Subtropics

The subtropics are the geographical and climate zones located just north and south of the tropics. While they are part of the temperate zones in both hemispheres, they span the middle latitudes, starting at about 23°26′09.0″ and extending to roughly 35° to 40° north and south. The horse latitudes are found within this range. These regions typically experience hot summers and mild winters, with frost being uncommon. Most subtropical climates are one of two main types: humid subtropical, where the heaviest rainfall occurs during the warmest months (seen in places like Southeast China and the Southeastern United States), or Mediterranean, where rain falls mostly during the cooler months (as in the Mediterranean Basin and California). Subtropical climates can also appear at high elevations within the tropics, such as the southern Mexican Plateau or Da Lat in Vietnam. Various climate classification systems use the term to define temperature and precipitation patterns across the planet.

A large share of the world’s deserts lie in the subtropics because this is where the semi-permanent subtropical anticyclone is located, typically inland on the southwestern sides of continents. On the southeastern sides of continents, areas bordering warm oceans have hot, wet summers with frequent but brief convective rain, and tropical cyclones can add to the yearly rainfall. On the southwestern sides of continents, areas bordering cool oceans are prone to fog, dryness, and dry summers. Crops like palms, citrus, mango, pistachio, lychee, and avocado are grown in the subtropics.

The tropics have historically been defined as the region between the Tropic of Cancer and the Tropic of Capricorn (23°26′09.0″ north and south). According to the American Meteorological Society, the poleward edge of the subtropics sits at roughly 35° north and south. Different climate systems define the subtropical climate in various ways. The Trewartha classification requires at least eight months with a mean temperature above 10 °C and at least one month below 18 °C. German climatologists Troll and Paffen defined warm temperate zones based on coldest-month averages, excluding oceanic and continental climates, and divided the warm-temperate subtropical zone into seven areas. The E. Neef classification splits the subtropics into rainy-winter west sides and eastern subtropical climates. Lauer and Frankenberg divided it into high-continental, continental, and maritime parts. The Siegmund/Frankenberg system lists subtropical as one of six global climate zones. Leslie Holdridge defined subtropical climates by mean annual biotemperature, between the frost line (16 °C to 18 °C, often simplified to 17 °C) and 24 °C. The frost line separates warm temperate from subtropical regions and marks a divide between plant groups: on the warmer side, most plants are sensitive to low temperatures and can be killed by frost, while on the colder side, flora are adapted to survive cold periods. Holdridge’s subtropical climates overlap with the warmest subtropical and less warm tropical climates in other systems. Wladimir Köppen distinguished hot subtropical and tropical arid climates (BWh or BSh) from cold temperate arid ones (BWk or BSk) using an annual average temperature of 18 °C, a definition similar to Holdridge’s for dry regions.

Heating near the equator causes upward motion and convection along the monsoon trough or Intertropical Convergence Zone. The resulting upper-level divergence pushes air away from the equator aloft. As this air moves toward the mid-latitudes, it...