Tropical ecology

From Wikipedia, the free encyclopedia

Tropical ecology is the study of the relationships between the biotic and abiotic components of the tropics, or the area of the Earth that lies between the Tropic of Cancer and the Tropic of Capricorn (23.4378° N and 23.4378° S, respectively). The tropical climate experiences hot, humid weather and rainfall year round. While many might associate the region solely with the rainforests, the tropics are home to a wide variety of ecosystems that boast a great wealth of biodiversity, from exotic animal species to seldom-found flora. Tropical ecology began with the work of early English naturalists and eventually saw the establishment of research stations throughout the tropics devoted to exploring and documenting these exotic landscapes. The burgeoning ecological study of the tropics has led to increased conservation education and programs devoted to the climate. This climatic zone offers numerous advantages to ecologists conducting a wide array of studies, from rich biodiversity to vast lands untainted by man.

Origins

The roots of tropical ecology can be traced to the voyages of European naturalists in the late 19th and early 20th centuries. Men who might be considered early ecologists such as Alexander Von Humboldt, Thomas Belt, Henry Walter Bates, and even Charles Darwin sailed to tropical locations and wrote extensively about the exotic flora and fauna they encountered. While many naturalists were simply drawn to the exotic nature of the tropics, some historians argue that the naturalists conducted their studies on tropical islands in order to increase the likelihood that their work might bring about social and political change.^[1] In any case, these early explorations and the subsequent writings that came from them comprise much of the early work of tropical ecology and served to spark further interest in the tropics among other naturalists. Henry Walter Bates, for example, wrote extensively about a species of toucan he encountered while traveling along the Amazon River. Bates discovered that if one toucan called out, the other surrounding toucans would mimic his or her call, and the forest would quickly fill with the sounds of toucans; this was one of the first documented studies of animal mimicry.^[2] Alexander Von Humboldt voyaged throughout South America, from Venezuela through the Andes Mountains. There, Humboldt and his associate, Aimé Bonpland, stumbled upon an interesting ecological concept. As the pair traveled from the base of the mountains to the peak, they noticed that the species of plants and animals would change according to which climatic zone they were in relative to their elevation. This simple discovery aided the theorization of the life zone concept, which would eventually give way to the popularization of the concept of ecosystems.^[2] Another voyager, William Beebe, researched many species of birds in tropical locations and published a large gamut of academic works on his findings that greatly shaped the field of ornithology. According to his biographer Carol Grant Gould, "The effects William Beebe had on science... are enormous and lasting. He made an effective transition between the Victorian natural historian, content to collect and classify the natural world, and the modern experimental biologist."^[3] The work of these early pioneers not only lead to an increased interest in the burgeoning field of tropical ecology, but also had far reaching implications for scientific study on the whole.

Conservation and management

B02 Ecuador 010 - Plants of the rainforest, February 1985

The tropics receive a lot of attention when it comes to conservation and management due to increased public awareness of the significance of tropical ecosystems and the delicacy with which they must be treated. The rainforests are subjects of heightened attention due to the excessive deforestation and logging that occurs in those ecosystems. In the 1980s, the United Nations Food and Agriculture Organization conducted a study that concluded that 15.4 million hectares (100 acres) of tropical forest was lost per year. In addition, 5.6 million hectares were logged each year.^[4] This landmark study sparked widespread interest in the tropical ecosystem, and a great number of non-profits and outspoken ecologists engaged in an extended fight to "save the rainforest" that continues today. This battle has manifested itself in a number of ways, one of which is the outcropping of biodiversity institutes in tropical locations dedicated to stopping the excessive deforestation of the landscape, one of the most notable of which was established in Costa Rica. The work of the Costa Rican National Biodiversity Institute (INBio) has served as a model for other biodiversity institutes. First, it must be noted that rainforests harbor the most alkaloid-producing plants of any biome; alkaloids are compounds that are crucial to the production of Western drugs.^[5] Due to the abundance of these compounds, pharmaceutical companies all over the world look to the rainforests for new medicinal treatments. In the early 1990s, the heads of INBio signed a deal with the pharmaceutical behemoth Merck that called for cooperation between the two entities in discovering and exploring new natural treatments in the Costa Rican rainforests.^[6] Ecologists, government officials, and corporations alike praised this decision as decisive progress in an ongoing struggle to work cooperatively in utilizing tropical biodiversity while ensuring the stability of tropical ecosystems.

Significance of ecology in the tropics

Vicuña Atacama, Chile

It is advantageous for ecologists and naturalists to study plants, animals, and ecosystems in the tropical climate for a number of reasons. For one, the tropics are home to a wide array of ecosystems, from rainforests to deserts. In that sense, the tropics are a great place for ecologists to conduct diverse studies without traveling too far from a research center. Secondly, the temperature in the tropics rarely hinders plant growth and activity; flora can be studied nearly year round, as cold weather never stunts plant activity. In addition to climatic reasons, the traditionally sparse population of the tropics has greatly aided research in the area, as the landscape is largely untainted by mankind and machinery. While this may not be the case so much as of late, the vast amounts of untapped land in the tropics still make for prime research territory. Finally, the tropics are valuable to ecologists because they are home to some of the oldest lands on Earth, including Chile's Atacama Desert and Australia's Peneplain. Thus, plant communities have been growing and evolving for millions of uninterrupted years, which makes for interesting study.^[7] That being said, while it may be advantageous to study ecology in the tropics, this is not to say that it is without difficulty. The ecosystems native to the tropics and the biodiversity they boast are dwindling. Half of the species located in biodiversity hotspots are in danger of extinction, and many of the plants with potential medicinal uses are dying off.^[8] In this sense, ecological study in the tropics is not as easily conducted as it once was; this is the reason why much of the modern ecological work in the field is aimed towards conservation and management as opposed to general research.

Tropics

From Wikipedia, the free encyclopedia

World map with the intertropical zone highlighted in red

Tropical climate zones of the Earth where all twelve months have mean temperatures above 18 °C (64.4 °F).

The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at 23°26′14.3″ (or 23.43731°) N and the Tropic of Capricorn in the southern hemisphere at 23°26′14.3″ (or 23.43731°) S; these latitudes correspond to the axial tilt of the Earth. The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone). The tropics include all the areas on the Earth where the Sun reaches a subsolar point, a point directly overhead at least once during the solar year.

The tropics are distinguished from the other climatic and biomatic regions of Earth, the middle latitudes and the polar regions on either side of the equatorial zone.

Seasons and climate

A graph showing the zonally averaged monthly precipitation. The tropics receive more precipitation than higher latitudes. The precipitation maximum, which follows the solar equator through the year, is under the rising branch of the Hadley circulation; the sub-tropical minima are under the descending branch and cause the desert areas.

Tropical sunset over the sea in Kota Kinabalu, Malaysia

"Tropical" is sometimes used in a general sense for a tropical climate to mean warm to hot and moist year-round, often with the sense of lush vegetation.

Many tropical areas have a dry and wet season. The wet season, rainy season or green season, is the time of year, covering one or more months, when most of the average annual rainfall in a region falls.^[1] Areas with wet seasons are disseminated across portions of the tropics and subtropics.^[2] Under the Köppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more.^[3] Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year.^[4] Some areas with pronounced rainy seasons see a break in rainfall during mid-season when the intertropical convergence zone or monsoon trough moves poleward of their location during the middle of the warm season.^[5]

When the wet season occurs during the warm season, or summer, precipitation falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves, freshwater quality improves and vegetation grows significantly, leading to crop yields late in the season. Floods cause rivers to overflow their banks, and some animals to retreat to higher ground. Soil nutrients diminish and erosion increases. The incidence of malaria increases in areas where the rainy season coincides with high temperatures. Animals have adaptation and survival strategies for the wetter regime. Unfortunately, the previous dry season leads to food shortages into the wet season, as the crops have yet to mature.

Regions within the tropics may well not have a tropical climate. There are alpine tundra and snow-capped peaks, including Mauna Kea, Mount Kilimanjaro, and the Andes as far south as the northernmost parts of Chile and Argentina. Under the Köppen climate classification, much of the area within the geographical tropics is classed not as "tropical" but as "dry" (arid or semi-arid) including the Sahara Desert, the Atacama Desert and Australian Outback.

Tropical ecosystems

Coconut palms in the warm, tropical climate in Brazil

Tropical forest near Fonds-Saint-Denis, Martinique, France

Tropical plants and animals are those species native to the tropics. Tropical ecosystems may consist of rainforests, dry deciduous forests, spiny forests, desert and other habitat types. There are often significant areas of biodiversity, and species endemism present, particularly in rainforests and dry deciduous forests. Some examples of important biodiversity and/or high endemism ecosystems are: El Yunque National Forest in Puerto Rico, Costa Rican and Nicaraguan rainforests, Amazon Rainforest territories of several South American countries, Madagascar dry deciduous forests, the Waterberg Biosphere of South Africa, and eastern Madagascar rainforests. Often the soils of tropical forests are low in nutrient content, making them quite vulnerable to slash-and-burn deforestation techniques, which are sometimes an element of shifting cultivation agricultural systems.

In biogeography, the tropics are divided into Paleotropics (Africa, Asia and Australia) and Neotropics (Caribbean, Central America, and South America). Together, they are sometimes referred to as the Pantropic. The Neotropical region should not be confused with the ecozone of the same name; in the Old World, there is no such ambiguity, as the Paleotropics correspond to the Afrotropical, Indomalayan, and partly the Australasian and Oceanic ecozones.

Image gallery

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  A Morpho peleides (Blue Morpho) tropical butterfly
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  A Graphium agamemnon (Tailed Jay) belongs to the swallowtail family.

Season

From Wikipedia, the free encyclopedia

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.^[1][2] 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, 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.

Various calendars used in South Asia define six seasons.

The six ecological seasons

The four calendar seasons, depicted in an ancient Roman mosaic from Tunisia.

An Empire style chariot clock depicting an allegory of the four seasons. France, c. 1822.

Hot regions have two or three seasons; the rainy (or wet, or monsoon) season and the dry season, and, in some tropical areas, a cool or mild season.

In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornado season, or a wildfire season.

Causes and effects

Illumination of the earth at each change of astronomical season

Fig. 1
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 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.

Axis tilt

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.^[3] (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 (see Fig. 1) 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 northern and southern hemispheres. At 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 noon (the culmination of the Sun) during a year. The low angle of Sun during the winter months means that incoming rays of solar radiation is spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Lower intensity light is less able to heat the ground. 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.

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  Illumination of Earth by Sun at the northern solstice.
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  Illumination of Earth by Sun at the southern solstice.
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  Diagram of the Earth's seasons as seen from the north. Far right: southern solstice
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  Diagram of the Earth's seasons as seen from the south. Far left: northern solstice
• Play media

  Animation of Earth as seen daily from the Sun looking at UTC+02:00, showing the solstice and changing seasons.
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  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.^[4] In fact, 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, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern hemisphere.^[5] 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.^[5] Any noticeable intensification of the southern hemisphere's winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the southern hemisphere.^[6]

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 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.

Tropics

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.

A study of temperature records over the past 300 years^{[7][page needed]} shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.

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.^[8]

Four-season calendar reckoning

Calendar-based reckoning defines the seasons in relative rather than absolute 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 coolest or coldest seasons which are separated by two intermediate seasons.

Modern mid-latitude meteorological

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.^[9] 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.^[10][11]

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.

Surface air temperature
Diagram was calculated (Abscisse: 21. of each month) Calculation based on data published by Jones et al. [12]	The picture shows Figure 7 as published by Jones et al.[12]

Mid-latitude astronomical

UT date and time of equinoxes and solstices on Earth[13]
event	equinox		solstice		equinox		solstice
month	March		June		September		December
year
	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	08:20	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 world-wide, 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. ^[14]

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 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.

Note: Distances are exaggerated and not to scale

The 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 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 calendar misalignment

The time 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 on 21 March as accurately as is practical, though it does not always achieve this. Also see: Gregorian calendar seasonal error.

Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century "leap year" rules of the Gregorian calendar). 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 UTC (roughly speaking, the time at Greenwich, ignoring British Summer Time). People living farther to the east (Asia and Australia), whose local times are in advance, will see the astronomical seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date which will not crop up again until 2103. On the other hand, people living far to the west (America) whose clocks run behind UTC may experience an equinox as early as March 19.

Change over time

Over thousands of years, the Earth's axial tilt and orbital eccentricity vary (see Milankovitch cycles). 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.

Traditional solar: Europe and East Asia

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 Celts, and is still ceremonially observed in 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 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). The Celtic dates corresponded to four Pagan agricultural festivals.

The traditional calendar in China forms the basis of other such systems in East Asia. Its seasons are traditionally based on 24 periods known as solar terms.^[15] The four seasons chūn (春), xià (夏), qiū (秋), and dōng (冬) are universally translated as "spring", "summer", "autumn", and "winter" but actually begin much earlier, with the solstices and equinoxes forming the midday of each season rather than their start. Astronomically, the seasons are said to begin on Lichun (立春, lit. "standing spring") on 7 February, Lixia (立夏) on 10 May, Liqiu (立秋) on 10 August, and Lidong (立冬) on 10 November. These dates were not part of the traditional lunar calendar, however, and moveable holidays such as Chinese New Year and the Mid-Autumn Festival are more closely associated with the seasons.

South Asian (mid-latitude and tropical) six-season calendars

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
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 Calendar is similar but differs in start and end time. It has the following seasons or ritu:

Bengali season	Start	End	Bengali Months	Mapping to English Names
Bosonto	mid-February	mid-April	Falgun, Choitro	Spring
Grishmo	Mid-April	Mid-June	Boishakh, Joishtho	Summer
Borsha	Mid-June	Mid-August	Asharh, Srabon	Monsoon
Shorot	Mid-August	Mid-October	Bhadro, Ashwin	Autumn
Hemonto	mid-October	mid-December	Kartik, Ogrohayon	Late Autumn
Sit	mid-December	mid-February	Poush, Magh	Winter

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 15	Maasi and Panguni

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 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.^[16]

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.

Non-calendar-based reckoning

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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.^[17] Mild temperate regions tend to experience the beginning of the hibernal season up to a month later than cool temperate areas, while the prevernal and vernal seasons begin up to a month earlier. For example, prevernal crocus blooms typically appear as early as February in mild coastal areas of British Columbia, the British Isles, and western and southern 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 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.
• 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.

Modern tropical ecological

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 Nicaragua the dry season (November to April) is called 'summer' and the rainy season (May to October) is called 'winter', even though it is located in the northern hemisphere. In some tropical areas a three-way division into hot, rainy, and cool season is used. There is no noticeable change in the amount of sunlight at different times of the year. However, many regions (such as the northern Indian ocean) are subject to monsoon rain and wind cycles.

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.

Indigenous ecological (polar, mid-latitude, and tropical)

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,^[10] 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.

"Official" designations

As noted, a variety of dates are used in different countries to mark the changes of seasons, especially those that are calendar based. These different observances are often declared "official" within their respective jurisdictions, especially by the media and promotional organizations in each nation.^[18][19] 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.^[20]