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 regimes, 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 palms, citrus, mango, pistachio, lychee, and avocado are grown within the 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. According to the American Meteorological Society, the poleward fringe of the subtropics is located at latitudes approximately 35° north and south, respectively.
Several methods have been used to define the subtropical climate. In the Trewartha climate classification, a subtropical region should have 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). 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 are the coastal areas of southern Africa (Namibia, South Africa), the south of the Canary Islands and the coasts of Peru and Chile.
The humid or monsoon 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 in the hot season. In the winter (dry season) the monsoon retreats, and the drier trade winds bring more stable airmass and often dry weather. Areas that have this type of subtropical climate include Australia, Southeast Asia, parts of South America, and the deep south of the United States. 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 in North America 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 palms and citrus to flourish. 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.
Tree ferns (pteridophytes) are grown within subtropical areas, primarily within the subtropics and within topography within the tropics. Dracaena and yucca can grow within the subtropics. Trees within the Taxaceae family grow within subtropical climate regimes. Apple, pear and pomegranates grow well within the subtropics.
The humid subtropical climate is a subtropical climate type characterized by hot, humid summers with frequent tropical downpours of short duration and warm, and frequently dry winters. In summer, the subtropical high pressure cells bring a sultry flow of tropical air with high dew points, and daily thundershowers are typical, though brief. Normally, rainfall is concentrated in the warmest months of the year. At times, the average annual precipitation may be more evenly distributed throughout the year, or a spring maximum is present. With decreasing latitude most humid subtropical climates typically have drier winters and wetter summers.
Humid subtropical climates lie on the lower east side of continents, roughly between latitudes 25° and 40° degrees away from the equator. In the United States, the humid subtropical zone is normally considered the region from central Florida north to Long Island in Southern New York.
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 rainfall in winter, in areas under the constant influence of the subtropical ridge. Inland from the immediate coastlines, mediterranean climates can take on extreme temperatures. This is seen in inland California as well as Andalusia in Spain. In Europe, the northernmost mediterranean climates are found along the French Riviera, located at 43° latitude. On the immediate Atlantic coastline, the mediterranean boundary goes between Porto and Vigo at around 41° latitude. Parts of southwestern Australia around Perth have a Mediterranean climate as does areas around coastal South Africa.
Arid subtropical climates are characterized by an annual average temperature above 18 °C (64.4 °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.