A
season is a division of the
year, marked by changes in
weather.
Seasons result from the yearly revolution of the Earth around the
Sun and the
tilt of the Earth's axis relative
to the plane of revolution. In temperate and polar regions, the
seasons are marked by changes in the intensity of sunlight that
reaches the Earth's surface, variations of which may cause animals
to go into hibernation or to migrate, and plants to be
dormant.
During June, July and August, the northern hemisphere is exposed to
more direct sunlight because the hemisphere faces the sun. The same
is true of the southern hemisphere in December, January, and
February. It is the tilt of the Earth that causes the Sun to be
higher in the sky during the summer months which increases the
solar flux.
In
temperate and
polar regions generally four seasons are
recognized:
spring,
summer,
autumn,
winter.
In some
tropical and
subtropical regions it is more common to speak
of the
rainy (or wet, or
monsoon) season versus the
dry
season, because the amount of
precipitation may vary more
dramatically than the average temperature. For example, in
Nicaragua, the dry season is called Summer (Oct to May) and the
rainy season is called Winter (Apr to Nov) even though it is
located in the northern hemisphere.
In other tropical areas a three-way division into hot, rainy and
cool season is used.
In some parts of the world, special "seasons" are loosely defined
based upon important events such as a
hurricane season,
tornado
season or a
wildfire season.
Chinese seasons are traditionally based on 24 periods known as
solar terms, and begin at the midpoint
of solstices and equinoxes.
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Causes and effects
Illumination of the earth during
various seasons
The seasons result from the
Earth's
axis being
tilted
to its
orbital plane; it
deviates by an angle of approximately 23.5
degree. Thus, at any given time
during summer or winter, one part of the planet is more directly
exposed to the rays of the
Sun (see
Fig. 1). This exposure alternates as the Earth
revolves in its orbit.
Therefore, at any given time, regardless of
season, the northern and southern hemispheres experience opposite seasons.
The effect of axis tilt is observable from the change in
day length, and
altitude of the Sun at
noon (the
culmination of the
Sun), during a
year.
Seasonal weather differences between hemispheres are further caused
by the
elliptical
orbit of Earth. Earth reaches
perihelion (the point in its orbit closest to the
Sun) in January, and it reaches
aphelion (farthest point from the Sun) in July.Even
though the effect this has on Earth's seasons is minor, it does
noticeably soften the northern hemisphere's winters and summers. In
the southern hemisphere, the opposite effect is observed.
Seasonal weather fluctuations (changes) also depend on factors such
as proximity to
oceans or other large bodies
of water,
current in those oceans,
El Niño/ENSO and
other oceanic cycles, and prevailing
winds.
In the temperate and polar regions, seasons are marked by changes
in the amount of
sunlight, which in turn
often causes
cycles of
dormancy in plants and
hibernation in
animals. These effects vary with latitude and with proximity to
bodies of water.
For example, the South Pole is in the middle of the continent of Antarctica and therefore a considerable distance from the
moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by
the water. The result is that the South Pole is consistently
colder during the southern winter than the North Pole during the
northern winter.
The cycle of seasons in the polar and temperate zones of one
hemisphere is opposite to that in the other.
When it is summer in
the Northern
Hemisphere, it is winter in the Southern Hemisphere, and vice versa.
In the
tropics, there is no noticeable change
in the amount of sunlight.
However, many regions (such as the northern
Indian
ocean) are subject to monsoon
rain and wind cycles.A study of
temperature records over the past 300 years shows that the climatic
seasons, and thus the
seasonal year,
are governed by the
anomalistic
year rather than the
tropical
year.
In meteorological terms, the summer
solstice and winter solstice (or the maximum and
minimum
insolation, respectively) do not
fall in the middles of summer and winter. The heights of these
seasons occur up to seven weeks later because of
seasonal lag. Seasons, though, are not always
defined in meteorological terms.
Compared to axial tilt, other factors contribute little to seasonal
temperature changes. The seasons are not the result of the
variation in
Earth’s distance to the sun
because of its
elliptical orbit.
Orbital eccentricity can
influence temperatures, but on Earth, this effect is small and is
more than counteracted by other factors; research shows that the
Earth as a whole is actually slightly warmer when
farther
from the sun. This is because the northern hemisphere has more land
than the southern, and land warms more readily than sea.
Mars however experiences wide temperature
variations and
violent dust storms every year at
perihelion.
Polar day and night
Any point north of the
Arctic Circle
or south of the
Antarctic Circle
will have one period in the summer when the sun does not set, and
one period in the winter when the sun does not rise. At
progressively higher latitudes, the maximum periods of "
midnight sun" and "
polar
night" are progressively longer.
For example, at the
military and weather station Alert 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 in
mid-February and each day it climbs higher and stays up longer; by
21 March, the sun is up for 12 hours. However, mid-February
is not first light. The sky (as seen from Alert) has
twilight, or at least a pre-dawn glow on the
horizon, for increasing hours each day, for more than a month
before the sun first appears.
In the weeks surrounding 21 June, the sun is at its highest, and it
appears to circle the sky without going below the horizon.
Eventually, it does go below the horizon, for progressively longer
periods each day until, around the middle of November, it
disappears for the last time. For a few more weeks, "day" is marked
by decreasing periods of twilight. Eventually, for the weeks
surrounding 21 December, it is continuously dark. In later winter,
the first faint wash of light briefly touches the horizon (for just
minutes per day), and then increases in duration and pre-dawn
brightness each day until sunrise in February.
Reckoning
Meteorological
Meteorological seasons are reckoned by temperature, with summer
being the hottest quarter of the year and winter the coldest
quarter of the year. Using this reckoning, the
Roman calendar began the year and the spring
season on the first of March, with each season occupying three
months. In 1780 the Societas Meteorologica Palatina, an early
international organization for meteorology, defined seasons as
groupings of three whole months. Ever since, professional
meteorologists all over the world have used this definition.So, in
meteorology for the Northern hemisphere: spring begins on 1 March,
summer on 1 June, autumn on 1 September, and winter on 1
December.
In Sweden, meteorologists use a different definition for the
seasons, based on the temperature: spring begins when the daily
averaged temperature permanently rises above 0° C, summer begins
when the temperature permanently rises above +10° C, summer ends
when the temperature permanently falls below +10° C and winter
begins when the temperature permanently falls below 0° C.
"Permanently" here means that the daily averaged temperature has
remained above or below the limit for seven consecutive days. This
implies two things: first, the seasons do not begin at fixed dates
but must be determined by observation and are known only after the
fact. And, second, a new season begins at different dates in
different parts of the country.
Astronomical
In
astronomical reckoning, the
solstices and
equinoxes
ought to be the middle of the respective seasons, but, because of
thermal lag, regions with a continental climate often consider
these four dates to be the start of the seasons as in the diagram,
with the
cross-quarter days
considered seasonal midpoints. The length of these seasons is not
uniform because of the elliptical orbit of the earth and its
different speeds along
that orbit.
From the March equinox it takes 92.75 days until the June solstice,
then 93.65 days until the September equinox, 89.85 days until the
December solstice and finally 88.99 days until the March
equinox.
Because of the differences in the Northern and Southern
Hemispheres, it is no longer considered appropriate to use the
northern-seasonal designations for the astronomical quarter days.
The modern convention for them is: March Equinox, June Solstice,
September Equinox and December Solstice. The oceanic climate of the
Southern Hemisphere produces a shorter temperature lag, so the
start of each season is usually considered to be several weeks
before the respective
solstice or
equinox in this hemisphere, in other countries with
oceanic climates, and in cultures with Celtic roots.
Ecological seasons
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.
The hot regions
Here there are two seasons:
- Rainy season (winter and spring)
- Dry season (summer and autumn)
The temperate areas
We can clearly distinguish six seasons. Mild temperate regions tend
to experience the beginnings of the prevernal and vernal seasons
about a month earlier than indicated below. Dates listed here are
for the Northern Hemisphere:
- Prevernal (ca.1 March–1 May)
- Vernal (ca.1 May–15 June)
- Estival (ca.15 June–15 August)
- Serotinal (ca.15 August–15 September)
- Autumnal (ca.15 September–1 November)
- Hibernal (ca.1 November–1 March)
Cold regions
There are again only two seasons:
- Polar Day (spring and summer)
- Polar Night (autumn and winter)
Traditional season divisions
Traditional seasons are reckoned by
insolation, with summer being the quarter of the
year with the greatest insolation and winter the quarter with the
least. These seasons begin about four weeks earlier than the
meteorological seasons and 7 weeks earlier than the astronomical
seasons.
In traditional reckoning, the seasons begin at the
cross-quarter days. The solstices and
equinoxes are the
midpoints of these seasons. For example,
the days of greatest and least insolation are considered the
"midsummer" and "midwinter" respectively.
This reckoning is used by various traditional cultures in the
Northern Hemisphere, including
East Asian
and
Irish cultures.
In Iran, Afghanistan and some other parts of Middle East the beginning of the astronomical
spring is the beginning of the new year which is called Nowruz.
So, according to traditional reckoning, winter begins between 5
November and 10 November,
Samhain,
立冬 (lìdōng or rittou); spring between 2
February and 7 February,
Imbolc,
立春 (lìchūn or rissyun); summer between 4
May and 10 May,
Beltane,
立夏 (lìxià or rikka); and autumn between 3
August and 10 August,
Lughnasadh,
立秋 (lìqiū or rissyuu). The middle
of each season is considered Mid-winter, between 20 December and 23
December,
冬至 (dōngzhì or touji);
Mid-spring, between 19 March and 22 March,
春分 (chūnfēn or syunbun); Mid-summer,
between 19 June and 23 June,
夏至
(xiàzhì or geshi); and Mid-autumn, between 21 September and 24
September,
秋分 (qiūfēn or
syuubun).
Australia
In
Australia, the traditional aboriginal
people defined the seasons by what was happening to the plants,
animals and weather around them. This led to each separate tribal
group having different seasons, some with up to eight seasons each
year. However, most modern
Aboriginal Australians follow either
four or six meteorological seasons, as do non-Aboriginal
Australians.
Official dates are as follows: 1st day of March, June, September
and December for the start of Autumn, Winter, Spring and Summer,
respectively.
India
In
India, and in the Hindu
calendar, there are six seasons or Ritu are Hemant (pre-winter), Shishira
(Winter), Basant (Spring), Grishma (Summer), Varsha
(Rainy) and Sharad (Autumn).
See also
References
(
,)()7. Maris, Mihaela, St. Luchian School, Bacau,
Romania, Seasonal Variations of the Bird Species, ref. ecological
seasons pp. 207-209.
External links