Axial parallelism

Axial parallelism is a characteristic of the Earth (and most other orbiting bodies in space) in which the direction of the axis remains parallel to itself throughout its orbit.

The Earth's orbit exhibits approximate axial parallelism, maintaining its direction towards Polaris (the "North Star") year-round. This is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right.^[5][6][7][8] Minor variation in the direction of the axis, known as axial precession, takes place over the course of 26,000 years, and therefore is not noticeable to modern human civilization.

Axial tilt

This diagram shows how the tilt of 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 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 of rotation being tilted with respect to its orbital plane by an angle of approximately 23.4 degrees.^[9] (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 Sun than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March 20 to around September 22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole 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 Northern Hemisphere will be experiencing spring as the hours of daylight increase, and the Southern Hemisphere is experiencing autumn as daylight hours shorten.

The effect of axial tilt is observable as the change in day length and altitude of the Sun at solar noon (the Sun's culmination) during the year. 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.

• 
  Illumination of Earth by Sun at the northern solstice.
• 
  Illumination of Earth by Sun at the southern solstice.
• 
  Illumination of Earth at each change of astronomical season
• Animation of Earth as seen daily from the Sun looking at UTC+02:00, showing the solstice and changing seasons.
• 
  Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m²).

Elliptical Earth orbit

Compared to axial parallelism and axial tilt, other factors contribute little to seasonal temperature changes.^[4] The seasons are not the result of the variation in Earth's distance to the Sun because of its elliptical orbit.^[10] In fact, Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (the point farthest from the Sun) in July, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere.^[11] 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.^[11] 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.^[12]

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.

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 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

Animation of seasonal differences especially snow cover through the year

The tropical and (to a lesser degree) subtropical regions see little annual fluctuation of sunlight and temperature due Earth's moderate 23.4-degree tilt being insufficient to appreciably effect the strength of the sun's rays annually. However, the slight differences between the solstices and the equinoxes are still enough to cause seasonal shifts to occur along a rainy, low-pressure belt called the Intertropical Convergence Zone (ICZ). As a result, the amount of precipitation 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 solstices (the maximum and minimum 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. 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 Earth's elliptical orbit and its different speeds along that orbit.^[13]

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.^[15] 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.^[16][17]

Meteorological

Four temperate and subpolar seasons: (above) winter, spring, (below) summer, autumn

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.^[18] According to this definition, 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.^[19][20] In Australasia the meteorological terms for seasons apply to the temperate zone that occupies all of New Zealand, New South Wales, Victoria, Tasmania, the south-eastern corner of South Australia and the south-west of Western Australia, and the south east Queensland areas south of Brisbane.

In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, which are defined based on mean daily temperatures.^[21] 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.

Surface air temperature
Diagram was calculated (abscisse: the 21st of each month). Calculation based on data published by Jones et al.[22]	The picture shows Figure 7 as published by Jones et al.[22]

The India Meteorological Department (IMD) designates four climatological seasons:^[23]

• Winter, occurring from December to February. The year's coldest months are December and January, when temperatures average around 10–15 °C (50–59 °F) in the northwest; temperatures rise as one proceeds towards the equator, peaking around 20–25 °C (68–77 °F) 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 32–40 °C (90–104 °F) 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.

In China, a common temperature-based reckoning holds that it is winter for the period when temperatures are below 10°C on average and summer for the period when temperatures are above 22°C on average. This means that areas with relatively extreme climates (such as the Paracel Islands and parts of the Tibetan plateau) may be said to have summer all year round or winter all year round.^[24]

Astronomical

Astronomical timing as the basis for designating the temperate seasons dates back at least to the Julian Calendar used by the ancient Romans. As mentioned above, Varro 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, respectively.^[14] The midpoints of these seasons were March 24 or 25, June 25, September 25 or 26, and December 24 or 25, which are near to the equinoxes and solstices of his day.

Pliny the Elder, in his Natural History, mentions the two equinoxes and the two solstices and gives the lengths of the intervals (values which were fairly correct in his day but are no longer very correct because the perihelion has moved from December into January). He then defines the seasons of autumn, winter, spring, and summer as starting half-way through these intervals.^[31] He gives "the eighth day to the Kalends of January" (December 25) as the date of the winter solstice, though actually it occurred on the 22nd or 23rd at that time.^[32]

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,^[33] 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 solstices and the times of the sun's transit over the equator for the equinoxes, or a traditional date close to these times.^[34]

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 (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 (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.

Exaggerated illustration of Earth's elliptical orbit around the Sun, marking that the orbital extreme points (apoapsis and periapsis) are not the same as the four seasonal extreme points (equinox and solstice)

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. 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.

Solar

The annual cycle of insolation (Sun energy, shown in blue) with key points for seasons (middle), quarter days (top) and cross-quarter days (bottom) along with months (lower) and Zodiac houses (upper). The cycle of temperature (shown in pink) is delayed by seasonal lag.

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 (see above). It was the method for reckoning seasons in medieval Europe, especially by the Celts, and is still ceremonially observed in 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's play A Midsummer Night's Dream, which is set on the summer solstice. On the Celtic calendar, the start of the seasons corresponds to four Pagan agricultural festivals - the traditional first day of winter is 1 November (Samhain, 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).

Modern mid-latitude ecological

Six ecological seasons can be distinguished without fixed calendar-based dates like the meteorological and astronomical seasons.^[36] Oceanic regions tend to experience the beginning of the hibernal season up to a month later than 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 do not appear until March or April in locations like the Midwest US 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,^[19] 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 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.

The Noongar people of South-West Western Australia recognise maar-keyen bonar,^[37] or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors^[38] 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.^[39]

Tropical

Wet and dry seasons

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 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.^[42]

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 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.^[43] 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 conscripts tended to melt away at harvest time, nor did it make economic sense in an agricultural society to neglect the sowing season.^[44] Any modern war of manoeuvre 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 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 Afghanistan fighting season.