Season 5 (2016-2018)
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A season is a division of the year based on changes in
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
,
ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
, and the number of
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
hours in a given region. On
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, seasons are the result of the
axial parallelism Axial parallelism (also known as gyroscopic stiffness, inertia or rigidity, or "rigidity in space") is the characteristic of a spinning body in which the direction of the axis of rotation remains fixed as the object moves through space. In astrono ...
of Earth's tilted orbit around the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. In temperate and polar regions, the seasons are marked by changes in the intensity of
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
that reaches the Earth's surface, variations of which may cause animals to undergo
hibernation Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...
or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical cultures whose number of seasons varies. The
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
experiences most direct sunlight during May, June, and July, as the hemisphere faces the Sun. The same is true of the Southern Hemisphere in November, December, and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months, which increases the
solar flux Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
. However, due to
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 (i.e. the summer solstice). This also applies to the minimum ...
, June, July, and August are the warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere. In
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
and sub-polar regions, four seasons based on the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years dif ...
are generally recognized: ''
spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a ...
'', ''
summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
'', ''
autumn Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September ( Northern Hemisphere) or March ( Sou ...
'' (or ''fall''), and ''
winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
''. Ecologists often use a six-season model for temperate
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
regions which are not tied to any fixed calendar dates: ''prevernal'', ''vernal'', ''estival'', ''serotinal'', ''autumnal'', and ''hibernal''. Many tropical regions have two seasons: the '' rainy'', '' wet'', or ''
monsoon A monsoon () is traditionally 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 annual latitudinal oscil ...
season'' and the ''
dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The te ...
''. Some have a third ''cool'', ''mild'', or ''
harmattan The Harmattan is a season in West Africa that occurs between the end of November and the middle of March. It is characterized by the dry and dusty northeasterly trade wind, of the same name, which blows from the Sahara over West Africa into the ...
season''. "Seasons" can also be dictated by the timing of important ecological events such as '' hurricane season'', ''
tornado season Tornadoes have been recorded on all continents except Antarctica. They are most common in the middle latitudes where conditions are often favorable for convective storm development. The United States has the most tornadoes of any country, as we ...
'', and ''
wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...
season''. Some examples of historical importance are the ancient Egyptian seasons—''
flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
'', ''
growth Growth may refer to: Biology * Auxology, the study of all aspects of human physical growth * Bacterial growth * Cell growth * Growth hormone, a peptide hormone that stimulates growth * Human development (biology) * Plant growth * Secondary growth ...
'', and ''
low water Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...
''—which were previously defined by the former annual flooding of the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin language, Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered ...
in
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Mediter ...
. Seasons often hold special significance for agrarian societies, whose lives revolve around
planting Sowing is the process of planting seeds. An area or object that has had seeds planted in it will be described as a sowed or sown area. Plants which are usually sown Among the major field crops, oats, wheat, and rye are sown, grasses and leg ...
and
harvest Harvesting is the process of gathering a ripe crop from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper. On smaller farms with minimal mechanization, harvesting is the most labor-i ...
times, and the change of seasons is often attended by
ritual A ritual is a sequence of activities involving gestures, words, actions, or objects, performed according to a set sequence. Rituals may be prescribed by the traditions of a community, including a religious community. Rituals are characterized, b ...
. The definition of seasons is also cultural. In India, from ancient times to the present day, six seasons or Ritu based on south Asian religious or cultural calendars are recognised and identified for purposes such as agriculture and trade.


Causes and effects


Axial parallelism

The Earth's orbit exhibits approximate
axial parallelism Axial parallelism (also known as gyroscopic stiffness, inertia or rigidity, or "rigidity in space") is the characteristic of a spinning body in which the direction of the axis of rotation remains fixed as the object moves through space. In astrono ...
, maintaining its direction towards
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude that ...
(the "North Star") year-round. This is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right. Minor variation in the direction of the axis, known as
axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particu ...
, takes place over the course of 26,000 years, and therefore is not noticeable to modern human civilization.


Axial tilt

The seasons result from the Earth's
axis of rotation Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rota ...
being tilted with respect to its
orbital plane The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) an ...
by an angle of approximately 23.4 degrees. (This tilt is also known as "obliquity of the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
".) Regardless of the time of year, the
northern Northern may refer to the following: Geography * North, a point in direction * Northern Europe, the northern part or region of Europe * Northern Highland, a region of Wisconsin, United States * Northern Province, Sri Lanka * Northern Range, a ra ...
and southern hemispheres always experience opposite seasons. This is because during
summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
or
winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
, one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March20 to around September22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December21. The two instants when the Sun is directly overhead at the
Equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
are the
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es. Also at that moment, both the
North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Mag ...
and the
South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
of the Earth are just on the
terminator Terminator may refer to: Science and technology Genetics * Terminator (genetics), the end of a gene for transcription * Terminator technology, proposed methods for restricting the use of genetically modified plants by causing second generation s ...
, and hence day and night are equally divided between the two hemispheres. Around the
March equinox The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the verna ...
, the Northern Hemisphere will be experiencing
spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a ...
as the hours of
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
increase, and the Southern Hemisphere is experiencing
autumn Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September ( Northern Hemisphere) or March ( Sou ...
as daylight hours shorten. The effect of axial tilt is observable as the change in
day length Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hem ...
and
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
of the Sun at solar
noon Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 (military time). Solar ...
(the Sun's
culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events were also known as meridian transits ...
) during the
year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hou ...
. The low angle of Sun during the winter months means that incoming rays of solar radiation are spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres. File:Earth-lighting-summer-solstice EN - corrected.png, Illumination of Earth by Sun at the northern solstice. File:Earth-lighting-winter-solstice EN.png, Illumination of Earth by Sun at the southern solstice. File:Earth seasons 2021-2022.jpg, Illumination of Earth at each change of astronomical season File:Earth seen from the sun.ogv, Animation of Earth as seen daily from the Sun looking at UTC+02:00, showing the solstice and changing seasons. File:ReflectedSolarRadiation Solstices.jpg, Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m2).


Elliptical Earth orbit

Compared to axial parallelism and axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth's distance to the Sun because of its
elliptical orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it ...
. In fact, Earth reaches
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
(the point in its orbit closest to the Sun) in January, and it reaches
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
(the point farthest from the Sun) in July, so the slight contribution of
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
opposes the temperature trends of the seasons in the Northern Hemisphere. In general, the effect of orbital eccentricity on Earth's seasons is a 7% variation 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 southern winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the Southern Hemisphere.


Maritime and hemispheric

Seasonal weather fluctuations (changes) also depend on factors such as proximity to
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
s or other large bodies of water,
currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
in those oceans,
El Niño El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date L ...
/ENSO and other oceanic cycles, and prevailing
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
s. In the
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
and
polar regions The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by float ...
, seasons are marked by changes in the amount of
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
, which in turn often causes cycles of dormancy in plants and
hibernation Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...
in animals. These effects vary with
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
and with proximity to bodies of water. For example, the
South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
is in the middle of the continent of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
and therefore a considerable distance from the moderating influence of the southern oceans. The
North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Mag ...
is in the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
, 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 seasonal cycle in the polar and temperate zones of one hemisphere is opposite to that of the other. When it is summer in the Northern Hemisphere, it is winter in the Southern, and vice versa.


Tropics

The
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
and
subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...
regions see little annual fluctuation of sunlight. However, seasonal shifts occur along a rainy, low-pressure belt called the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
(ICZ). As a result, the amount of
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
tends to vary more dramatically than the average temperature. When the Zone is north of the Equator, the northern tropics experience their wet season while the southern tropics have their dry season. This pattern reverses when the Zone migrates to a position south of the Equator.


Mid-latitude thermal lag

In meteorological terms, the
solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countr ...
s (the maximum and minimum
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
) do not fall in the middles of summer and winter. The heights of these seasons occur up to 7 weeks later because of
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 (i.e. the summer solstice). This also applies to the minimum ...
. Seasons, though, are not always defined in meteorological terms. In
astronomical Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies ...
reckoning by hours of
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
alone, the solstices and
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es are in the ''middle'' of the respective seasons. Because of seasonal lag due to
thermal absorption A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
and release by the oceans, regions with a
continental climate Continental climates often have a significant annual variation in temperature (warm summers and cold winters). They tend to occur in the middle latitudes (40 to 55 north), within large landmasses where prevailing winds blow overland bringing som ...
, which predominate in the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, often consider these four dates to be the ''start'' of the seasons as in the diagram, with the
cross-quarter day The Wheel of the Year is an annual cycle of seasonal festivals, observed by many modern pagans, consisting of the year's chief solar events (solstices and equinoxes) and the midpoints between them. While names for each festival vary among dive ...
s considered seasonal midpoints. The length of these seasons is not uniform because of Earth's
elliptical orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it ...
and its different speeds along that orbit.


Four-season reckoning

Most calendar-based partitions use a four-season model to demarcate the warmest and coldest seasons, which are further separated by two intermediate seasons. Calendar-based reckoning defines the seasons in relative rather than absolute terms, so the coldest quarter-year is considered winter even if floral activity is regularly observed during it, despite the traditional association of flowers with spring and summer. The major exception is in the tropics where, as already noted, the winter season is not observed. The four seasons have been in use since at least Roman times, as in
Rerum rusticarum Rerum may refer to : *Lacrimae rerum is the Latin for tears for things. *Rerum novarum is an encyclical issued by Pope Leo XIII on May 16, 1891. * Rerum Moscoviticarum Commentarii was a Latin book by Baron Sigismund von Herberstein on the geograph ...
of
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath and a prolific author. He is regarded as ancient Rome's greatest scholar, and was described by Petrarch as "the third great light of Rome" (after Vergil and Cicero). He is sometimes calle ...
De Re Rustica
28

Varro says that spring, summer, autumn, and winter start on the 23rd day of the sun's passage through Aquarius, Taurus, Leo, and Scorpio, respectively. Nine years before he wrote,
Julius Caesar Gaius Julius Caesar (; ; 12 July 100 BC – 15 March 44 BC), was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in a civil war, and ...
had reformed the calendar, so Varro was able to assign the dates of February 7, May 9, August 11, and November 10 to the start of spring, summer, autumn, and winter.


Official

As noted, a variety of dates and even exact times are used in different countries or regions to mark changes of the calendar seasons. These observances are often declared "official" within their respective areas by the local or national media, even when the weather or climate is contradictory. However, they are mainly a matter of custom only, and have not generally been proclaimed by governments north or south of the equator for civil purposes.


Meteorological

Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. In 1780 the Societas Meteorologica Palatina (which became defunct in 1795), an early international organization for meteorology, defined seasons as groupings of three whole months as identified by the Gregorian calendar. Ever since, professional meteorologists all over the world have used this definition. Therefore, for temperate areas in the northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December. For the southern hemisphere temperate zone, spring begins on 1 September, summer on 1 December, autumn on 1 March, and winter on 1 June. In
Australasia Australasia is a region that comprises Australia, New Zealand and some neighbouring islands in the Pacific Ocean. The term is used in a number of different contexts, including geopolitically, physiogeographically, philologically, and ecologica ...
the meteorological terms for seasons apply to the temperate zone that occupies all of
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
,
New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...
,
Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...
,
Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...
, the south-eastern corner of
South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
and the south-west of Western Australia, and the south east
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...
areas south of
Brisbane Brisbane ( ) is the capital and most populous city of the states and territories of Australia, Australian state of Queensland, and the list of cities in Australia by population, third-most populous city in Australia and Oceania, with a populati ...
. In Sweden and Finland, meteorologists and news outlets use the concept of
thermal season A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
s, which are defined based on mean daily temperatures. The beginning of spring is defined as when the mean daily temperature permanently rises above 0 °C. The beginning of summer is defined as when the temperature permanently rises above +10 °C, autumn as when the temperature permanently falls below +10 °C, and winter as when the temperature permanently falls below 0 °C. In Finland, "permanently" is defined as when the mean daily averaged temperature remains above or below the defined limit for seven consecutive days. (In Sweden the number of days ranges from 5 to 7 depending on the season.) This implies two things: * the seasons do not begin on fixed dates and must be determined by observation and are known only after the fact, * the seasons begin on different dates in different parts of the country. The
India Meteorological Department The India Meteorological Department (IMD) is an agency of the Ministry of Earth Sciences of the Government of India. It is the principal agency responsible for meteorological observations, weather forecasting and seismology. IMD is headquarter ...
(IMD) designates four climatological seasons: * Winter, occurring from December to February. The year's coldest months are December and January, when temperatures average around in the northwest; temperatures rise as one proceeds towards the equator, peaking around in mainland India's southeast. * Summer or pre-monsoon season, lasting from March to May. In western and southern regions, the hottest month is April; for northern regions of India, May is the hottest month. Temperatures average around in most of the interior. * Monsoon or rainy season, lasting from June to September. The season is dominated by the humid southwest summer monsoon, which slowly sweeps across the country beginning in late May or early June. Monsoon rains begin to recede from North India at the beginning of October. South India typically receives more rainfall. * Post-monsoon or autumn season, lasting from October to November. In the northwest of India, October and November are usually cloudless. Tamil Nadu receives most of its annual precipitation in the northeast monsoon season.


Astronomical

Astronomical timing as the basis for designating the temperate seasons dates back at least to the
Julian Calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...
used by the ancient Romans. As mentioned above,
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath and a prolific author. He is regarded as ancient Rome's greatest scholar, and was described by Petrarch as "the third great light of Rome" (after Vergil and Cicero). He is sometimes calle ...
wrote that spring, summer, autumn, and winter start on the 23rd day of the sun's passage through Aquarius, Taurus, Leo, and Scorpio, respectively, and that (in the Julian Calendar) these days were February 7, May 9, August 11, and November 10. He points out that the lengths are not equal, being 91 (in non-leap years), 94, 91, and 89 days for spring, summer, autumn, and winter, respectively. The midpoints of these seasons were March 24 or 25, June 25, September 25 or 26, and December 24 or 25, which are near to the equinoxes and solstices of his day.
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic '' ...
, in his '' Natural History'', mentions the two equinoxes and the two solstices and gives the lengths of the intervals (values which were fairly correct in his day but are no longer very correct because the
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
has moved from December into January). He then defines the seasons of autumn, winter, spring, and summer as starting half-way through these intervals. He gives "the eighth day to the Kalends of January" (December 25) as the date of the winter solstice, though actually it occurred on the 22nd or 23rd at that time. Nowadays the astronomical timing has winter starting at the winter solstice, spring at the spring equinox, and so on. This is used worldwide, although some countries like Australia, New Zealand, Pakistan and Russia prefer to use meteorological reckoning. The precise timing of the seasons is determined by the exact times of the sun reaching the tropics of Cancer and Capricorn for the
solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countr ...
s and the times of the sun's transit over the equator for the
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es, or a traditional date close to these times. 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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
(periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 19, 20 or 21 March, the point of June solstice on 20 or 21 June, the
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
(apoapsis—farthest point from the sun) on anywhere from 3 July to 6 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December. These "astronomical" seasons are not of equal length, because of the elliptical nature of the orbit of the Earth, as discovered by
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
. From the March equinox it currently takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. Thus the time from the March equinox to the September equinox is 7.56 days longer than from the September equinox to the March equinox.


Variation due to calendar misalignment

The times of the equinoxes and solstices are not fixed with respect to the modern Gregorian calendar, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to keep the March equinox no later than 21 March as accurately as is practical. ''Also see: Gregorian calendar seasonal error.'' The calendar equinox (used in the calculation of Easter) is 21 March, the same date as in the Easter tables current at the time of the Council of Nicaea in AD 325. The calendar is therefore framed to prevent the astronomical equinox wandering onto 22 March. From Nicaea to the date of the reform, the years 500, 600, 700, 900, 1000, 1100, 1300, 1400, and 1500, which would not have been leap years in the Gregorian calendar, amount to nine extra days, but astronomers directed that ten days be removed. Because of this, the (
proleptic Proleptic may refer to: * Prolepsis Prolepsis may refer to: * Prolepsis (rhetoric), a figure of speech in which the speaker raises an objection and then immediately answers it *Prolepsis (literary), anticipating action, a flash forward, see For ...
) Gregorian calendar agrees with the Julian calendar in the
third century The 3rd century was the period from 201 ( CCI) to 300 ( CCC) Anno Domini (AD) or Common Era (CE) in the Julian calendar.. In this century, the Roman Empire saw a crisis, starting with the assassination of the Roman Emperor Severus Alexander i ...
of the
Christian era The terms (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. The term is Medieval Latin and means 'in the year of the Lord', but is often presented using "our Lord" instead of "the Lord", ...
, rather than in the fourth. Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23, and December 21; the four-year average slowly shifts to earlier times as a century progresses. This shift is a full day in about 128 years (compensated mainly by the century "leap year" rules of the Gregorian calendar); as 2000 was a leap year, the current shift has been progressing since the beginning of the last century, when equinoxes and solstices were relatively late. This also means that in many years of the twentieth century, the dates March 21, June 22, September 23, and December 22 were much more common, so older books teach (and older people may still remember) these dates. Note that all the times are given in UTC (roughly speaking, the time at
Greenwich Greenwich ( , ,) is a town in south-east London, England, within the ceremonial county of Greater London. It is situated east-southeast of Charing Cross. Greenwich is notable for its maritime history and for giving its name to the Greenwich ...
, ignoring
British Summer Time During British Summer Time (BST), civil time in the United Kingdom is advanced one hour forward of Greenwich Mean Time (GMT), in effect changing the time zone from UTC±00:00 to UTC+01:00, so that mornings have one hour less daylight, and ev ...
). People living farther to the east (Asia and Australia), whose local times are in advance, see the astronomical seasons apparently start later; for example, in
Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...
(UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall again until 2103. On the other hand, people living far to the west (America), whose clocks run behind UTC, may experience an equinox as early as March 19.


Change over time

Over thousands of years, the Earth's
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...
and orbital eccentricity vary (see
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypot ...
). The equinoxes and solstices move westward relative to the stars while the perihelion and aphelion move eastward. Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change — the average temperature difference between summer and winter in location — will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees. Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.


Solar

Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. This was the case with the seasons described by the Roman scholar
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath and a prolific author. He is regarded as ancient Rome's greatest scholar, and was described by Petrarch as "the third great light of Rome" (after Vergil and Cicero). He is sometimes calle ...
(see above). It was the method for reckoning seasons in medieval Europe, especially by the
Celts The Celts (, see pronunciation for different usages) or Celtic peoples () are. "CELTS location: Greater Europe time period: Second millennium B.C.E. to present ancestry: Celtic a collection of Indo-European peoples. "The Celts, an ancien ...
, and is still ceremonially observed in Republic of Ireland, Ireland and 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 seasons starting at equinoxes and solstices. 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 start of the seasons corresponds to four Pagan agricultural festivals - 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 day of autumn is 1 August (Celtic Lughnasadh).


Solar terms

The Chinese calendar, traditional calendar in China has 4 seasons based on 24 periods known as solar terms. The four seasons ''chūn'' (), ''xià'' (), ''qiū'' (), and ''dōng'' ()—universally translated as "spring", "summer", "autumn", and "winter"—each center around the respective solstice or equinox. Astronomically, the seasons are said to begin on Lichun (, "the start of spring") on about 4 February, Lixia () on about 6 May, Liqiu () on about 8 August, and Lidong () on about 7 November. These dates were not part of the traditional lunar calendar, however, and moveable holidays such as Chinese New Year and the Mid-Autumn Festival are more closely associated with the seasons. It forms the basis of other such systems in East Asian lunisolar calendars.


Six-season reckoning

Some calendars in south Asia use a six-season partition where the number of seasons between summer and winter can number from one to three. The dates are fixed at even intervals of months. In the Hindu calendar of tropical and subtropical India, there are six seasons or Ritu that are calendar-based in the sense of having fixed dates: Vasanta (Ritu), Vasanta (spring), Greeshma (summer), Ritu (Indian season), Varsha (
monsoon A monsoon () is traditionally 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 annual latitudinal oscil ...
), Sharad (autumn), Hemanta (early winter), and Shishira (prevernal or late winter). The six seasons are ascribed to two months each of the twelve months in the Hindu calendar. The rough correspondences are: The Bengali Calendar is similar but differs in start and end times. It has the following seasons or ritu: The Odia calendar, Odia Calendar is similar but differs in start and end times. The Tamil calendar follows a similar pattern of six seasons


Non-calendar-based reckoning

Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; hedgehogs hibernate—winter). So, if we can observe a change in daily floral and 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. In Great Britain, the onset of spring used to be defined as when the maximum daily temperature reached 50 °F (10 °C) in a defined sequence of days. This almost always occurred in March. However, with global warming this temperature is now not uncommon in the winter.


Modern mid-latitude ecological

Six ecological seasons can be distinguished without fixed calendar-based dates like the meteorological and astronomical seasons. Oceanic climate, Oceanic regions tend to experience the beginning of the hibernal season up to a month later than Köppen climate classification#Group D: Continental/microthermal climates, continental climates. Conversely, prevernal and vernal seasons begin up to a month earlier near oceanic and coastal areas. For example, prevernal crocus blooms typically appear as early as February in coastal areas of British Columbia, the British Isles, but generally don't appear until March or April in locations like the Midwest USA or parts of eastern Europe. The actual dates for each season vary by climate region and can shift from one year to the next. Average dates listed here are for mild and cool temperate climate zones in the Northern Hemisphere: * Prevernal (early or pre-spring): Begins February (mild temperate), to March (cool temperate). Deciduous tree buds begin to swell. Some types of migrating birds fly from winter to summer habitats. * Vernal (spring): Begins mid March (mild temperate), to late April (cool temperate). Tree buds burst into leaves. Birds establish territories and begin mating and nesting. * Estival (high summer): Begins June in most temperate climates. Trees in full leaf. Birds hatch and raise offspring. * Serotinal (late summer): Generally begins mid to late August. Deciduous leaves begin to change color in higher latitude locations (above 45 north). Young birds reach maturity and join other adult birds preparing for autumn migration. The traditional "harvest season" begins by early September. * Autumnal (autumn): Generally begins mid to late September. Tree leaves in full color then turn brown and fall to the ground. Birds migrate back to wintering areas. * Hibernal (winter): Begins December (mild temperate), November (cool temperate). Deciduous trees are bare and fallen leaves begin to decay. Migrating birds settled in winter habitats.


Indigenous ecological

Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined the seasons ecologically by observing the activity of the plants, animals and weather around them. Each separate tribal group traditionally observes different seasons determined according to local criteria that can vary from the hibernation of polar bears on the arctic tundras to the growing seasons of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year, as do the Sami people in Scandinavia. Many indigenous people who no longer live directly off the land in traditional often nomadic styles, now observe modern methods of seasonal reckoning according to what is customary in their particular country or region. The North American Cree and possibly other Algonquian languages, Algonquian speaking peoples used or still use a 6-season system. The extra two seasons denoting the freezing and breaking up of the ice on rivers and lakes. The Noongar people of South-West Western Australia recognise maar-keyen bonar, or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors such as changing winds, flowering plants, temperature and migration patterns and lasts approximately two standard calendar months. The seasons also correlate to aspects of the human condition, intrinsically linking the lives of the people to the world that surrounds them and also dictating their movements, as with each season, various parts of country would be visited which were particularly abundant or safe from the elements.


Tropical


Two seasons

In the tropics, where seasonal dates also vary, it is more common to speak of the rainy (or wet, or
monsoon A monsoon () is traditionally 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 annual latitudinal oscil ...
) season versus the
dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The te ...
. For example, in Nicaragua the dry season (November to April) is called "summer" and the rainy season (May to October) is called "winter", even though it is located in the northern hemisphere. There is no noticeable change in the amount of sunlight at different times of the year. However, many regions (such as the northern Indian ocean) are subject to
monsoon A monsoon () is traditionally 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 annual latitudinal oscil ...
rain and wind cycles. Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season. Thus, the tropics are characterized by numerous "mini-seasons" within the larger seasonal blocks of time. In the tropical parts of Australia in the northern parts of
Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...
, Western Australia, and the Northern Territory, wet and dry seasons are observed in addition to or in place of temperate season names.


Three seasons

The most historically important of these are the three seasons—''
flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
'', ''
growth Growth may refer to: Biology * Auxology, the study of all aspects of human physical growth * Bacterial growth * Cell growth * Growth hormone, a peptide hormone that stimulates growth * Human development (biology) * Plant growth * Secondary growth ...
'', and ''
low water Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...
''—which were previously defined by the former annual flooding of the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin language, Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered ...
in
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Mediter ...
. In some tropical areas a three-way division into hot, rainy, and cool season is used. In Thailand three seasons are recognised


Polar

Any point north of the Arctic Circle or south of the Antarctic Circle will have one period in the summer called "polar day" when the sun does not set, and one period in the winter called 'polar night' when the sun does not rise. At progressively higher latitudes, the maximum periods of "midnight sun" and "polar night" are progressively longer. For example, at the military and weather station Alert, Nunavut, 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 6 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. First light comes in late January because the sky has twilight, being a glow on the horizon, for increasing hours each day, for more than a month before the sun first appears with its disc above the horizon. From mid-November to mid-January, there is no twilight. In the weeks surrounding 21 June, in the northern polar region, the sun is at its highest elevation, appearing to circle the sky there without going below the horizon. Eventually, it does go below the horizon, for progressively longer periods each day until around the middle of October, when it disappears for the last time until the following February. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, from mid-November to mid-January, there is no twilight and it is continuously dark. In mid January the first faint wash of twilight briefly touches the horizon (for just minutes per day), and then twilight increases in duration with increasing brightness each day until sunrise at end of February, then on 6 April the sun remains above the horizon until mid October.


Military campaigning seasons

Seasonal weather and climate conditions can become important in the context of military operations. Seasonal reckoning in the military of any country or region tends to be very fluid and based mainly on short to medium term weather conditions that are independent of the calendar. For navies, the presence of accessible ports and bases can allow naval operations during certain (variable) seasons of the year. The availability of ice-free harbor , ice-free or warm-water ports can make navies much more effective. Thus Russia, historically navally constrained when confined to using Arkhangelsk (before the 18th century) and even Kronstadt, has particular interests in maintaining and in preserving access to Baltiysk, Vladivostok, and Sevastopol. Storm seasons or polar winter-weather conditions can inhibit surface warships at sea. Pre-modern armies, especially in Europe, tended to campaign in the summer months - peasant conscription , conscripts tended to melt away at harvest time, nor did it make economic sense in an agricultural society to neglect the sowing season. Any modern Manoeuvre warfare , war of manoeuvre profits from firm ground - summer can provide dry conditions suitable for marching and transport, frozen snow in winter can also offer a reliable surface for a period, but rasputitsa , spring thaws or autumn rains can inhibit campaigning. Rainy-season floods may make rivers temporarily impassable, and winter snow tends to block mountain passes. Taliban offensives are usually confined to the Afghanistan fighting season.


See also

* Horae, Greek goddesses of seasons * Indian summer * Persephone, Greek mythological figure associated with the rebirth of vegetation in the spring * Sun path * Vertumnus, Roman god of the seasons


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)
Why the Earth has seasons
article on h2g2.
Aboriginal seasons of Kakadu

Indigenous seasons (Australian Bureau of Meteorology)

Mt Stirling Seasons

The Lost Seasons



Tutorial on Earth/Sun Relations and Seasons

Sunpreview Season Forecast Project

Satellite photo demonstrating seasons changes in 2004 on NASA website
{{Authority control Calendars Climate patterns Seasons, Units of time Articles containing video clips