Subtropical climates can also occur at high elevations within thetropics, such as in the southern end of theMexican Plateau and inDa Lat of the Vietnamese Central Highlands. The six climate classifications use the term to help define the varioustemperature andprecipitation regimes for planetEarth.
A great portion of the world's deserts are within the subtropics, as this is where the semi-permanent subtropical anticyclone resides (typically inland on the southwest sides of continents). Areas bordering warm oceans (typically on the southeast sides of continents) have hot and wet summers with frequent (but brief) convective rainfall (tropical cyclones can also contribute to annual rainfall). Areas bordering cool oceans (typically on the southwest sides of continents) are prone to fog, aridity, and dry summers. Plants such aspalms,citrus,mango,pistachio,lychee, andavocado are grown in the subtropics.
The tropics have beenhistorically defined as lying between theTropic of Cancer andTropic of Capricorn, at latitudes 23°26′09.3″ (or 23.43593°) north and south, respectively.[2] According to the American Meteorological Society, the poleward fringe of the subtropics is at latitudes approximately35° north andsouth, respectively.[3]
Houses in subtropical Charleston, South Carolina, alongThe Battery
Several methods have been used to define the subtropical climate depending on the climate system used.[citation needed]
The most well known[4] is theTrewartha 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).[5]
German climatologists Carl Troll and Karlheinz Paffen definedwarm 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 theNorthern Hemisphere and between 6 °C (42.8 °F) and 13 °C (55.4 °F) in theSouthern Hemisphere, excludingoceanic andcontinental climates. According to the Troll-Paffen climate classification, there generally exists one large subtropical zone named the warm-temperate subtropical zone,[6] which is subdivided into seven smaller areas.[7]
According to the E. Neef climate classification, the subtropical zone is divided into two parts:rainy winters of the west sides andeastern subtropical climate.[8] According to the Wilhelm Lauer & Peter Frankenberg climate classification, the subtropical zone is divided into three parts:high-continental,continental, andmaritime.[9] According to the Siegmund/Frankenberg climate classification, subtropical is one of six climate zones in the world.[10]
Leslie Holdridge defined the subtropical climates as having a mean annualbiotemperature between the frost line or critical temperature line, 16 °C to 18 °C (depending on locations in the world), and 24 °C.[11] 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. Most of the plants are sensitive to low temperatures on the warmer side of the line. 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 lengths of low temperatures, whether as seeds in the case of the annuals or as perennial plants that can withstand the cold. The 16 °C–18 °C segment is often "simplified" as 17 °C.[12]
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.[citation needed]
However,Wladimir Köppen has distinguished the hot or subtropical and tropical (semi-)arid climates (BWh orBSh) having an average annual temperature greater than or equal to 18 °C (64.4 °F) from the cold or temperate (semi-)arid climates (BWk orBSk) whose annual temperature average is lower.[13] This definition, though restricted to dry regions, is almost similar to Holdridge's.
Hadley cells on the Earth's atmospheric circulation
Heating of the earth by thesun near the equator leads to large amounts of upward motion and convection winds along themonsoon trough orIntertropical 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, gets denser and sinks, which leads to subsidence near the30th parallel of both hemispheres. This circulation is known as theHadley cell and leads to the formation of thesubtropical ridge.[14] Many of the world's deserts are caused by these climatologicalhigh-pressure areas,[15] within the subtropics. This regime is known as asemiarid/arid subtropical climate, which is generally in areas adjacent to powerful cold ocean currents. Examples of this climate are the coastal areas ofSouthern Africa and the west coast ofSouth America.[16]
Thehumid subtropical climate is often 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 driertrade winds bring more stable airmass and often dry weather, and frequent sunny skies. Areas that have this type of subtropical climate include Australia, theIndo-Gangetic Plain in South Asia, and parts of South America.[17][18][19] In areas bounded by warm ocean like the southeastern United States and East Asia, tropical cyclones can contribute significantly to local rainfall within the subtropics.[20]Japan receives over half of its rainfall from typhoons.[21]
These climates do not routinely see hard freezes or snow due to winter on average being above freezing, which allows plants such as palms and citrus to flourish.[26][27] 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 asmango,lychee,avocado andaloe vera, are also cultivated in the subtropics. Pest control of the crops is easier than in the tropics, due to the cooler winters.[28]
In most humid subtropical climates, summer is the wettest season. In summer, the subtropical high pressure cells provide a sultry southernly flow of tropical air with high dew points, and frequent (but brief) convective showers are common. With decreasing latitude most humid subtropical climates typically have drier winters and wetter summers, however some sectors with this climate see a more even rhythm of seasonal rainfall. Tropical lows and weakening tropical storms often contribute to seasonal rainfall in most humid subtropical climates. In the cool season (winter) when the subtropical highs retreat, the humid subtropics are more influenced by the westerlies and the fronts and storms that move with them.[citation needed]
TheSubtropical highland climate is a climate variant often grouped together with oceanic climates found in some mountainous areas of either the subtropics or tropics. It has characteristically mild temperatures year-round, featuring the four seasons in the subtropics and no marked seasons in the tropics, the latter usually remaining mild to cool through most of the year. Subtropical highland climates under the Cfb classification usually have rainfall spread relatively evenly in all months of the year similar to most oceanic climates[31] while climates under the Cwb classification have significant monsoon influence, usually having dry winters and wet summers.[32]
The Mediterranean climate regime resembles the climate of the lands in theMediterranean Basin, parts of lower West Coast of the United States, parts ofWestern andSouth Australia, in southwesternSouth Africa and in parts of centralChile. The climate is characterized by hot dry summers and cooler winters with rainfall.[33] In Europe, the northernmost mediterranean climates are found along theItalian Riviera, at 44° latitude.
According toKöppen, arid subtropical climates are characterized by an annual average temperature above 18 °C (64.4 °F), the absence of regular rainfall, and high humidity.[16]
