Season
Encyclopedia
A season is a division 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, generally four calendar
-based seasons (with their adjectives) are recognized: spring
(vernal), summer
(estival), autumn
(autumnal) and winter
(hibernal). However, ecologists are increasingly using 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 (See Ecological Seasons below).
In some 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.
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 seasons result from the Earth
's axis being tilted
to its orbital plane
; it deviates by an angle of approximately 23.5 degree
s. 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 hemisphere
s experience 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
.
Seasonal weather differences between hemispheres are further caused by the elliptical orbit
of Earth. 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.
Even though the effect this has on Earth's seasons is minor, it does noticeably soften the northern hemisphere's winters and summers. In the southern hemisphere, the opposite effect is observed.
Seasonal weather fluctuations (changes) also depend on factors such as proximity to ocean
s or other large bodies of water, current
s in those oceans, El Niño
/ENSO and other oceanic cycles, and prevailing wind
s.
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.
In the tropics
, there is no noticeable change in the amount of sunlight. However, many regions (such as the northern Indian ocean
) are subject to monsoon
rain
and wind cycles.
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
.
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.
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. 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. Mars however experiences wide temperature variations and violent dust storms every year at perihelion.
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 062°20′20″W, on the northern tip of Ellesmere Island
, Canada
(about 450 nautical mile
s 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 September 6 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.
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, in meteorology for the Northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December.
In Sweden and Finland, meteorologists use a different 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.
s and the times of the sun's transit over the equator for the equinox
es.
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.
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 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
.
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. In North America
and most of Europe
the educational systems and media consider the astronomical seasons "official
" over all other reckonings.
Because of the differences in the Northern and Southern Hemispheres, it is no longer considered appropriate to use the northern-seasonal designations for the astronomical quarter days. The modern convention for them is: March Equinox, June Solstice, September Equinox and December Solstice. The oceanic climate of the Southern Hemisphere produces a shorter temperature lag, so the start of each season is usually considered to be several weeks before the respective solstice
or equinox
in this hemisphere, in other countries with oceanic climates, and in cultures with Celtic roots.
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 cool temperate climate zones in the Northern Hemisphere:
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Traditional temperate seasonal changes on a city road in Manchester, UK:
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Spring
Summer
Autumn
Winter
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. 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).
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
follow either four or six meteorological seasons, as do non-Aboriginal Australians.
The commonly followed dates are as follows: 1st day of March, June, September and December for the start of Autumn, Winter, Spring and Summer, respectively.
, Beltaine, Lughnasa and Samhain
occurring on the cross-quarter days. Today, those festivals are celebrated on the 1st of February, May, August and November respectively.
, 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:
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
, marked by changes in weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
, ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, and hours of daylight
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
.
Seasons result from the yearly revolution of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the tilt of the Earth's axis
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
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
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
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
Seasonal lag
Seasonal lag is the phenomenon whereby the date of maximum average air temperature at a geographical location on a planet is delayed until some time after the date of maximum insolation...
, 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
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
and subpolar regions, generally four calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...
-based seasons (with their adjectives) are recognized: spring
Spring (season)
Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...
(vernal), summer
Summer
Summer is the warmest of the four temperate seasons, between spring and autumn. At the summer solstice, the days are longest and the nights are shortest, with day-length decreasing as the season progresses after the solstice...
(estival), autumn
Autumn
Autumn is one of the four temperate seasons. Autumn marks the transition from summer into winter usually in September or March when the arrival of night becomes noticeably earlier....
(autumnal) and winter
Winter
Winter is the coldest season of the year in temperate climates, between autumn and spring. At the winter solstice, the days are shortest and the nights are longest, with days lengthening as the season progresses after the solstice.-Meteorology:...
(hibernal). However, ecologists are increasingly using a six-season model for temperate climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
regions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four (See Ecological Seasons below).
In some tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
and subtropical regions it is more common to speak of the rainy
Wet season
The the wet season, or rainy season, is the time of year, covering one or more months, when most of the average annual rainfall in a region occurs. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the...
(or wet, or monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
) season versus the dry season
Dry season
The dry season is a term commonly used when describing the weather in the tropics. The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year...
, because the amount of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms 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.
In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
season or a wildfire
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...
season.
Causes and effects
The seasons result from the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's axis being tilted
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
to its orbital plane
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...
; it deviates by an angle of approximately 23.5 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s. Thus, at any given time during summer or winter, one part of the planet is more directly exposed to the rays of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(see Fig. 1). This exposure alternates as the Earth revolves in its orbit. Therefore, at any given time, regardless of season, the northern
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
and southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
s experience opposite seasons.
The effect of axis tilt is observable from the change in day length
Day length
Day length, or length of day, or length of daytime, refers to the time each day from the moment the upper limb of the sun's disk appears above the horizon during sunrise to the moment when the upper limb disappears below the horizon during sunset...
, and altitude of the Sun at noon
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...
(the culmination
Culmination
In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....
of the Sun), during a year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
.
Seasonal weather differences between hemispheres are further caused by the elliptical orbit
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
of Earth. Earth reaches perihelion (the point in its orbit closest to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
) 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, it does noticeably soften the northern hemisphere's winters and summers. In the southern hemisphere, the opposite effect is observed.
Seasonal weather fluctuations (changes) also depend on factors such as proximity to ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s or other large bodies of water, current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s in those oceans, El Niño
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...
/ENSO and other oceanic cycles, and prevailing wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
s.
In the temperate and polar regions, seasons are marked by changes in the amount of sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, which in turn often causes cycles
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
of dormancy in plants and hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
in animals. These effects vary with latitude and with proximity to bodies of water. For example, the South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North 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
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
is in the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...
, 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
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
, it is winter in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, and vice versa.
In the tropics
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
, there is no noticeable change in the amount of sunlight. However, many regions (such as the northern Indian ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
) are subject to monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
and wind cycles.
A study of temperature records over the past 300 years shows that the climatic seasons, and thus the seasonal year
Seasonal year
The seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant....
, are governed by the anomalistic year rather than the tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...
.
In meteorological terms, the summer solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
and winter solstice (or the maximum and minimum insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
, 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
Seasonal lag
Seasonal lag is the phenomenon whereby the date of maximum average air temperature at a geographical location on a planet is delayed until some time after the date of maximum insolation...
. Seasons, though, are not always defined in meteorological terms.
Compared to axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
’s distance to the sun because of its elliptical orbit. Orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
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. Mars however experiences wide temperature variations and violent dust storms every year at perihelion.
Polar day and night
Any point north of the Arctic CircleArctic Circle
The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....
or south of the Antarctic Circle
Antarctic Circle
The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. For 2011, it is the parallel of latitude that runs south of the Equator.-Description:...
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
Midnight sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...
" and "polar night
Polar night
The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.-Description:...
" are progressively longer.
For example, at the military and weather station Alert
Alert, Nunavut
Alert, in the Qikiqtaaluk Region, Nunavut, Canada, is the northernmost permanently inhabited place in the world, from the North Pole. It takes its name from HMS Alert, which wintered east of the present station, off what is now Cape Sheridan, in 1875–1876.Alert was reported to have five permanent...
located at 82°30′05″N 062°20′20″W, on the northern tip of Ellesmere Island
Ellesmere Island
Ellesmere Island is part of the Qikiqtaaluk Region of the Canadian territory of Nunavut. Lying within the Canadian Arctic Archipelago, it is considered part of the Queen Elizabeth Islands, with Cape Columbia being the most northerly point of land in Canada...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
(about 450 nautical mile
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...
s or 830 km from the North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
), 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 September 6 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
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...
, 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.
Meteorological
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 calendarRoman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...
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, in meteorology for the Northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December.
In Sweden and Finland, meteorologists use a different 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 | |
|---|---|
Astronomical
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 solsticeSolstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
s and the times of the sun's transit over the equator for the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
es.
For 2011 these times are:
Equinoxes Solstices Mar 20 @2321 UTC June 21 @1716 UTC Sept 23 @0905 UTC Dec 22 @0530 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
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
on 20 or 21 March, the point of June solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
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.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
reckoning, the solstices and equinoxes ought to be the middle of the respective seasons, but, because of thermal lag, regions with a continental climate 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
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
.
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. In North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and most of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
the educational systems and media consider the astronomical seasons "official
Official
An official is someone who holds an office in an organization or government and participates in the exercise of authority .A government official or functionary is an official who is involved in public...
" over all other reckonings.
Because of the differences in the Northern and Southern Hemispheres, it is no longer considered appropriate to use the northern-seasonal designations for the astronomical quarter days. The modern convention for them is: March Equinox, June Solstice, September Equinox and December Solstice. The oceanic climate of the Southern Hemisphere produces a shorter temperature lag, so the start of each season is usually considered to be several weeks before the respective solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
or equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
in this hemisphere, in other countries with oceanic climates, and in cultures with Celtic roots.
Ecological seasons
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.Temperate areas
Six seasons can be distinguished. 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 crocusCrocus
Crocus is a genus in the iris family comprising about 80 species of perennials growing from corms. Many are cultivated for their flowers appearing in autumn, winter, or spring...
blooms typically appear as early as February in mild coastal areas of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, the British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
, and western and southern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. 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 cool temperate climate zones in the Northern Hemisphere:
- Prevernal (ca.1 March–1 May)
- Vernal (ca.1 May–15 June)
- Estival (ca.15 June–15 August)
- Serotinal (ca.15 August–15 September)
- Autumnal (ca.15 September–1 November)
- Hibernal (ca.1 November–1 March)
Cold regions
There are again only two seasons:- Polar Day (spring and summer)
- Polar Night (autumn and winter)
Traditional season divisions
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Traditional seasons are reckoned by insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
, 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
Solar term
A solar term is any of 24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons. Solar terms...
and Irish
Irish calendar
The Irish calendar is a pre-Christian Celtic system of timekeeping used during Ireland's Gaelic era and still in popular use today to define the beginning and length of the day, the week, the month, the seasons, quarter days, and festivals...
cultures. In Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
, Afghanistan
Afghanistan
Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...
and some other parts of Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
the beginning of the astronomical spring is the beginning of the new year which is called Nowruz
Nowruz
Nowrūz is the name of the Iranian New Year in Iranian calendars and the corresponding traditional celebrations. Nowruz is also widely referred to as the Persian New Year....
.
So, according to traditional reckoning, winter begins between 5 November and 10 November, Samhain
Samhain
Samhain is a Gaelic harvest festival held on October 31–November 1. It was linked to festivals held around the same time in other Celtic cultures, and was popularised as the "Celtic New Year" from the late 19th century, following Sir John Rhys and Sir James Frazer...
, 立冬 (lìdōng or rittou); spring between 2 February and 7 February, Imbolc
Imbolc
Imbolc , or St Brigid’s Day , is an Irish festival marking the beginning of spring. Most commonly it is celebrated on 1 or 2 February in the northern hemisphere and 1 August in the southern hemisphere...
, 立春 (lìchūn or risshun); summer between 4 May and 10 May, Beltane
Beltane
Beltane or Beltaine is the anglicised spelling of Old Irish Beltaine or Beltine , the Gaelic name for either the month of May or the festival that takes place on the first day of May.Bealtaine was historically a Gaelic festival celebrated in Ireland, Scotland and the Isle of Man.Bealtaine...
, 立夏 (lìxià or rikka); and autumn between 3 August and 10 August, Lughnasadh
Lughnasadh
Lughnasadh is a traditional Gaelic holiday celebrated on 1 August. It is in origin a harvest festival, corresponding to the Welsh Calan Awst and the English Lammas.-Name:...
, 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).
Australia
The indigenous people of AustraliaAustralia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
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
Australian Aborigines
Australian Aborigines , also called Aboriginal Australians, from the latin ab originem , are people who are indigenous to most of the Australian continentthat is, to mainland Australia and the island of Tasmania...
follow either four or six meteorological seasons, as do non-Aboriginal Australians.
The commonly followed dates are as follows: 1st day of March, June, September and December for the start of Autumn, Winter, Spring and Summer, respectively.
Celts
The ancient Celts used a solar calendar, with the four Pagan agricultural festivals, ImbolcImbolc
Imbolc , or St Brigid’s Day , is an Irish festival marking the beginning of spring. Most commonly it is celebrated on 1 or 2 February in the northern hemisphere and 1 August in the southern hemisphere...
, Beltaine, Lughnasa and Samhain
Samhain
Samhain is a Gaelic harvest festival held on October 31–November 1. It was linked to festivals held around the same time in other Celtic cultures, and was popularised as the "Celtic New Year" from the late 19th century, following Sir John Rhys and Sir James Frazer...
occurring on the cross-quarter days. Today, those festivals are celebrated on the 1st of February, May, August and November respectively.
China
Chinese seasons are traditionally based on 24 periods known as solar terms, and begin at the midpoint of solstices and equinoxes.India
In the Hindu calendarHindu calendar
The hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. Nepali calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Telugu calendar, Kannada...
, there are six seasons or Ritu: Vasanta
Vasanta
Vasanta or Vasantha can mean* Vasanta, spring season in Hindu calendar.* Vasanta, a scouting group in the Netherlands, founded in 1933.* Vasanta was also the original name of the USS Aquamarine ....
(Spring), Greeshma (Summer), Varsha
Varsha
Varsha may refer to:* Varsha, Rainy season or ritu in Hindu calendar-Given names:*Bob Varsha, American sports announcer*Varsha Bhosle, Indian journalist*Varsha Soni, Indian field hockey player*Varsha Usgaonkar, Indian actress...
(Monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
) and Sharad
Sharad
Sharad in North India, variantly referred to as Sharat or Sharath or Sarath in South India, is the early autumn ritu, or season in the Hindu calendar. It roughly corresponds to the western months of mid-September to mid-November. Sharad is preceded by Varsha, followed by Hemant...
(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 season | Start | End | Hindu Months |
|---|---|---|---|
| Greeshma | mid-April | mid-June | Vaishakha, Jyestha |
| Varsha | mid-June | mid-August | Ashadha, Shravana Shravana Shravana is the 22nd nakshatra or lunar mansion as used in Hindu astronomy and astrology. It belongs to the constellation Makara or Capricorn.... |
| Sharad | mid-August | mid-October | Bhadrapada, Ashwayuja |
| Hemanta | mid-October | mid-December | Kartika Kartika Kartika or Karthika may refer to:* alternative transliteration of Kartik** Kartikeya, the son of Shiva in the Indian mythology* alternative transliteration of Karthika, a feminine name in Indian languages** Kartika Rane, Indian actress... , Maargashirsha |
| Shishira | mid-December | mid-February | Pushya Pushya Pushya is a nakshatra in Indian astronomy. It corresponds to γ, δ and θ Cancri, in the Cancer .-Naming practices:Under the traditional Hindu principle of naming individuals according to their birthstar , the following Sanskrit syllables correspond with this Nakshatra, and would belong at the... , Magha Magha Magha may refer to:* Magha , a month in the Hindu calendar* Magh , the same month in the Bengali calendar* Magha , an 8th century Sanskrit poet, who wrote Shishupala-vadha... |
| Vasanta | mid-February | mid-April | Phalguna Phalguna Phalguna is a month of the Hindu calendar. In India's national civil calendar, Phaalgun is the twelfth month of the year, beginning on 20 February and ending on 21 March .In lunar religious calendars, Phaalgun may begin on either the new moon or the full moon... , Chaitra Chaitra Chaitra is a month of the Hindu calendar.... |
See also
- Indian SummerIndian summerAn Indian summer is a meteorological phenomenon that occurs in the autumn. It refers to a period of considerably above normal temperatures, accompanied by dry and hazy conditions, usually after there has been a killing frost...
- Perennial tea ceremonyPerennial tea ceremonyPerennial tea ceremony / Four Seson Tea Ceremony is a Chinese tea ceremony, created by Lin Easu , of the Ten Ren Teaism Foundation. The first two characters of the Chinese term literally mean four steps or sequences that are linked together, the latter two simply meaning "tea ceremony". With...
- Months
- PersephonePersephoneIn Greek mythology, Persephone , also called Kore , is the daughter of Zeus and the harvest-goddess Demeter, and queen of the underworld; she was abducted by Hades, the god-king of the underworld....
- Risks and benefits of sun exposureRisks and benefits of sun exposureThe ultraviolet radiation in sunlight, though a principal source of vitamin D3 compared to diet, is mutagenic. Supplementing diet with vitamin D3 supplies vitamin D without this mutagenic effect, but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from...
- Indigenous Australian seasonsIndigenous Australian seasonsIndigenous Australians have distinct ways of dividing the year up. Naming and understanding of seasons differed between groups, and depending on where in Australia the group lives. Below are a few examples of different groups and their seasons....
External links
- Australian Weather and Seasons
- When do the Seasons Begin? (from the Bad Astronomer)
- Solstice does not signal season's start (from The Straight Dope)
- Why the Earth has seasons article on h2g2H2g2h2g2 is a British-based collaborative online encyclopedia project engaged in the construction of, in its own words, "an unconventional guide to life, the universe, and everything", in the spirit of the fictional publication The Hitchhiker's Guide to the Galaxy from the science fiction comedy series...
. - 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