Misplaced Pages

#109890

75-763: The Harmattan is a season in West Africa that occurs between the end of November and the middle of March. It is characterized by the dry and dusty northeasterly trade wind , of the same name, which blows from the Sahara over West Africa into the Gulf of Guinea . The name is related to the word haramata in the Twi language. The temperature is cold mostly at night in some places but can be very hot in certain places during daytime. Generally, temperature differences can also depend on local circumstances. The Harmattan blows during

150-493: A change in daily floral and animal events can be observed, 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 do not normally occur in a particular region, then that area cannot be said to experience that season on a regular basis. Six ecological seasons can be distinguished without fixed calendar-based dates like

225-584: A distinct rainy season. Also subtropical areas like Florida , South and Southeast Texas, and southern Louisiana in the United States have a rainy season. Monsoon regions include the Indian subcontinent , Southeast Asia (including Indonesia and Philippines ), northern sections of Australia , Polynesia , Central America , western and southern Mexico , the Desert Southwest of

300-404: A four-season model to demarcate the warmest and coldest seasons, which are further separated by two intermediate seasons. Calendar-based reckoning defines the seasons in relative rather than absolute terms, so the coldest quarter-year is considered winter even if floral activity is regularly observed during it, despite the traditional association of flowers with spring and summer. The major exception

375-408: A month where average precipitation is 60 millimetres (2.4 in) or more. In contrast to areas with savanna climates and monsoon regimes, Mediterranean climates have wet winters and dry summers. Dry and rainy months are characteristic of tropical seasonal forests : in contrast to tropical rainforests , which do not have dry or wet seasons, since their rainfall is equally distributed throughout

450-639: A wet season in the winter months. Similarly, the wet season in the Negev Desert of Israel extends from October through May. At the boundary between the Mediterranean and monsoon climates lies the Sonoran Desert , which receives the two rainy seasons associated with each climate regime. The wet season is known by many different local names throughout the world. For example, in Mexico it

525-787: Is 1 November ( Samhain , the Celtic origin of Halloween ); spring starts 1 February (Celtic Imbolc ); summer begins 1 May ( Beltane , the Celtic origin of May Day ); the first day of autumn is 1 August (Celtic Lughnasadh ). The traditional calendar in China has 4 seasons based on 24 periods, twelve of which are called zhōngqi and twelve of which are known as jiéqi . These periods are collectively known in English as "solar terms" or "solar breaths". The four seasons chūn ( 春 ), xià ( 夏 ), qiū ( 秋 ), and dōng ( 冬 )—translated as "spring", "summer", "autumn", and "winter" —each center on

600-657: 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. Any noticeable intensification of southern winters and summers due to Earth's elliptical orbit

675-504: 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),

750-425: Is called "summer" and the rainy season (May to October) is called "winter", even though it is located in the northern hemisphere. There is no noticeable change in the amount of sunlight at different times of the year. Instead, many regions (such as the northern Indian Ocean ) have varying monsoon rain and wind cycles. Wet season The wet season (sometimes called the rainy season or monsoon season )

825-460: Is characterized by cold, dry, dust-laden wind, and also wide fluctuations in the ambient temperatures of the day and night. Temperatures can easily be as low as 9 °C (48 °F) all day, but sometimes in the afternoon the temperature can also soar to as high as 30 °C (86 °F), while the relative humidity drops under 5%. It can also be hot in some regions, like in the Sahara. The air

SECTION 10

#1732765961110

900-585: Is contradictory. These are mainly a matter of custom and not generally proclaimed by governments north or south of the equator for civil purposes. 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

975-418: 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

1050-461: Is in the tropics where, as already noted, the winter season is not observed. The four seasons have been in use since at least Roman times, as in Rerum rusticarum of Varro Varro says that spring, summer, autumn, and winter start on the 23rd day of the sun's passage through Aquarius, Taurus, Leo, and Scorpio, respectively. Nine years before he wrote, Julius Caesar had reformed the calendar, so Varro

1125-510: Is known as "storm season". Different names are given to the various short "seasons" of the year by the First Nations of Northern Australia : the wet season typically experienced there from December to March is called Gudjewg . The precise meaning of the word is disputed, although it is widely accepted to relate to the severe thunderstorms, flooding, and abundant vegetation growth commonly experienced at this time. In tropical areas, when

1200-634: Is mitigated by the abundance of water in the Southern Hemisphere. 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,

1275-449: Is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right. 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. 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 . (This tilt

1350-590: Is particularly dry and desiccating when the Harmattan blows over the region. The Harmattan brings desert-like weather conditions: it lowers the humidity, dissipates cloud cover, prevents rainfall formation and sometimes creates big clouds of dust which can result in dust storms or sandstorms . The wind can increase fire risk and cause severe crop damage. The interaction of the Harmattan with monsoon winds can cause tornadoes . In some countries in West Africa,

1425-462: Is the time of year when most of a region's average annual rainfall occurs. Generally, the season lasts at least one month. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and subtropics . Under the Köppen climate classification , for tropical climates , a wet season month is defined as

1500-421: The rainy / wet / monsoon season and the dry season . Some have a third cool , mild , or harmattan season . "Seasons" can also be dictated by the timing of important ecological events such as hurricane season , tornado season , and wildfire season . Some examples of historical importance are the ancient Egyptian seasons— flood , growth , and low water —which were previously defined by

1575-633: The Midwestern United States and 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: Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined

SECTION 20

#1732765961110

1650-642: The Paracel Islands and parts of the Tibetan plateau ) may be said to have summer all year round or winter all year round. 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

1725-654: 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

1800-630: 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

1875-720: The former annual flooding of the Nile in Egypt . Seasons often hold special significance for agrarian societies, whose lives revolve around planting and harvest times, and the change of seasons is often attended by ritual . The definition of seasons is also cultural. In India, from ancient times to the present day, six seasons or Ritu based on south Asian religious or cultural calendars are recognised and identified for purposes such as agriculture and trade. The Earth's orbit exhibits approximate axial parallelism , maintaining its direction toward Polaris (the "North Star") year-round. This

1950-581: 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 . Most calendar-based partitions use

2025-512: The ( proleptic ) Gregorian calendar agrees with the Julian calendar in the third century of the Christian era , rather than in the fourth. Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23, and December 21; the four-year average slowly shifts to earlier times as a century progresses. This shift is a full day in about 128 years (compensated mainly by

2100-455: 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 the altitude of

2175-460: 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. He gives "the eighth day to

2250-494: The Gregorian calendar. 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. In Australasia the meteorological terms for seasons apply to

2325-507: The Harmattan increases infant and child mortality, as well as has persistent adverse health impacts on surviving children. Humidity can drop lower than 15%, which can result in spontaneous nosebleeds for some people. Other health effects on humans may include conditions of the skin (dryness of the skin), dried or chapped lips, eyes, and respiratory system, including aggravation of asthma . Season The Northern Hemisphere experiences most direct sunlight during May, June, and July (thus

Harmattan - Misplaced Pages Continue

2400-531: The ICZ migrates to a position south of the Equator. 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

2475-470: 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. 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

2550-471: 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. 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, Pakistan and Russia prefer to use meteorological reckoning. The precise timing of

2625-403: The March equinox. The times 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 no later than 21 March as accurately as is practical. The calendar equinox (used in

2700-577: 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

2775-539: 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

2850-571: The Sun at solar noon (the Sun's culmination ) during the year . The low angle of the 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. Compared to axial parallelism and axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not

2925-807: The United States, southern Guyana , and northeast Brazil . Northern Guyana has two wet seasons: one in early spring and the other in early winter. In western Africa, there are two rainy seasons across southern sections, but only one across the north. Within the Mediterranean climate regime, the west coast of the United States, the southwest coast of Australia and South Africa, the Mediterranean coastline of Italy, Spain, Greece, Lebanon, Syria, Algeria, Morocco, Tunisia, and Turkey, as well as areas further inland in Western Asia which include Jordan, Northern Iraq and most parts of Iran, experience

3000-452: 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. Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. This was

3075-403: The beginning of the wet season. The onset of the rainy season signals the departure of the monarch butterfly from Mexico. Tropical species of butterflies show larger dot markings on their wings to fend off possible predators and are more active during the wet season than the dry season . Within the tropics and warmer areas of the subtropics, decreased salinity of near shore wetlands due to

Harmattan - Misplaced Pages Continue

3150-646: The calculation of Easter) is 21 March, the same date as in the Easter tables current at the time of the Council of Nicaea in AD 325. The calendar is therefore framed to prevent the astronomical equinox wandering onto 22 March. From Nicaea to the date of the reform, the years 500, 600, 700, 900, 1000, 1100, 1300, 1400, and 1500, which would not have been leap years in the Gregorian calendar, amount to nine extra days, but astronomers directed that ten days be removed. Because of this,

3225-609: 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

3300-434: The century "leap year" rules of the Gregorian calendar); as 2000 was a leap year, the current shift has been progressing since the beginning of the last century, when equinoxes and solstices were relatively late. This also means that in many years of the twentieth century, the dates 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. All

3375-554: The charging of local aquifers during the wet season. Water also softens, as the concentration of dissolved materials reduces during the rainy season. Erosion is also increased during rainy periods. Arroyos that are dry at other times of the year fill with runoff, in some cases with water as deep as 10 feet (3.0 m). Leaching of soils during periods of heavy rainfall depletes nutrients. The higher runoff from land masses affects nearby ocean areas, which are more stratified , or less mixed, due to stronger surface currents forced by

3450-500: 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 India Meteorological Department (IMD) designates four climatological seasons: 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

3525-566: The dry season, which occurs during the months with the lowest sun. In this season, the subtropical ridge of high pressure stays over the central Sahara and the low-pressure Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. On its passage over the Sahara, the Harmattan picks fine dust and sand particles (between 0.5 and 10 microns). It is also known as the "doctor wind", because of its invigorating dryness compared with humid tropical air. This season differs from winter because it

3600-410: The heavy amount of dust in the air can severely limit visibility and block the sun for several days, comparable to a heavy fog . This effect is known as the Harmattan haze. It costs airlines millions of dollars in cancelled and diverted flights each year. When the haze is weak, the skies are clear. The extreme dryness of the air may cause branches of trees to die. A 2024 study found that dust carried by

3675-740: The heavy rainfall runoff. High rainfall can cause widespread flooding, which can lead to landslides and mudflows in mountainous areas. Such floods cause rivers to burst their banks and submerge homes. The Ghaggar-Hakra River , which only flows during India's monsoon season, can flood and severely damage local crops. Floods can be exacerbated by fires that occurred during the previous dry season, which cause soils which are sandy or composed of loam to become hydrophobic, or water repellent. In various ways governments may help people deal with wet season floods. Flood plain mapping identifies which areas are more prone to flooding. Instructions on controlling erosion through outreach are also provided by telephone or

3750-432: The ice on rivers and lakes. The Noongar people of South-West Western Australia recognise maar-keyen bonar, or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors 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

3825-565: The internet. The wet season is the main period of vegetation growth within the Savanna climate regime. However, this also means that wet season is a time for food shortages before crops reach their full maturity. This causes seasonal weight changes for people in developing countries, with a drop occurring during the wet season until the time of the first harvest, when weights rebound. Malaria incidence increases during periods of high temperature and heavy rainfall. Cows calve, or give birth, at

SECTION 50

#1732765961110

3900-482: 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. 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)

3975-536: The meteorological and astronomical seasons. 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

4050-534: 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

4125-421: The monsoon arrives, high daytime high temperatures drop and overnight low temperatures increase, thus reducing diurnal temperature variation . During the wet season, a combination of heavy rainfall and, in some places such as Hong Kong , an onshore wind, improve air quality . In Brazil, the wet season is correlated with weaker trade winds off the ocean. The pH level of water becomes more balanced due to

4200-532: 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), Grishma (summer), Varsha ( monsoon ), Sharada (autumn), Hemanta (early winter), and Shishira (prevernal or late winter). The six seasons are ascribed to two months each of

4275-449: 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. The tropical and (to a lesser degree) subtropical regions see little annual fluctuation of sunlight and temperature due to Earth's moderate 23.4-degree tilt being insufficient to appreciably affect the strength of the sun's rays annually. The slight differences between

4350-472: The respective solstice or equinox. Astronomically, the seasons are said to begin on Lichun ( 立春 , "the start of spring") on about 4 February, Lixia ( 立夏 ) on about 6 May, Liqiu ( 立秋 ) on about 8 August, and Lidong ( 立冬 ) on about 8 November. This system forms the basis of other such systems in East Asian lunisolar calendars. Some calendars in south Asia use a six-season partition where

4425-604: The result of the variation in Earth's distance to the Sun because of its elliptical orbit . 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. In general, the effect of orbital eccentricity on Earth's seasons

4500-412: The season. Rivers overflow their banks, and some animals retreat to higher ground. Soil nutrients diminish and erosion increases. The incidence of malaria and dengue increases in areas where the rainy season coincides with high temperatures, particularly in tropical areas. Some animals have adaptation and survival strategies for the wet season. Often, the previous dry season leads to food shortages in

4575-411: 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, as do

SECTION 60

#1732765961110

4650-450: 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. 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

4725-483: 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. These "astronomical" seasons are not of equal length, because of

4800-489: The solstices and the equinoxes cause seasonal shifts 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 ICZ is north of the Equator, the northern tropics experience their wet season while the southern tropics have their dry season. This pattern reverses when

4875-413: 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. The beginning of spring

4950-524: 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, see the astronomical seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall again until 2103. On the other hand, people living far to

5025-611: The traditional celebration of Midsummer in June), as the hemisphere faces the Sun. For the Southern Hemisphere it is instead in November, December, and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months , which increases the solar flux . Due to seasonal lag , June, July, and August are the warmest months in the Northern Hemisphere while December, January, and February are

5100-480: The tropics can have two wet seasons, because the monsoon trough, or Intertropical Convergence Zone , can pass over locations in the tropics twice per year. However, since rain forests have rainfall spread evenly through the year, they do not have a wet season. Areas with a savanna climate in Sub-Saharan Africa , such as Ghana , Burkina Faso , Darfur , Eritrea , Ethiopia , and Botswana have

5175-554: The twelve months in the Hindu calendar. The rough correspondences are: The Bengali Calendar is similar but differs in start and end times. It has the following seasons or ritu: The Odia Calendar is similar but differs in start and end times. The Tamil calendar follows a similar pattern of six seasons 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

5250-513: The warmest months in the Southern Hemisphere. In temperate and sub-polar regions, four seasons based on the Gregorian calendar are generally recognized: spring , summer , autumn ( fall ), and winter . Ecologists often use a six-season model for temperate climate regions which are not tied to any fixed calendar dates: prevernal , vernal , estival , serotinal , autumnal , and hibernal . Many tropical regions have two seasons:

5325-470: The west (America), whose clocks run behind UTC, may experience an equinox as early as March 19. Over thousands of years, the Earth's axial tilt and orbital eccentricity vary (see Milankovitch cycles ). The equinoxes and solstices move westward relative to the stars while the perihelion and aphelion move eastward. Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change —

5400-414: The wet season is mainly due to daytime heating, which leads to diurnal thunderstorm activity within a pre-existing moist airmass , so the rain mainly falls in late afternoon and early evening in savanna and monsoon regions. Much of the total rainfall each day occurs in the first minutes of the downpour, before the storms mature into their stratiform stage. Most places have only one wet season, but areas of

5475-474: The wet season, as the crops have yet to mature. Crops which can be successfully planted during the wet or rainy season are cassava , maize , groundnut , millet , rice and yam . The temperate counterpart to the tropical wet season is spring or autumn . In areas where the heavy rainfall is associated with a wind shift, the wet season is known as the monsoon season. Many tropical and subtropical climates experience monsoon rainfall patterns. Rainfall in

5550-497: The year. Some areas with pronounced rainy seasons will see a break in rainfall mid-season, when the Intertropical Convergence Zone or monsoon trough moves to higher latitudes in the middle of the warm season. When the wet season occurs during a warm season, or summer , precipitation falls mainly during the late afternoon and early evening. In the wet season, air quality improves, fresh water quality improves, and vegetation grows substantially, leading to crop yields late in

5625-408: Was able to assign the dates of February 7, May 9, August 11, and November 10 to the start of spring, summer, autumn, and winter. 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

#109890