A SEASON is a division of the year marked by changes in weather ,
ecology , and amount of daylight . Seasons result from Earth\'s orbit
During May, June, and July, the
Northern Hemisphere is exposed to
more direct sunlight because the hemisphere faces the Sun. The same is
true of the
Southern Hemisphere in November, December, and January. It
is Earth's axial tilt that causes the
In temperate and subpolar regions, four calendar -based seasons are
generally recognized: _spring _, _summer _, _autumn _ or _fall_, and
_winter _. Ecologists often use a six-season model for temperate
climate regions: _prevernal_, _vernal_, _estival_, _serotinal_,
_autumnal_, and _hibernal_. Many tropical regions have two seasons:
the _rainy _, _wet _, or _monsoon season_ and the _dry season _. Some
have a third _cool_, _mild_, or _harmattan season_. Seasons often held
special significance for agrarian societies, whose lives revolved
around planting and harvest times, and the change of seasons was often
attended by ritual . A deciduous tree in winter The six
ecological seasons The four calendar seasons, depicted in an
ancient Roman mosaic from
In some parts of the world, some other "seasons" capture the timing
of important ecological events such as _hurricane season _, _tornado
season _, and _wildfire season_. The most historically important of
these are the three seasons—_flood _, _growth _, and _low water
_—which were previously defined by the former annual flooding of the
* 1 Causes and effects
* 2 Four-season calendar reckoning
* 2.1 Meteorological
* 2.2 Astronomical
* 2.2.1 Variation due to calendar misalignment * 2.2.2 Change over time
* 2.3 Solar * 2.4 Australasian seasons
* 3 Six-season calendar reckoning * 4 Polar day and night
* 5 Non-calendar-based reckoning
* 5.1 Modern mid-latitude ecological * 5.2 Tropical ecological * 5.3 Indigenous ecological
* 6 "Official" designations * 7 See also * 8 References * 9 External links
CAUSES AND EFFECTS
Illumination of the earth at each change of astronomical season
This diagram shows how the tilt of the Earth's axis aligns with
incoming sunlight around the winter solstice of the northern
hemisphere. Regardless of the time of day (i.e. the Earth's rotation
on its axis), the
The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.5 degrees . (This tilt is also known as "obliquity of the ecliptic ".)
Regardless of the time of year, the northern and southern hemispheres
always experience opposite seasons. This is because during summer or
winter, one part of the planet is more directly exposed to the rays of
The effect of axial tilt is observable as the change in day length
and altitude of the
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
Orbital eccentricity can influence temperatures, but on Earth, this
effect is small and is more than counteracted by other factors;
research shows that the
MARITIME AND HEMISPHERIC
Seasonal weather fluctuations (changes) also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño /ENSO and other oceanic cycles, and prevailing winds . A deciduous tree in autumn (fall)
In the temperate and polar regions, seasons are marked by changes in
the amount of sunlight , which in turn often causes cycles of dormancy
in plants and hibernation in animals. These effects vary with latitude
and with proximity to bodies of water. For example, the
The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and vice versa.
In tropical and subtropical regions there is little annual fluctuation of sunlight. However, there are seasonal shifts of a rainy global-scale low pressure belt called the Intertropical convergence zone . As a result, the amount of precipitation tends to vary more dramatically than the average temperature. When the convergence zone is north of the equator, the tropical areas of the northern hemisphere experience their wet season while the tropics south of the equator have their dry season. This pattern reverses when the convergence zone migrates to a position south of the equator.
MID-LATITUDE THERMAL LAG
In meteorological terms, the summer solstice and winter solstice (or the maximum and minimum insolation , respectively) do not fall in the middles of summer and winter. The heights of these seasons occur up to seven weeks later because of seasonal lag. Seasons, though, are not always defined in meteorological terms.
In astronomical reckoning by hours of daylight alone, the solstices and equinoxes 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 which predominate in the northern hemisphere often consider these four dates to be the _start_ of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. The length of these seasons is not uniform because of the elliptical orbit of the earth and its different speeds along that orbit .
FOUR-SEASON CALENDAR RECKONING
Calendar-based reckoning defines the seasons in absolute rather than relative terms. Accordingly, if floral activity is regularly observed during the coolest quarter of the year in a particular area, it is still considered winter despite the traditional association of flowers with spring and summer. Additionally, the seasons are considered to change on the same dates everywhere that uses a particular calendar method regardless of variations in climate from one area to another. Most calendar-based methods use a four-season model to identify the warmest and coldest seasons, which are separated by two intermediate seasons. A deciduous tree in autumn (fall)
Animation of seasonal differences especially snow cover through the year
Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. 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. A tree in summer
In Sweden and Finland, meteorologists use a non-calendar based definition for the seasons based on the temperature. Spring begins when the daily averaged temperature permanently rises above 0 °C, summer begins when the temperature permanently rises above +10 °C, summer ends when the temperature permanently falls below +10 °C and winter begins when the temperature permanently falls below 0 °C. "Permanently" here means that the daily averaged temperature has remained above or below the limit for seven consecutive days. This implies two things: first, the seasons do not begin at fixed dates but must be determined by observation and are known only after the fact; and second, a new season begins at different dates in different parts of the country. 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.
SURFACE AIR TEMPERATURE
UT date and time of
equinoxes and solstices on
EVENT EQUINOX SOLSTICE EQUINOX SOLSTICE
MONTH MARCH JUNE SEPTEMBER DECEMBER
DAY TIME DAY TIME DAY TIME DAY TIME
2010 20 17:32 21 11:28 23 03:09 21 23:38
2011 20 23:21 21 17:16 23 09:04 22 05:30
2012 20 05:14 20 23:09 22 14:49 21 11:12
2013 20 11:02 21 05:04 22 20:44 21 17:11
2014 20 16:57 21 10:51 23 02:29 21 23:03
2015 20 22:45 21 16:38 23 08:21 22 04:48
2016 20 04:30 20 22:34 22 14:21 21 10:44
2017 20 10:28 21 04:24 22 20:02 21 16:28
2018 20 16:15 21 10:07 23 01:54 21 22:23
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:44 22 13:31 21 10:02
Astronomical timing as the basis for designating the temperate seasons dates back at least to the Julian calendar used by the ancient Romans. It continues to be used on many modern Gregorian calendars worldwide, although some countries like Australia, New Zealand, and Russia prefer to use meteorological reckoning. The precise timing of the seasons is determined by the exact times of transit of the sun over the tropics of Cancer and Capricorn for the solstices and the times of the sun's transit over the equator for the equinoxes , 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
These "astronomical" seasons are not of equal length, because of the elliptical nature of the orbit of the Earth, as discovered by 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.
Variation Due To
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 days, but astronomers directed that ten days be removed.
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 the century progresses. This shift is a full day in about 128 years (compensated mainly by the century "leap year" rules of the Gregorian calendar) and 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 of 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
Change Over Time
Over thousands of years, the Earth's axial tilt and orbital eccentricity vary (see 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.
The annual cycle of insolation (
Solar timing is based on insolation in which the solstices and
equinoxes are seen as the midpoints of the seasons. It was the method
for reckoning seasons in medieval Europe, especially by the
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
The traditional calendar in China forms the basis of other such
In Australasia the traditional terms for seasons apply to the
temperate zone that occupies all of
SEASON START DATE END DATE
Spring 1 September 30 November
In the tropical parts of Australia in the northern parts of
SIX-SEASON CALENDAR 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 (spring), Greeshma (summer), Varsha (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:
HINDU SEASON START END HINDU MONTHS MAPPING TO ENGLISH NAMES
Sharad Mid-September Mid-November Ashwin , Kartika autumn
Shishira Mid-January Mid-March Magh , Phalguna prevernal or late winter
BENGALI SEASON START END BENGALI MONTHS MAPPING TO ENGLISH NAMES
The Tamil calendar follows a similar pattern of six seasons
TAMIL SEASON GREGORIAN MONTHS TAMIL MONTHS
IlaVenil (Spring) April 15 to June 14 Chithirai and Vaikasi
MuthuVenil (Summer) June 15 to August 14 Aani and Aadi
Kaar (Monsoon) August 15 to October 14 Avani and Purattasi
Kulir (Autumn) October 15 to December 14 Aipasi and Karthikai
MunPani (Winter) December 15 to February 14 Margazhi and Thai
PinPani (Prevernal) February 15 to April 14 Maasi and Panguni
POLAR DAY AND NIGHT
Any point north of the
For example, at the military and weather station Alert located at
82°30′05″N and 62°20′20″W, on the northern tip of Ellesmere
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.
Seasonal changes regarding a tree over a year
Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; hedgehogs hibernate—winter). So, if we can observe a change in daily floral/animal events, the season is changing. In this sense, ecological seasons are defined in absolute terms, unlike calendar-based methods in which the seasons are relative. If specific conditions associated with a particular ecological season don't normally occur in a particular region, then that area cannot be said to experience that season on a regular basis.
MODERN MID-LATITUDE ECOLOGICAL
Six seasons can be distinguished which do not have fixed
calendar-based dates like the meteorological and astronomical seasons.
Mild oceanic regions tend to experience the beginning of the hibernal
season up to a month later than continental climates , while the
prevernal and vernal seasons begin up to a month earlier near to the
sea. For example, prevernal crocus blooms typically appear as early as
February in mild coastal areas of
* PREVERNAL (early or pre-spring): Begins February or late January (mild temperate), March (cool temperate). Deciduous tree buds begin to swell. Migrating birds fly from winter to summer habitats. * VERNAL (spring): Begins March (mild temperate), April (cool temperate). Tree buds burst into leaves. Birds establish territories and begin mating and nesting. * ESTIVAL (high summer): Begins June in most temperate climates. Trees in full leaf. Birds hatch and raise offspring. * SEROTINAL (late summer): Generally begins mid to late August. Deciduous leaves begin to change color. Young birds reach maturity and join other adult birds preparing for autumn migration. The traditional "harvest season" begins. * 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.
In the tropics, where seasonal dates also vary, it is more common to
speak of the rainy (or wet, or monsoon ) season versus the dry season
. For example, in
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.
See also: Indigenous Australian seasons
Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined the seasons ecologically by observing the activity of the plants, animals and weather around them. Each separate tribal group traditionally observes different seasons determined according to local criteria that can vary from the hibernation of polar bears on the arctic tundras to the growing seasons of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year, as do the Sami people in Scandinavia. Many indigenous people who no longer live directly off the land in traditional often nomadic styles, now observe modern methods of seasonal reckoning according to what is customary in their particular country or region.
As noted, a variety of dates are used in different countries to mark
the changes of seasons, especially those that are calendar based.
These observances are often declared "official" within their
respective jurisdictions 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. The
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* ^ Cain, Fraiser. "Tilt of the Earth". Retrieved 2 May 2014.
* ^ "Fundamentals of physical geography", _PhysicalGeography.net_,
Ch. 6: Energy and Matter:(h) Earth-