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Season

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Everyday life

                                                          CAPTION: Seasons

                                                                  Tropical

                                                                Dry season

                                                                Wet season

                                                          CAPTION: Seasons

                                                                 Temperate

                                                                    Spring

                                                                    Summer

                                                                    Autumn

                                                                    Winter

   A season is one of the major divisions of the year, generally based on
   yearly periodic changes in weather.

   In temperate and polar regions generally four seasons are recognized:
   spring, summer, autumn (fall), and 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, as the
   amount of precipitation may vary more dramatically than the average
   temperature.

   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, wildfire season or a sport season.
   Diagram of the Earth's seasons as seen from the north. Far right:
   December solstice
   Enlarge
   Diagram of the Earth's seasons as seen from the north. Far right:
   December solstice
   Diagram of the Earth's seasons as seen from the south. Far left: June
   solstice
   Enlarge
   Diagram of the Earth's seasons as seen from the south. Far left: June
   solstice

Causes and climatic effects

   Fig. 1This is a diagram of the seasons. Note that, regardless of the
   time of day (i.e. the Earth's rotation on its axis), the North Pole
   will be dark, and the South Pole will be illuminated; see also arctic
   winter. In addition to the density of incident light, the dissipation
   of light in the atmosphere is greater when it falls at a shallow angle.
   Fig. 1
   This is a diagram of the seasons. Note that, regardless of the time of
   day (i.e. the Earth's rotation on its axis), the North Pole will be
   dark, and the South Pole will be illuminated; see also arctic winter.
   In addition to the density of incident light, the dissipation of light
   in the atmosphere is greater when it falls at a shallow angle.

   The seasons result from the Earth's axis being tilted to its orbital
   plane; it deviates by an angle of approximately 23.5 degrees of arc.
   Thus, at any given time during the 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. At any
   given time, regardless of season, the northern and southern hemispheres
   experience opposite seasons (see Fig. 2 and Month ranges of seasons
   (below) and Effect of sun angle on climate).

   Seasonal weather fluctuations 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.

   In the temperate and polar regions, seasons are marked by changes in
   the amount of sunlight, which in turn often cause 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 presence of all that water. The result is
   that the South Pole is consistently colder during the southern winter
   than the North Pole during the northern winter.
   Fig. 2As the Earth revolves around the Sun, the seasons in the northern
   and southern hemispheres are opposite.
   Fig. 2
   As the Earth revolves around the Sun, the seasons in the northern and
   southern hemispheres are opposite.

   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,
   and when it is spring in the Northern hemisphere it is autumn in the
   Southern hemisphere, and vice versa.

   In the tropics, there is no noticeable change in the amount of
   sunlight. However, many regions (famously the northern Indian Ocean)
   are subject to monsoon rain and wind cycles. Curiously, a study of
   temperature records over the past 300 years (David Thompson, Science,
   April 1995) 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 winter solstice and summer solstice (or
   the date maximum/minimum insolation) do not fall in the middle of
   winter and summer respectively. The heights of these seasons occur up
   to a month later due to seasonal lag. Seasons though, are not always
   defined in meteorological terms; see reckoning

   Compared to axial tilt, other factors contribute little to seasonal
   temperature changes. It's a common misconception that the seasons are
   the result of the variation in Earth’s distance to the sun due to 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 a few
   degrees warmer when farther from the sun. Mars however experiences wide
   temperature variations and violent dust storms every year at
   perihelion.

Polar day and night

   Illumination of the earth during various seasons
   Enlarge
   Illumination of the earth during various seasons

   A common misconception is that, within the Arctic and Antarctic
   Circles, the sun rises once in the spring and sets once in the fall;
   thus, the day and night are erroneously thought to last uninterrupted
   for 183 calendar days each. This is true only in the immediate region
   of the poles themselves.

   What does happen is that 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 periods of " midnight sun" (or
   "midday dark" for the other side of the globe) are progressively
   longer. For example, at the military and weather station called 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 a bit higher, and stays
   up a bit 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 been
   showing twilight, or at least a pre-dawn glow on the horizon, for
   increasing hours each day, for more than a month before that first
   sliver of sun appears.

   In the weeks surrounding 21 June, the sun is at its highest, and it
   appears to circle the sky without ever going below the horizon.
   Eventually, it does go below the horizon, for progressively longer and
   longer periods each day until, around the middle of October, 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, nothing breaks the darkness. 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

   The four season mahjong tiles on the right and the four flower tiles on
   the other side. The flower tiles are arranged in accord to their
   growing seasons.
   Enlarge
   The four season mahjong tiles on the right and the four flower tiles on
   the other side. The flower tiles are arranged in accord to their
   growing seasons.
   Personifications of the Four Seasons are a frequent theme in Roman
   mosaics, like this from Complutum.
   Enlarge
   Personifications of the Four Seasons are a frequent theme in Roman
   mosaics, like this from Complutum.

   The date at which each season begins depends on how it is defined. In
   the United States, the seasons are often considered to begin at the
   astronomical solstices and equinoxes: these are sometimes known as the
   " astronomical seasons". By this reckoning, summer begins at summer
   solstice, winter at winter solstice, spring at the vernal equinox and
   autumn at the autumnal equinox.

   United Kingdom and Ireland have no hard and fast rules about seasons,
   but most follow the equinoxes and solstices; However informally many
   people use three calendar months for each season with Spring being
   March, April and May, etc.

   It is interesting to note that historically seasons were considered to
   begin about 4 weeks earlier than the meteorological seasons, and seven
   weeks earlier than the astronomical seasons:
     * spring began on February 2 / Candlemas or February 1 / Imbolc,
     * summer on May 1st / May Day/ Beltane ,
     * autumn on August 1st / Lammas/ Lughnasadh, and
     * winter on November 1st / All Hallows / Samhain.

   Accordingly, midsummer and midwinter were, as their names suggest, the
   middle of summer and winter.

   So, in meteorology for the Northern hemisphere:
     * spring begins by convention on March 1,
     * summer on June 1,
     * autumn on September 1 and
     * winter on December 1.

   This definition is also followed in Denmark and former USSR.

   Conversely, for the Southern hemisphere, meteorological
     * summer begins on December 1,
     * autumn on March 1,
     * winter on June 1 and
     * spring on September 1.

   This definition is also followed in Australia.

   The Chinese, Japanese, and Korean calendars are based on a lunisolar
   calendar, where the solstices and equinoxes mark the middle of each
   season. This is very close to the British & Irish definitions of
   seasons. See below, using solstices and equinoxes as mid-days.

   CAPTION: Month ranges of seasons

   Northern Hemisphere Month Southern Hemisphere
   Traditional Meteorological Astronomical Meteorological Astronomical
                 Winter Winter Winter January Summer Summer
                               Spring February
                             Spring March Autumn
                             Spring April Autumn
                                 Summer May
                             Summer June Winter
                             Summer July Winter
                                Autumn August
                           Autumn September Spring
                            Autumn October Spring
                               Winter November
                           Winter December Summer

Mid-season

   CAPTION: UTC Date and Time of
   Solstice and Equinox

   year Equinox
   Mar Solstice
   June Equinox
   Sept Solstice
   Dec
   day time day time day time day time
   2002 20 19:16 21 13:24 23 04:55 22 01:14
   2003 21 01:00 21 19:10 23 10:47 22 07:04
   2004 20 06:49 21 00:57 22 16:30 21 12:42
   2005 20 12:33 21 06:46 22 22:23 21 18:35
   2006 20 18:26 21 12:26 23 04:03 22 00:22
   2007 21 00:07 21 18:06 23 09:51 22 06:08
   2008 20 05:48 20 23:59 22 15:44 21 12:04
   2009 20 11:44 21 05:45 22 21:18 21 17:47
   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 11:11
   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

   In the conventional US calendar, the following dates are considered to
   be halfway through a season:
     * Winter ( February 3)
     * Spring ( May 5 or May 6)
     * Summer ( August 7)
     * Autumn ( November 6)

Using solstices and equinoxes as Middle of Season

   When the solstices (peaks) are used as the middle of each season, as is
   in Jieqi and East Asian countries, the longest day of the year occurs
   in the middle of summer, and conversely the shortest day of the year in
   the middle of winter.

   So, in the Northern Hemisphere:
     * Spring begins ( February 3)
     * Summer begins ( May 5 or May 6)
     * Autumn begins ( August 7)
     * Winter begins ( November 6)

   In the Southern Hemisphere:
     * Autumn begins ( February 3)
     * Winter begins ( May 5 or May 6)
     * Spring begins ( August 7)
     * Summer begins ( November 6)

Seasons in images

   In hemiboreal and temperate climates:

   In the winter, the plants are at rest.

   In the spring, the plants start to grow again.

   In the summer the plants grow.

   In the autumn, the trees turn yellow and drop their leaves.

   Retrieved from " http://en.wikipedia.org/wiki/Season"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
