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A season is a division of the year marked by changes in weather, ecology and hours of daylight. Seasons result from the yearly orbit of the Earth around the Sun and the tilt of the Earth's rotational axis relative to the plane of the orbit. 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 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 the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July, and August are the hottest months in the northern hemisphere and December, January, and February are the hottest months in the southern hemisphere.

In temperate and subpolar regions, four calendar-based seasons (with their adjectives) are generally recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal). In American English and Canadian English, fall is sometimes used as a synonym for both autumn and autumnal. Ecologists often use a six-season model for temperate climate regions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four.

Surface air temperature
Diagram was calculated (Abscisse: 21. of each month)
Calculation based on data published by Jones et al.
The picture shows Figure 7 as published by Jones et al.
UT date and time of
equinoxes and solstices on Earth
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
Australasian seasons
Season Start date End date
Summer 1 December 28/29 February
Autumn 1 March 31 May
Winter 1 June 31 August
Spring 1 September 30 November
Hindu season Start End Hindu Months Mapping to English Names
Vasanta Mid-March Mid-May Chaitra, Vaishakha spring
Greeshma Mid-May Mid-July Jyeshtha, Ashadha summer
Varsha Mid-July Mid-September Shraavana, Bhadrapada monsoon
Sharad Mid-September Mid-November Ashwin, Kartika autumn
Hemanta Mid-November Mid-January Maargashirsha, Pausha early winter
Shishira Mid-January Mid-March Magh, Phalguna prevernal or late winter
Bengali season Start End Bengali Months Mapping to English Names
Bosonto বসন্ত (ঋতু) Mid-February Mid-April Falgun, Choitro Spring
Grishmo (গ্রীষ্ম) Mid-April Mid-June Boishakh, Joishtho Summer
Borsha (বর্ষা) Mid-June Mid-August Asharh, Srabon Monsoon
Shorot (শরৎ) Mid-August Mid-October Bhadro, Ashwin Autumn
Hemonto (হেমন্ত) Mid-October Mid-December Kartik, Ogrohayon Late Autumn
Sit (শীত) Mid-December Mid-February Poush, Magh Winter
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

  • 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.
  • 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.


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