A tropical climate in the Köppen climate classification is a non-arid climate in which all twelve months have mean temperatures of warmer than 18 °C (64 °F). In tropical climates there are often only two seasons: a wet season and a dry season. Tropical climates are frost-free, and changes in the solar angle are small. In tropical climates temperature remains relatively constant (hot) throughout the year. Sunlight is intense.
Within the tropical climate zone there are three distinct varieties based on precipitation:
Tropical rainforest climates (Af) have heavy annual precipitation distributed throughout the year, so that there is either no dry season or at most two dry months. All 12 months have average precipitation of at least 60 mm (2.4 in). These climates usually occur within 10° latitude of the equator. This climate is dominated by the doldrums low-pressure system all year round, so has no natural seasons in terms of thermal and moisture changes.
The Amazon Basin of South America, the Congo Basin of equatorial Africa and oceanic margins of parts of tropical West Africa have tropical rainforests climates. The insular and peninsular area of Southeast Asia (Thailand, Philippines, New Guinea..etc) comprise the most extensive region with tropical rainforest climate. Eastern Central America and parts of the Caribbean islands, the Atlantic margins of Brazil, eastern Madagascar island, and southern Ceylon comprise smaller areas with tropical rainforest climate.
Cities featuring a Tropical rainforest climate
Some of the places with this climate are indeed uniformly and monotonously wet throughout the year (e.g., the northwest Pacific coast of South and Central America, from Ecuador to Costa Rica; see, for instance, Andagoya, Colombia), but in many cases, the period of higher sun and longer days is distinctly wettest (as at Palembang, Indonesia) or the time of lower sun and shorter days may have more rain (as at Sitiawan, Malaysia).
(Note. The term aseasonal refers to the lack in the tropical zone of large differences in daylight hours and mean monthly (or daily) temperature throughout the year. Annual cyclic changes occur in the tropics, but not as predictably as those in the temperate zone, albeit unrelated to temperature, but to water availability whether as rain, mist, soil, or ground water. Plant response (e. g., phenology), animal (feeding, migration, reproduction, etc.), and human activities (plant sowing, harvesting, hunting, fishing, etc.) are tuned to this 'seasonality'. Indeed, in tropical South America and Central America, the 'rainy season' (and the 'high water season') is called invierno or inverno, though it could occur in the Northern Hemisphere summer; likewise, the 'dry season' (and 'low water season') is called verano or verão, and can occur in the Northern Hemisphere winter).
This type of climate results from the monsoon winds which change direction according to the seasons. In this type of tropical climate, there is a short but distinct dry season. This climate has a driest month (which nearly always occurs at or soon after the "winter" solstice for that side of the equator) with rainfall less than 60 mm, but more than 1/25 the total annual precipitation.:200–1:208
Cities featuring a Tropical monsoon climate
Aw climates have a pronounced dry season, with the driest month having precipitation less than 60 mm and less than 1/25 of the total annual precipitation. In the Aw tropical climate the dry season is often several months and can be severe, with most plant and animal life adapting to drought conditions for several months. Aw climates often (but not always as in Africa) occur in higher tropical latitudes, between 15 and 20 north and south. Aw climates often feature tree-studded grasslands, rather than thick jungle. It is this widespread occurrence of tall , course grass (called savanna) which has lead to Aw climates often being referred to as tropical savanna. However, there is some doubt whether tropical grasslands are climatically induced. Additionally, pure savannas, with out trees, are the exception rather than the rule.
This type of tropical climate occurs in northern Australia, across the Sudan and eastern Africa, and parts south-central Brazil and adjacent parts of Bolivia and Paraguay and western Madagascar in the southern hemisphere. In the northern hemisphere tropical savanna climate occurs north of the Amazon in Venezuela and Colombia, as well as parts of western Central America, western Cuba, and the extreme southern part of Florida in the United States.
Cities featuring a Tropical savanna climate
Sometimes As is used in place of Aw if the dry season occurs during the time of higher sun and longer days. This is the case in parts of Hawaii, northwestern Dominican Republic, East Africa, and the Brazilian Northeastern Coast. In most places that have tropical wet and dry climates, however, the dry season occurs during the time of lower sun and shorter days because of rain shadow effects during the 'high-sun' part of the year.
Owing to different causes certain places within the tropics do not have a tropical climate, such as some desert areas and alpine regions.
Because of the effect of sun angle on climate most areas within the tropics are hot year-round, with diurnal variations in temperature exceeding seasonal variations. Seasonal variations in tropical climate are dominated by changes in precipitation, which are in turn largely influenced by the tropical rain belt or Intertropical Convergence Zone (ITCZ), a portion of the Hadley cell. The ITCZ is shown, for July average, in the graphic. Areas of ascending air have heavy rainfall; areas of descending air are dry. The ITCZ somewhat follows the solar equator throughout the year, but with geographical variations, and in some areas (India) is heavily influenced by local large-scale monsoons.