Thetropics are the regions ofEarth surrounding theequator, where the sun may shinedirectly overhead. This contrasts with thetemperate orpolar regions of Earth, where the Sun can never be directly overhead. Because ofEarth's axial tilt, the width of the tropics (in latitude) is twice the tilt. The tropics are also referred to as thetropical zone and thetorrid zone (seegeographical zone).
Due to the sun's high angle throughout the year, the tropicsreceive the mostsolar energy over the course of the year, and consequently have the highest temperatures on the planet. Even when not directly overhead, the sun is still close to overhead throughout the year, therefore the tropics also have the lowestseasonal variation on the planet; "winter" and "summer" lose their temperature contrast. Instead,seasons are more commonly dividedby precipitation variations than by temperature variations or daylight hours.
The Earth's axial tilt iscurrently around 23.4°, and therefore so are the latitudes of thetropical circles, marking the boundary of the tropics: specifically, ±23°26′09.4″ (or 23.43594°). The northern one is called theTropic of Cancer, and the southern is theTropic of Capricorn. As the Earth's axial tilt changes,[a] so too do the tropical andpolar circles.
The tropics constitute 39.8% of Earth's surface area[1] and contain 36% of Earth'slandmass.[2] As of 2014[update], the region was home also to 40% of the world'spopulation, and this figure was then projected to reach 50% by 2050. Because ofglobal warming, the weather conditions of the tropics are expanding with areas in thesubtropics,[3] having more extreme weather events such as heatwaves and more intense storms.[4][3] These changes in weather conditions may make certain parts of the tropics uninhabitable.[5]
Relationship of Earth's axial tilt (ε) to the tropical and polar circles: the Tropic of Cancer is a subsolar point only at theJune solstice, and the Tropic of Capricorn is only at theDecember solstice[7]
The Tropic of Cancer is the Northernmost latitude from which theSun can ever be seendirectly overhead, and the Tropic of Capricorn is the Southernmost.[8] This means that the tropical zone includes everywhere on Earth which is asubsolar point at least once during thesolar year. Thus the maximum latitudes of the tropics have equal distances from the equator on either side. Likewise, they approximate the angle of the Earth's axial tilt. This angle is not perfectly fixed, mainly due to the influence of the moon, but the limits of the tropics are a geographic convention, and their variance from the true latitudes is very small.
Many tropical areas have both a dry and a wet season. Thewet season, rainy season or green season is the time of year, ranging from one or more months when most of the average annualrainfall in a region falls.[9] Areas with wet seasons are disseminated across portions of the tropics andsubtropics, some even intemperate regions.[10] Under theKöppen climate classification, fortropical climates, a wet-season month is defined as one or more months where average precipitation is 60 mm (2.4 in) or more.[11] Some areas with pronounced rainy seasons see a break in rainfall during mid-season when theIntertropical Convergence Zone ormonsoon trough moves poleward of their location during the middle of the warm season;[12] Typical vegetation in these areas ranges from moistseasonal tropical forests tosavannahs.
Graph showing the zonally averaged monthly precipitation. The tropics receive more precipitation than higher latitudes. The precipitation maximum, which follows the solar equator through the year, is under the rising branch of the Hadley circulation. The sub-tropical minima are under the descending branch and cause the formation of desert areas.
When the wet season occurs during the warm season, orsummer,precipitation falls mainly during the late afternoon and early evening hours. The wet season is a time whenair quality improves, freshwater quality improves and vegetation grows significantly due to the wet season supplementing flora, leading to crop yields late in the season. Floods and rains cause rivers to overflow their banks, and some animals to retreat to higher ground.Soil nutrients are washed away and erosion increases. The incidence ofmalaria increases in areas where the rainy season coincides with high temperatures. Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature.
The climate is changing in the tropics, as it is in the rest of the world.[13] The effects of steadily rising concentrations ofgreenhouse gases on the climate may be less obvious to tropical residents, however, because they are overlain by considerable natural variability. Much of this variability is driven by theEl Niño-Southern Oscillation (ENSO). The Tropics has warmed by 0.7–0.8 °C over the last century—only slightly less than the global average—but a strongEl Niño made 1998 the warmest year in most areas, with no significant warming since. Climate models predict a further 1–2 °C warming by 2050 and 1–4 °C by 2100.
Flora are plants found in a specific region at a specific time. Some well-known plants that are exclusively found in, originate from, or are often associated with the tropics include:
Tropicality refers to the image of the tropics that people from outside the tropics have of the region, ranging from critical to verging on fetishism.[14] Tropicality gained renewed interest in geographical discourse when French geographerPierre Gourou publishedLes pays tropicaux (The Tropical World in English), in the late 1940s.[15]
Tropicality encompassed two major images. One, is that the tropics represent a 'Garden of Eden', a heaven on Earth, a land of rich biodiversity or a tropical paradise.[16] The alternative is that the tropics consist of wild, unconquerable nature. The latter view was often discussed in old Western literature more so than the first.[16] Evidence suggests over time that the view of the tropics as such in popular literature has been supplanted by more well-rounded and sophisticated interpretations.[17]
Western scholars tried to theorise why tropical areas were relatively more inhospitable to human civilisations than colder regions of the Northern Hemisphere. A popular explanation focused on the differences in climate. Tropical jungles and rainforests have much more humid and hotter weather than colder and drier temperaments of the Northern Hemisphere, giving to a more diverse biosphere. This theme led some scholars to suggest that humid hot climates correlate to human populations lacking control over nature e.g. 'the wild Amazonian rainforests'.[18]
