Subtropics

The tropics have been historically defined as lying between the Tropic of Cancer and Tropic of Capricorn, located at latitudes 23.45° north and south, respectively.[1] According to the American Meteorological Society, the poleward fringe of the subtropics is located at latitudes approximately 35° north and south, respectively.[2]

Tadrart Acacus desert, part of the Sahara, in western Libya.

Several methods have been used to define the subtropical climate. The most well known is the Trewartha climate classification, which defines a subtropical region as one that has at least eight months with a mean temperature greater than 10 °C (50.0 °F) and at least one month with a mean temperature under 18 °C (64.4 °F).[3] In most regions in this climate zone the coldest month has a mean temperature of above 7 °C (45 °F). In the Trewartha climate classification, most of these climates are located in the most equatorial portions of the temperate zone (latitudes between 23.5 and 35 north and south).

German climatologists Carl Troll and Karlheinz Paffen defined warm temperate zones as plain and hilly lands having an average temperature of the coldest month between 2 °C (35.6 °F) and 13 °C (55.4 °F) in the Northern Hemisphere and between 6 °C (42.8 °F) and 13 °C (55.4 °F) in the Southern Hemisphere, excluding oceanic and continental climates. According to the Troll-Paffen climate classification, there generally exists one large subtropical zone named the warm-temperate subtropical zone,[4] which is subdivided into seven smaller areas.[5]

According to the E. Neef climate classification, the subtropical zone is divided into two parts: rainy winters of the west sides and eastern subtropical climate.[6] According to the Wilhelm Lauer & Peter Frankenberg climate classification, the subtropical zone is divided into three parts: high-continental, continental, and maritime.[7] According to the Siegmund/Frankenberg climate classification, subtropical is one of six climate zones in the world.[8]

Leslie Holdridge defined the subtropical climates as having a mean annual biotemperature between the frost line or critical temperature line, 16 °C to 18 °C (depending on locations in the world) and 24 °C.[9] The frost line separates the warm temperate region from the subtropical region. It represents the dividing line between two major physiological groups of evolved plants. On the warmer side of the line, the majority of the plants are sensitive to low temperatures. They can be killed back by frosts as they have not evolved to withstand periods of cold. On the colder temperate side of the line, the total flora is adapted to survive periods of variable length of low temperatures, whether as seeds in the case of the annuals or as perennial plants which can withstand the cold. The 16 °C-18 °C segment is often "simplified" as 17 °C (= 2^{(log₂12+0;5)} ≈ 16.97 °C).[10]

The Holdridge subtropical climates straddle more or less the warmest subtropical climates and the less warm tropical climates as defined by the Köppen-Geiger or Trewartha climate classifications.

However Wladimir Köppen has distinguished the hot or subtropical and tropical (semi-)arid climates (BWh or BSh) having an average annual temperature greater than or equal to 18 °C (64.4 °F) from the cold or temperate (semi-)arid climates (BWk or BSk) whose annual temperature average is lower.[11] This definition, though restricted to dry regions, is almost similar to Holdridge's.

Hadley cells located on the Earth's atmospheric circulation.

Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or intertropical convergence zone. The upper-level divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As the air moves towards the mid-latitudes, it cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres. This circulation is known as the Hadley cell and leads to the formation of the subtropical ridge.[12] Many of the world's deserts are caused by these climatological high-pressure areas,[13] located within the subtropics. This regime is known as an arid subtropical climate, which is generally located in areas adjacent to powerful cold ocean currents. Examples of this climate are the coastal areas of Southern Africa and the west coast of South America.[14]

The humid subtropical climate is often located on the western side of the subtropical high. Here, unstable tropical airmasses in summer bring convective overturning and frequent tropical downpours, and summer is normally the season of peak annual rainfall. In the winter (dry season) the monsoon retreats, and the drier trade winds bring more stable airmass and often dry weather, and frequent sunny skies. Areas that have this type of subtropical climate include Australia, Southeast Asia, and parts of South America.[15][16][17] In areas bounded by warm ocean like the southeastern North America and East Asia, tropical cyclones can contribute significantly to local rainfall within the subtropics.[18] Japan receives over half of its rainfall from typhoons.[19]

The Mediterranean climate is a subtropical climate with a wet season in winter and a dry season in the summer. Regions with this type of climate include the rim lands of the Mediterranean Sea, southwestern Australia, parts of the west coast of South America around Santiago, the coastal areas of southern North America.[20][21][22][23]

Live oak with araucarias in Curitiba, Brazil

These climates do not routinely see hard freezes or snow, which allows plants such as palms and citrus to flourish.[24][25] As one moves toward the tropical side the slight winter cool season disappears, while at the poleward threshold of the subtropics the winters become cooler. Some crops which have been traditionally farmed in tropical climates, such as mango, litchi, and avocado, are cultivated in the subtropics. Pest control of the crops is easier than in the tropics, due to the cooler winters.[26]

Tree ferns (pteridophytes) are grown in subtropical areas, as are dracaena and yucca, and trees in the Taxaceae. Apple, pear and pomegranate also grow well in the subtropics.[27]

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• Geographical zone
• Subtropical cyclone