pattern of 6 Seasons

Tamil season	Gregorian Months	Tamil Months
IlaVenil (Spring)	April 15 to June 14	Chithirai and Vaikasi
MuthuVenil (Summer)	June 15 to August 14	Aani and Aadi
Kaar (Monsoon)	August 15 to October 14	Avani and Purattasi
Kulir (Autumn)	October 15 to December 14	Aipasi and Karthikai
MunPani (Winter)	December 15 to February 14	Margazhi and Thai
PinPani (Prevernal)	February 15 to April 15	Maasi and Panguni

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.

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.

Non-calendar-based reckoning

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. 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.

Modern mid-latitude ecological

Six seasons can be distinguished which do not have fixed calendar-based dates like the meteorological and astronomical seasons. 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.

Modern tropical ecological

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. In some 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 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.

Indigenous ecological (polar, mid-latitude, and tropical)

Main article: 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 seasons of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year, 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.

"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 or "officially" observed in the largely maritime mid-latitude climates of the Southern hemisphere. This is because of the milder winter temperature regime over and near water that leads to an earlier arrival of spring weather. With the earlier spring date established, the other seasons are timed accordingly at three month intervals. Because of the cooler temperatures during the run-up to spring observed in the predominately continental mid-latitude climates north of the equator, the common convention for many countries of the Northern hemisphere is to observe the later-arriving astronomical seasons. In the colder high-latitude countries of northern Europe, the preference shifts back to the meteorological dates to coincide more closely with the pre-solstice arrival of winter weather.

These different observances are largely informal and have not generally been legally proclaimed by governments north or south of the equator. Instead, any official status of the meteorological seasons comes from the World Meteorological Organization which can only make official designations for the meteorological profession. Similarly, the astronomical seasons derive their status from the International Astronomical Union which can only make official designations for the astronomical profession. In these contexts, official, a largely media-driven term when used for the start of any calendar-based season, has to be taken as meaning customary or symbolic rather than legal.

See also

• Geocentric view of the seasons
• Heliocentric view of the seasons
• Indian summer
• Growing season
• Persephone, Greek mythological figure associated with the rebirth of vegetation in the spring
• Vertumnus, Roman god of the seasons

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

• When do the Seasons Begin? (from the Bad Astronomer)
• Southern Hemisphere calendar
• Solstice does not signal season's start (from The Straight Dope)
• 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
• The Lengths of the Seasons (numerical integration analysis)
• Earth distance vs tilt
• Tutorial on Earth/Sun Relations and Seasons