The subtropics are geographic and climate zones located roughly between the tropic circle of latitude (the Tropic of Cancer and Tropic of Capricorn) and the 38th parallel in each hemisphere. Subtropical climate regimes can exist at high elevations within the tropics, such as across the Mexican Plateau and in Vietnam and Taiwan. Six climate classifications utilize the term to help define the various temperature and precipitation regimes for the planet Earth. Eight months of the year within the subtropics have an average temperature at or above 10 °C (50.0 °F), with their coldest month averaging between 2 and 13 °C (35.6 and 55.4 °F).
A great portion of the world's deserts are located within the subtropics, due to the development of the subtropical ridge. Within savanna regimes in the subtropics, a wet season is seen annually during the summer, which is when most of the yearly rainfall falls. Within Mediterranean climate regime, the wet season occurs during the winter. Areas bordering warm oceans are prone to locally heavy rainfall from tropical cyclones, which can contribute a significant percentage of the annual rainfall. Plants such as date palms, citrus, mango, litchi, and avocado are grown within the subtropics. Tree ferns and sequoia also grow within subtropical climate regimes.
The subtropics have been historically defined as lying between the Tropic of Cancer and Tropic of Capricorn, located at 23.45° north and south latitude respectively. Recent studies suggest that the tropics migrated an additional 2.5° of latitude outward from the equator between 1982 and 2007. The poleward fringe of the subtropics is close to the 38th parallel north and south, though it extends farther poleward along western sides of continents and less poleward near eastern sides of continents.
Several methods have been used to define the subtropical climate. In the Trewartha climate classification system, a subtropical region should have at least eight months with a mean temperature of 10 °C (50.0 °F) or above. 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, which is subdivided into seven smaller areas.
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. According to the Wilhelm Lauer & Peter Frankenberg climate classification, the subtropical zone is divided into three parts: high-continental, continental, and maritime. According to the Siegmund/Frankenberg climate classification, subtropical is one of six climate zones in the world.
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. Many of the world's deserts are caused by these climatological high-pressure areas, 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, the coastal areas of southern Africa (Namibia, South Africa), south of the Canary Islands and the coasts of Peru and Chile.
A savanna is a grasslandbiome located in semi-arid to semi-humid climate regions of subtropical and tropicallatitudes, with rainfall between 750 millimetres (30 in) and 1,270 millimetres (50 in) a year. They are widespread on Africa, and are also found in India, and Australia. The savannah climate regime in the subtropics, Florida and East Texas have a rainy season. Monsoon regions with a wet season include western Mexico. the Desert Southwest of the United States, Within the Mediterranean climate regime, the west coast of the United States and the Mediterranean coastline of Italy, Greece, and Turkey experience 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.
In areas bounded by warm ocean, tropical cyclones can contribute significantly to local rainfall within the subtropics.Japan receives over half of its rainfall from typhoons.
These climates do not routinely see hard frosts or snow, which allows plants such as date palms and citrus to flourish. The zone where the orange was grown was originally defined as the area where subtropical plants could be grown. 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 less difficult than within the tropics, due to the cooler winters.
The subtropical highland variety (Köppen climate classification: Cfb, Cwb) of the oceanic climate exists in elevated portions of the world that are either within the tropics or subtropics, though it is typically found in mountainous locations in some tropical countries. Despite the latitude, due to the higher altitudes of these regions, it tends to share characteristics with oceanic climates.
In locations outside the tropics, other than the drying trend in the winter, subtropical highland climates tend to be essentially identical to an oceanic climate, complete with mild summers, noticeably cooler winters and in some instances, some snowfall. In the tropics, a subtropical highland climate tends to feature spring-like weather year-round. Temperatures here remain relatively constant throughout the year and snowfall is seldom seen. Areas with this climate feature monthly averages below 22 °C (72 °F) but above −3 °C (27 °F) (or 0 °C (32 °F) using American standards). At least one month's average temperature is below 18 °C (64 °F). Without the elevation, many of these regions would likely feature either tropical or humid subtropical climates. These regions usually carry a Cwb or Cfb designation.
This type of climate exists in parts of east, south and south-eastern Africa, some mountainous areas across southern Europe, sections of mountainous Latin America, some mountainous areas across Southeast Asia, higher elevations of the southern Appalachians, the Mexican Plateau down past the 19th parallel to Morelos, and parts of the Himalayas. It also occurs in a few areas of Australia, although average high temperatures during summers there tend to be higher and the climate drier than is typical of subtropical highland climates, with maximums sometimes exceeding 40 °C (104 °F).
The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of coastal southwestern North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters, in areas under the constant influence of the subtropical ridge.
The humid subtropical climate is a subtropical climate type characterized by hot, humid summers and warm to cool dry winters. The average annual precipitation may either be evenly distributed throughout the year (Köppen climate classification: Cfa) or marked by a dry season or drying trend during winter (Köppen climate classification: Cwa).
The humid subtropical climate zone where winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones. Humid subtropical climates lie on the east side of continents, roughly between latitudes 20° and 40° degrees away from the equator.
Arid subtropical climates are characterized by an annual average temperature of 18.2 °C (64.8 °F), the absence of regular rainfall and high humidity. Mild climate variants are generally located in areas adjacent to powerful cold ocean currents. Examples of this climate, the coastal areas of southern Africa (Namibia, South Africa), south of the Canary Islands and the coasts of Peru and Chile.