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A humid subtropical climate is a zone of climate characterized by hot, humid summers and mild winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 35° and are located poleward from adjacent tropical climates, and south of temperate climates. While many subtropical climates tend to be on or near a coast, in some cases they extend inland, most notably in China and the United States (US).

The subtropical climate was created in the 1966 update of the Koppen climate classification. The Trewartha climate classification sought to redefine middle latitude climates into smaller zones (the original Köppen system grouped all middle latitude climates into a single zone, which was the major criticism). Under the Trewartha system, climates are termed subtropical when they have monthly mean air temperatures higher than 10 °C (50 °F) for eight or more months a year and at least one month with mean temperature below 18 °C (64.4 °F). It classifies humid subtropical climates as typically occupying the southernmost portions of the temperate zone from 23.5 to 35.0 north and south latitude, and they often have a mean temperature of 7 °C (45 °F) or higher in the coldest month.[1]

Rainfall often shows a summer peak, especially where monsoons are well developed, as in Southeast Asia and Florida (US). Other areas have a more uniform or varying rainfall cycles, but consistently lack any predictably dry summer months. Most summer rainfall occurs during thunderstorms that build up due to the intense surface heating and strong subtropical angle of the sun. Weak tropical lows that move in from adjacent warm tropical oceans, as well as infrequent tropical storms, often contribute to summer seasonal rainfall peaks. Winter rainfall is often associated with large storms in the westerlies that have fronts reaching down into subtropical latitudes. However, many subtropical climates such as southeast Asia and Florida have very dry winters, with frequent brush fires and water shortages.

Africa

Durban, South Africa
Climate chart (explanation)
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In Africa, the humid subtropical climates is found only is one small area on the southern hemisphere of the continent. This narrow area spans from Port Elizabeth to Maputo and inland about 200 km. South Africa's version of this climate features heavy oceanic influences resulting in generally milder winters and cooler summers. This is particularly evident in its winters when temperatures do not drop as low as in many other regions within the humid subtropical category.

Asia

East Asia

In East and Southeast Asia, this climate type is found in the southeastern quarter of mainland China from Hong Kong north to Nanjing, the northern half of Taiwan, northern Myanmar, northern Vietnam, north through southern and central Japan ((Kyushu, Shikoku and half of Honshu), and the extreme southern tip of South Korea around Pusan. Cities near the equatorward boundary of this zone include Hong Kong, Hanoi and Taipei; while Qingdao and Tokyo are near the northern boundary.

The influence of the strong Siberian anticyclone in East Asia brings colder winter temperatures than the humid subtropical zones in North America, South America, and Australia. The 0 °C isotherm reaches as far south as the valleys of the Yellow and Wei, roughly latitude 34° N. At Hainan Island and in Taiwan, the climate transitions from subtropical into tropical. In most of this region, the winter monsoon is very well developed, as such eastern Asian humid subtropical zones have a strong winter dry season and heavy summer rainfall.

Only in inland areas below the Yangtze River and coastal areas between approximately the Huai River and the beginning of the coast of Guangdong is there sufficient winter rainfall to produce a Cfa climate; even in these areas, rainfall and streamflow display a highly pronounced summer peak, unlike other regions of this climate type. Drought can be severe and often catastrophic to agriculture in the Cwa zone.

The only area where winter precipitation equals or even exceeds the summer rain is on the "San-in" (Sea of Japan), or western, coast of Japan, which during winter is on the windward side of the westerlies. The winter precipitation in these regions is usually produced by low-pressure systems off the east coast that develop in the onshore flow from the Siberian high. Summer rainfall comes from the East Asian Monsoon and from frequent typhoons. Annual rainfall is generally over 1,000 millimetres (39 in), and in areas below the Himalayas can be much higher still.

Tokyo, Japan
Climate chart (explanation)
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Source: Japan Meteorological Agency


Western Asia

Kutaisi, Georgia
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Source: [3]

Although humid subtropical climates in Asia are mostly confined to the southeastern quarter of the continent, there are two narrow areas along the coast of the Caspian Sea and Black Sea with humid subtropical climates. Summers in these locations are cooler than typical humid subtropical climates and snowfall in winter is relatively common, but is usually of a short duration.

In Western Asia, the climate is prevalent in the Gilan and Māzandarān Provinces of Iran, in parts of the Caucasus, in Azerbaijan and in Georgia wedged between the Caspian and Black seas and coastal (Black Sea) Turkey, albeit having more oceanic influence.

Annual rainfall ranges from around 740 mm (29 inches) at Sari to over 2,000 mm (78 inches) at Bandar-e Anzali, and is heavy throughout the year, with a maximum in October or November when Bandar-e Anzali can average 400 millimetres (16 inches). Temperatures are generally moderate in comparison with other parts of Western Asia. During winter, the coastal areas can receive snowfall, which is usually of a short duration.

In Rasht, the average maximum in July is around 28 °C (82 °F) but with near-saturation humidity, whilst in January it is around 9 °C (48 °F). The heavy, evenly distributed rainfall extends north into the Caspian coastal strip of Azerbaijan up to its northern border but this climate in Azerbaijan is, however, a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case.[4]

Western Georgia (Batumi and Kutaisi) in the Kolkheti Lowland and the northeast coast of Turkey (Giresun), have a climate similar to that of Gilan and Mazandaran in Iran and very similar to that of southeastern and northern Azerbaijan. Temperatures range from 22 °C in summer to 5 °C in winter and rainfall is even heavier than in Caspian Iran, up to 2,300 millimetres per year in Hopa (Turkey). These climates are a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case.

North America

Houston, United States
Climate chart (explanation)
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Source: [5]

In North America, humid subtropical climates are almost exclusively the domain of the American Gulf and south Atlantic states. This zone extends for extreme southeast Virginia, through eastern North Carolina, South Carolina, Georgia, and Florida, then westward through Alabama, Mississippi, Louisiana, Arkansas, and eastern Texas. On the Florida peninsula, the humid subtropical climate gives way to the tropical climate of south Florida and the Florida Keys.

In Mexico, there are small areas of Cfa and Cwa climates. The climate can be found in small areas scattered around the northeastern part of the country, in proximity to the Gulf of Mexico. Other areas where the climate can be found are in the high elevations of Trans-Mexican Volcanic Belt and Sierra Madre Oriental. Despite being located at higher elevations, these locations have summers that are too warm to qualify as a subtropical highland climate. Guadalajara's climate is a major example of this.

Outside of isolated sections of Mexico, the southernmost limits of this climate in North America lie just north of South Florida and around southern coastal Texas. Cities at these southernmost limits, such as Orlando, Tampa, Corpus Christi, and Brownsville, Texas generally feature warm weather year-round and minimal temperature differences between seasons. In contrast, cities at the northernmost limits of the humid subtropical region such Dallas, Memphis, Atlanta, and Norfolk feature hot, humid summers and chilly winters. These areas have average winter temperatures at the coldest limit of climates classed as humid subtropical.

Precipitation is plentiful in the humid subtropical climate zone in North America – but with significant variations in terms of wettest/driest months and seasons. Much of the interior areas including the northern halves of Mississippi and Alabama, tend to have a winter or spring (not summer) precipitation maximum. Closer to the South Atlantic and Gulf coasts, there gradually becomes a more summer maximum, with July or August usually the wettest month – as at Norfolk, Cape Hatteras and Jacksonville, North Carolina, Charleston, South Carolina, Mobile, Alabama and New Orleans. The monsoon pattern becomes much stronger in the southern portion of the region on the Florida peninsula, as most locations in Florida have quite dry winters and wet summers.

In addition, areas in Texas that are slightly inland from the Gulf of Mexico, such as Austin and San Antonio, that border the semi-arid climate zone, generally see a peak of precipitation in May, a drought-like nadir in mid-summer and a secondary, if not equal, precipitation peak in September or October. Districts further south along South Texas' Gulf Coast (Corpus Christi and Brownsville), which closely border the tropical climate classification or semi-arid zone, typically have a strong September precipitation maximum, and a tendency toward dry conditions in late winter and spring, with March or April often the driest months.

The freeze free season ranges from 200 days in the far northern areas to 330 days a year in southern areas. Snowfall is rare in winter, with locations in the southern portions of this zone like Jacksonville, Houston, New Orleans, or Savannah seeing snow a few times per generation, while far northern areas like Atlanta, Memphis, Nashville, Dallas, or Raleigh seeing small amounts of snow once or twice each winter season. Hurricanes are a hazard along the coast from June through October.

South America

Buenos Aires, Argentina
Climate chart (explanation)
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Source: National Meteorological Service

Humid subtropical climates are found in a sizable portion of South America. The climate extends over a few states of southern Brazil, including Paraná, into sections of Paraguay, all of Uruguay, and the Río de la Plata region in Argentina. Major cities such as São Paulo, Buenos Aires, Porto Alegre, Montevideo and Campo Grande, have a humid subtropical climate, generally in the form of hot humid summers and mild to cool winters. These areas, which include the Pampas, generally feature a Cfa climate categorization.

The Cwa climate occurs in parts of tropical highlands of São Paulo state, Mato Grosso do Sul and near the Andean highland in northwestern Argentina. These highland areas feature summer temperatures that are warm enough to fall outside the subtropical highland climate category.

Australia

Brisbane, Queensland
Climate chart (explanation)
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Source: [6]

The humid subtropical climate zone predominantly lies in eastern Australia. The coastal strip from Mackay, Queensland to the southern coast of Sydney, where it grades into the cooler and wet oceanic climates.[7][8]

From Newcastle, just above Sydney, the Cfa zone would extend to inland New South Wales, excluding the highland regions (which have an oceanic climate), stretching towards Dubbo to the northwest and Wagga Wagga to the south, ending at the New South Wales/Victoria border (Albury–Wodonga).[9] To note, these places would have characteristics of the semi-arid and/or Mediterranean climates. Furthermore, the inland Cfa climates generally have drier summers, or at least summers with low humidity.[10]

Extreme heat is more often experienced in Sydney than in other large cities in Australia's Cfa zone, especially in the western suburbs, where highs over 40 °C are not uncommon. Frost is prevalent in the more inland areas of Sydney, such as Richmond. Average annual rainfall in the Sydney region ranges between 800 mm and 1200 mm.[11]

There is usually a distinct summer rainfall maximum that becomes more pronounced moving northwards: in Brisbane the wettest month (February) receives five times the rainfall of the driest (September). Temperatures are very warm to hot but not excessive: the average maximum in February is usually around 29 °C (84 °F) and in July around 21 °C (70 °F). Frosts are extremely rare except at higher elevations, but temperatures over 35˚C (95˚F) are not common on the coast.

North of the Cfa climate zone there is a zone centred upon Rockhampton and extending up to the Atherton Tableland of Köppen Cwa climate. This has a very pronounced dry winter with often negligible rainfall between June and October, and winter temperatures generally only slightly below 18 °C, above which one would have a tropical savanna, or Aw, climate.

Annual rainfall on the coast can reach as high as 2,000 mm (80 inches) in coastal locations and is generally above 1,000 mm (40 inches). However, because most of the heaviest two- and three-day rainfalls in the world occur in this coastal zone as a result of east coast lows forming to the north of a large high pressure system, there can be great variation in rainfall from year to year. At Lismore in the centre of this zone, the annual rainfall can range from less than 550 mm (22 inches) in 1915 to more than 2,780 mm (110 inches) in 1950.[12]

See also

References

  1. ^ Belda et al. "Climate classification revisited: from Köppen to Trewartha", Clim Res. Vol 59:1–13, 2014.
  2. ^ http://www.saexplorer.co.za/south-africa/climate/durban_climate.asp
  3. ^ http://www.dwd.de/DWD/klima/beratung/ak/ak_373950_kt.pdf
  4. ^ Climate in Azerbaijan
  5. ^ NOAA
  6. ^ "Brisbane Regional Office". Climate statistics for Australian locations. Bureau of Meteorology. Retrieved 2 August 2011. 
  7. ^ Donohue, R. J., McVicar, T. R., and Roderick, M. L.: Climaterelated trends in Australian vegetation cover as inferred from satellite observations, 1981–2006, Glob. Change Biol., 15, 1025–39, 2009.
  8. ^ Gentilli, J. (Ed.): Climates of Australia and New Zealand, World Survey of Climatology, Vol. 13. Elsevier, Amsterdam, 405p, 1971.
  9. ^ R.L. Specht, Philip Rundel, W.E. Westman, P.C. Catling, Jonathan Majer, Penelope Greenslade (6 December 2012). Mediterranean-type Ecosystems: A data source book. Springer Science & Business Media. p. 95. ISBN 978-94-009-3099-5. 
  10. ^ Stern, H., De Hoedt, G., and Ernst, J.: Objective classification of Australian climates, Aust. Meteorol. Mag., 49, 87–96, 2000.
  11. ^ Colls, K. & Whitaker, R. (2001) The Australian Weather Book. New Holland Publishers, Sydney.
  12. ^ 058037 Lismore (Centre Street) monthly rainfall