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A season is a division of the year based on changes in
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

weather
,
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers at the individual, , , , an ...
, 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 ...

daylight
hours in a given region. On
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, seasons are the result of
Earth's orbit Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), and one complete orbit takes  days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). Jean Meeus, ''Astron ...
around the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
and Earth's
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
relative to the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit 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 against the ...

ecliptic
plane. In temperate and polar regions, the seasons are marked by changes in the intensity of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
that reaches the Earth's surface, variations of which may cause animals to undergo
hibernation Hibernation is a state of minimal activity and metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological ...

hibernation
or to
migrate Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum len ...
, 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 vary. The
Northern Hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
experiences more direct sunlight during May, June, and July, as the hemisphere faces the Sun. The same is true of the
Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western He ...

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
month A month is a unit of time, used with calendars, which is approximately as long as a natural Orbital period, period related to the motion of the Moon; ''month'' and ''Moon'' are cognates. The traditional concept arose with the cycle of Moon phases; s ...
s, which increases the
solar flux Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one jou ...
. However, due to
seasonal lag Seasonal lag is the phenomenon A phenomenon (; plural phenomena) is an observable fact or event. The term came into its modern philosophical usage through Immanuel Kant Immanuel Kant (, ; ; 22 April 1724 – 12 February 1804) was a G ...
, 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 Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use ...
and sub-polar regions, four seasons based on the
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
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 heli ...
'', ''
summer Summer is the hottest of the four temperate In geography, the temperate climates of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land ...

summer
'', ''
autumn Autumn, also known as fall in American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Curr ...

autumn
'' (or ''fall''), and ''
winter Winter is the est of the year in and . It occurs between and . The tilt of Earth's axis causes seasons; winter occurs when a is oriented away from the . Different cultures define different dates as the start of winter, and some use a de ...

winter
''. Ecologists often use a six-season model for temperate
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
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 In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
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. Rain ...
''. Some have a third ''cool'', ''mild'', or ''
harmattan 300px, Harmattan haze surrounding Abuja National Mosque in Abuja ">Abuja.html" ;"title="Abuja National Mosque in Abuja">Abuja National Mosque in Abuja The Harmattan is a season in West Africa, which occurs between the end of November and the m ...

harmattan
season''. "Seasons" can also be dictated by the timing of important ecological events such as '' hurricane season'', ''
tornado season 300px, Areas worldwide with the highest frequency of tornadoes are indicated by orange shading. Tornado A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in ra ...
'', and ''
wildfire A wildfire, bushfire, wildland fire or rural fire is an unplanned, unwanted, uncontrolled fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction ...

wildfire
season''. Some examples of historical importance are the ancient Egyptian seasons—''
flood A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the . Floods are an area of study of the discipline and are of significant concern in , a ...
'', '' growth'', and ''
low water (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift. Tides are the rise and fall of sea level ...
''—which were previously defined by the former annual flooding of the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin Nobiin, or Mahas, is a Northern Nubian languages, Nubian language of the Nilo-Saharan languages, Nilo-Saharan language family. "Nobiin" is the genitive case, genitive form of ''Nòòbíí'' ("Nub ...

Nile
in
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identi ...

Egypt
. Seasons often hold special significance for agrarian societies, whose lives revolve around
planting Sowing is the process of planting. An area or object that has had seeds planted in it will be described as a sowed area. Plants which are usually sown Among the major field crops, oats, wheat Wheat is a grass widely Agriculture, cultiva ...

planting
and
harvest Harvesting is the process of gathering a ripe crop from the field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an ...

harvest
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, ...

ritual
. 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 tilt

The seasons result from the Earth's
axis of rotation Rotation around a fixed axis is a special case of rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotati ...
being
tilted
tilted
with respect to its
orbital plane The orbital plane of a revolving body is the geometric plane in which its orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satel ...
by an angle of approximately 23.4
degrees Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
. (This tilt is also known as "obliquity of the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit 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 against the ...

ecliptic
".) Regardless of the time of year, the
northern
northern
and
southern hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western He ...

southern hemisphere
s always experience opposite seasons. This is because during
summer Summer is the hottest of the four temperate In geography, the temperate climates of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land ...

summer
or
winter Winter is the est of the year in and . It occurs between and . The tilt of Earth's axis causes seasons; winter occurs when a is oriented away from the . Different cultures define different dates as the start of winter, and some use a de ...

winter
, 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 Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

Equator
are the
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

equinox
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 The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only ast ...
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 where intersects its surface. It is the southernmost point on Earth and lies on the of Earth from the . Situated on the ...
of the Earth are just on the terminator, 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 An equinox is the instant of time when the plane of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's s ...
, 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 heli ...
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 ...

daylight
increase, and the Southern Hemisphere is experiencing
autumn Autumn, also known as fall in American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Curr ...

autumn
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 Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's ...
and
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference and a point or object. The exact definition and reference datum varies according to the context (e.g. ...
of the Sun at solar
noon Noon (or midday) is 12 o'clock The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the In ...

noon
(the Sun's
culmination In observational astronomy, culmination is the instant of time of the transit of a astronomical object, celestial object (the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's meridian (astronomy), local me ...
) during the
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, 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 ...
. 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:North season.jpg, Diagram of the Earth's seasons as seen from the north. Far right: southern solstice 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 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 orbital eccentricity, eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a ...
. In fact, Earth reaches
perihelion upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...

perihelion
(the point in its orbit closest to the Sun) in January, and it reaches
aphelion upright=1.15, The two-body system of interacting primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object consti ...

aphelion
(the point farthest from the Sun) in July, so the slight contribution of
orbital eccentricity In astrodynamics Orbital mechanics or astrodynamics is the application of ballistics Ballistics is the field of mechanics Mechanics (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ε ...
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 The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
s or other large bodies of water,
currents Currents 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 (stream), c ...
in those oceans,
El Niño es, El Niño, translation=The Boy (; ) is the warm phase of the El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the Tropics, tropical easte ...
/ENSO and other oceanic cycles, and prevailing
wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by th ...

wind
s. In the
temperate In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use ...
and
polar regions The polar regions, also called the frigid zones, of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent ...
, seasons are marked by changes in the amount of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
, which in turn often causes cycles of dormancy in plants and
hibernation Hibernation is a state of minimal activity and metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological ...

hibernation
in animals. These effects vary with
latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

latitude
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 where intersects its surface. It is the southernmost point on Earth and lies on the of Earth from the . Situated on the ...
is in the middle of the continent of
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

Antarctica
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 The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only ast ...
is in the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major s. It spans an area of approximately and is also known as the coldest of all the oceans. The (IHO) recognizes it as an ocean, although some call it the Arctic Medit ...

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 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 region of Earth surrounding the Equator. They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S; these latitudes correspond to ...

tropical
and
subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones located to the north and south of the tropics, Torrid Zone. Geographically part of the Geographical zone#Temperate zones, ...

subtropical
regions see little annual fluctuation of sunlight. However, seasonal shifts occur along a rainy, low-pressure belt called the Intertropical Convergence Zone (ICZ). As a result, the amount of
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
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 In astronomy and navigation, the celestial sphere is an abstraction, abstr ...

solstice
s (the maximum and minimum
insolation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...

insolation
) 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 A phenomenon (; plural phenomena) is an observable fact or event. The term came into its modern philosophical usage through Immanuel Kant Immanuel Kant (, ; ; 22 April 1724 – 12 February 1804) was a G ...
. Seasons, though, are not always defined in meteorological terms. In
astronomical Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics Mathematics (from Ancient Greek, Gr ...
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 ...

daylight
alone, the solstices and
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

equinox
es are in the ''middle'' of the respective seasons. Because of seasonal lag due to thermal absorption and release by the oceans, regions with a
continental climate Continental climates often have a significant annual variation in temperature (hot summers and cold winters). They tend to occur in the middle latitudes (40 to 55 north), within large landmasses where prevailing winds In meteorology Mete ...
, which predominate in the
Northern Hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
, often consider these four dates to be the ''start'' of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. The length of these seasons is not uniform because of Earth's
elliptical orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an orbital eccentricity, eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a ...
and its different speeds along that orbit.


Four-season reckoning

Most calendar-based methods use a four-season model to identify the warmest and coldest seasons, which are 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. Additionally, the seasons are considered to change on the same dates everywhere that uses a particular calendar method regardless of variations in climate from one area to another. The four seasons have been in use since at least Roman times, as in Rerum rusticarum of
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known ...
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 Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened ...

Julius Caesar
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 the meteorological terms for seasons apply to the temperate zone that occupies all of
New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon Engl ...

New Zealand
,
New South Wales New South Wales (abbreviated as NSW) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...
,
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, Seychelles ...
,
Tasmania Tasmania (), abbreviated as TAS, is an island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atol ...
, the south-eastern corner of
South Australia South Australia (abbreviated as SA) is a States and territories of Australia, 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 Austral ...

South Australia
and the south-west of Western Australia, and the south east
Queensland Queensland ( ) is a state situated in northeastern Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the ...

Queensland
areas south of
Brisbane Brisbane ( ) is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller low ...

Brisbane
. In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, 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 headquartered ...

India Meteorological Department
(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 Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
used by the ancient Romans. As mentioned above,
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known ...
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. The midpoints of these seasons were March 24 or 25, June 25, September 25 or 26, and December 24 or 25, which correspond to the equinoctes and solstices of his day. 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 In astronomy and navigation, the celestial sphere is an abstraction, abstr ...

solstice
s and the times of the sun's transit over the equator for the
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

equinox
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 upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...
(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 upright=1.15, The two-body system of interacting primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object consti ...
(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 of the orbit of the Earth, as discovered by
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe as ...

Johannes Kepler
. From the March equinox it currently takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. Thus the time from the March equinox to the September equinox is 7.56 days longer than from the September equinox to the March equinox.


Variation due to calendar misalignment

The times of the equinoxes and solstices are not fixed with respect to the modern Gregorian calendar, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to keep the March equinox no later than 21 March as accurately as is practical. ''Also see: Gregorian calendar seasonal error.'' The calendar equinox (used in the calculation of Easter) is 21 March, the same date as in the Easter tables current at the time of the Council of Nicaea in AD 325. The calendar is therefore framed to prevent the astronomical equinox wandering onto 22 March. From Nicaea to the date of the reform, the years 500, 600, 700, 900, 1000, 1100, 1300, 1400, and 1500, which would not have been leap years in the Gregorian calendar, amount to nine extra days, but astronomers directed that ten days be removed. Because of this, the { proleptic) Gregorian calendar agrees with the Julian calendar in the
third century The 3rd century AD was the period from 201 (Roman numerals, CCI) to 300 (Roman numerals, CCC). In this century, the Roman Empire saw a Crisis of the Third Century, 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 calendar The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a ...
, 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 ( , , , or ) is a town in South London, south-east London, England, located in the Historic county of England, historic county of Kent and the Ceremonial counties of England, ceremonial county of Greater London. It is situated east ...

Greenwich
, ignoring
British Summer Time During British Summer Time (BST), civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at ...
). 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 named the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesia Polynesia (, ; from grc, πολύς "many" and grc, νῆσος "island") ( to, Faka-Polinisia; mi, Porinihia; haw, Polenekia; ...

Tonga
(UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall again until 2103. On the other hand, people living far to the west (America), whose clocks run behind UTC, may experience an equinox as early as March 19.


Change over time

Over thousands of years, the Earth's
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
and orbital eccentricity vary (see
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the 's movements on its over thousands of years. The term is named for n and . In the 1920s, he hypothesized that variations in , , and resulted in cyclical variation in the r ...

Milankovitch cycles
). The equinoxes and solstices move westward relative to the stars while the perihelion and aphelion move eastward. Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change — the average temperature difference between summer and winter in location — will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees. Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.


Solar

Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. This was the case with the seasons described by the Roman scholar
Varro Marcus Terentius Varro (; 116–27 BC) was a Roman polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known ...
(see above). It was the method for reckoning seasons in medieval Europe, especially by the
Celts The Celts (, see pronunciation of ''Celt'' for different usages) are. "CELTS location: Greater Europe time period: Second millennium B.C.E. to present ancestry: Celtic a collection of Indo-European peoples The Indo-European languages ar ...

Celts
, and is still ceremonially observed in
Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic. It is separated from Great Britain to its east by the North Channel (Great Britain and Ireland), North Channel, the Irish Sea ...

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 William Shakespeare (bapt. 26 April 1564 – 23 April 1616) was an English playwright, poet and actor, widely regarded as the greatest writer in the English language and the world's greatest dramatist. He is often called England's national p ...

William Shakespeare
's play ''
A Midsummer Night's Dream ''A Midsummer Night's Dream'' is a comedy Comedy (from the el, κωμῳδία, ''kōmōdía'') is a genre of fiction that consists of discourses or works intended to be humor Humour (Commonwealth English The use of the Eng ...

A Midsummer Night's Dream
'', which is set on the summer solstice. On the
Celtic calendar The Celtic calendar is a compilation of pre-Christian Celtic systems of timekeeping, including the Gaulish Coligny calendar, used by Celtic countries to define the beginning and length of the day, the week, the month, the seasons, quarter days, a ...
, the start of the seasons corresponds to four
Pagan Paganism (from classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, includ ...

Pagan
agricultural festivals - the traditional first day of winter is 1 November (
Samhain Samhain (, , ; gv, Sauin ) is a Gaels, Gaelic festival marking the end of the harvest season and beginning of winter or "Celtic calendar#Medieval Irish and Welsh calendars, darker-half" of the year. It is held on 1 November but with celebration ...
, 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). {, class="wikitable" , +Irish seasons , - ! Season !! Start date !! End date , - , Winter , , 1 November (All Saints' Day) , , 31 January , - , Spring (season), Spring , , 1 February (St. Brigid's Day) , , 30 April , - , Summer , , 1 May (May Day) , , 31 July , - , Autumn , , 1 August (Lughnasadh) , , 31 October (Hallowe'en)


Solar terms

The 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 method 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 In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
), 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: {, class="wikitable" , - ! Hindu season ! Start ! End ! Hindu months ! Mapping to English names , - , Vasanta (Ritu), Vasanta , Mid-March , Mid-May , Chaitra, Vaishakha , spring , - , Greeshma , Mid-May , Mid-July , Jyeshtha, Ashadha , summer , - , Varshā , Mid-July , Mid-September , Shraavana, Bhadrapada , monsoon , - , Sharad , Mid-September , Mid-November , Ashvin, Kartika (month), Kartika , autumn , - , Hemant , Mid-November , Mid-January , Maargashirsha, Pushya , early winter , - , Shishir , Mid-January , Mid-March , Maagha, Magh, Phalguna , prevernal or late winter The Bengali Calendar is similar but differs in start and end times. It has the following seasons or ritu: {, class="wikitable" , - ! Bengali season (ঋতু) ! Start ! End ! Bengali months ! Mapping to English names , - , Bôsônto বসন্ত(spring) , Mid-February , Mid-April , Falgun, Choitro , Spring (Season), Spring , - , Greeshmo (গ্রীষ্ম)(summer) , Mid-April , Mid-June , Boishakh, Joishtho , Summer , - , Bôrsha (বর্ষা) (monsoon) , Mid-June , Mid-August , Asharh, Srabon , Monsoon , - , Shôrôt (শরৎ) (autumn/ fall) , Mid-August , Mid-October , Bhadro, Ashwin , Autumn , - , Hemônto (হেমন্ত) (frost/ late autumn) , Mid-October , Mid-December , Kartika (month), Kartik, Ogrohayon , Dry season, Late Autumn , - , Sheet (শীত) (winter) , Mid-December , Mid-February , Poush, Magh (Bengali calendar), Magh , Winter The Odia calendar, Odia Calendar is similar but differs in start and end times. {, class="wikitable" , - ! Odia Season
(ଋତୁ) !! Season !! Odia months !! Gregorian , - , ଗ୍ରୀଷ୍ମ
Grīṣmå , , Summer , , Båiśākhå–Jyeṣṭhå , , April–June , - , ବର୍ଷା
Bårṣā , , Monsoon , , Āṣāṛhå–Śrābåṇ , , June–August , - , ଶରତ
Śåråt , , Autumn , , Bhādråb–Āświn , , August–October , - , ହେମନ୍ତ
Hemåntå , , Pre-Winter , , Kārtik–Mārgåśir , , October–December , - , ଶୀତ
Śīt , , Winter , , Pouṣå–Māghå , , December–February , - , ବସନ୍ତ
Båsåntå , , Spring (season), Spring , , Fālgun–Chåitrå , , February–April , - The Tamil calendar follows a similar pattern of six seasons {, class="wikitable" , - ! Tamil season ! Gregorian months ! Tamil months , - , MuthuVenil (summer) , April 15 to June 14 , Chithirai and Vaikasi , - , Kaar (monsoon) , June 15 to August 14 , Aani and Aadi , - , Kulir (autumn) , August 15 to October 14 , Avani and Purattasi , - , MunPani (winter) , October 15 to December 14 , Aipasi and Karthikai , - , PinPani (prevernal) , December 15 to February 14 , Margazhi and Thai , - , IlaVenil (spring) , February 15 to April 14 , Maasi and Panguni


Non-calendar-based reckoning

Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; hedgehogs hibernate—winter). So, if 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. {, class="wikitable" , - !Cree season !Approximate months !English translation , - , Pipon , Jan/Feb , Winter , - , Sekwun , Mar/Apr , Break-up , - , Mithoskumin , May/Jun , Spring , - , Nepin , Jul/Aug , Summer , - , Tukwakin , Sep/Oct , Autumn , - , Mikiskaw , Nov/Dec , Freeze-up The Noongar people of South-West Western Australia recognise maar-keyen bonar, 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. {, class="wikitable" , - !Noongar season !Approximate months !Cultural parallel , - , Birak (first summer) , December to January , Season of the young , - , Bunuru (second summer) , February to March , Season of adolescence , - , Djeran (autumn) , April to May , Season of adulthood , - , Makuru (the first rains) , June to July , Fertility season , - , Djilba (the second rains) , August to September , Season of conception , - , Kambarang (wildflower season) , October to November , Season of birth


Tropical


Two seasons

In the tropics, where seasonal dates also vary, it is more common to speak of the rainy (or wet, or
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
) 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. Rain ...
. 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 In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
rain and wind cycles. Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season. Thus, the tropics are characterized by numerous "mini-seasons" within the larger seasonal blocks of time. In the tropical parts of Australia in the northern parts of
Queensland Queensland ( ) is a state situated in northeastern Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the ...

Queensland
, Western Australia, and the Northern Territory, wet and dry seasons are observed in addition to or in place of temperate season names. {, class="wikitable" , +Meteorological Tropical seasons , - ! Northern Hemisphere !! Southern Hemisphere !! Start date !! End date , - , Dry season , , Wet season , , 1 November , , 30 April , - , Wet season , , Dry season , , 1 May , , 31 October


Three seasons

The most historically important of these are the three seasons—''
flood A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the . Floods are an area of study of the discipline and are of significant concern in , a ...
'', '' growth'', and ''
low water (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift. Tides are the rise and fall of sea level ...
''—which were previously defined by the former annual flooding of the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin Nobiin, or Mahas, is a Northern Nubian languages, Nubian language of the Nilo-Saharan languages, Nilo-Saharan language family. "Nobiin" is the genitive case, genitive form of ''Nòòbíí'' ("Nub ...

Nile
in
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identi ...

Egypt
. In some tropical areas a three-way division into hot, rainy, and cool season is used. In Thailand three seasons are recognised {, class="wikitable" , - !Thai season !Months , - , Ruedu nao (cold season) , mid October to mid February , - , Ruedu ron (hot season) , mid February to mid May , - , Ruedu fon (rainy season) , mid May to mid October


Polar

Any point north of the Arctic Circle or south of the Antarctic Circle will have one period in the summer called "polar day" when the sun does not set, and one period in the winter called 'polar night' when the sun does not rise. At progressively higher latitudes, the maximum periods of "midnight sun" and "polar night" are progressively longer. For example, at the military and weather station 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 manouevre 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 Afghan 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


Notes


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