Evaporation is the physical process by which a substance transitions from a liquid state into a gaseous state, or vapor, requiring a significant input of energy. The tropics are the geographical zone encircling the Earth, situated between the Tropic of Cancer and the Tropic of Capricorn, roughly 23.5 degrees north and south of the equator. The highest rates of evaporation on the planet occur within this tropical belt, resulting from a powerful combination of intense solar energy, abundant water availability, and dynamic atmospheric circulation.
The Intensity of Solar Radiation
The primary physical driver for increased evaporation in the tropics is the intensity of solar radiation the region receives year-round. Solar energy provides the thermal force necessary for water molecules to gain enough kinetic energy to escape the liquid surface as vapor. This belt receives the most direct sunlight because the sun is positioned nearly overhead for most of the year.
This direct angle of sunlight, close to 90 degrees, means the incoming solar energy is concentrated onto the smallest possible surface area. At higher latitudes, the sun’s rays strike the surface at a much lower angle, spreading the same amount of energy over a larger area. This maximized energy concentration in the tropics results in higher surface temperatures, directly fueling the rapid phase change of water into vapor.
This constant, high-power energy input from the sun ensures that water surfaces, such as the vast tropical oceans, are continually warmed. This heat drives the thermodynamic engine for the entire water cycle in the region. This mechanism maintains high evaporation rates throughout the year, overriding seasonal variations.
Abundance of Surface Water
The rate of evaporation is limited by the availability of the substance being evaporated, a condition rarely limiting in the tropics. This region is dominated by massive tropical oceans, which cover a significant portion of the Earth’s surface and act as a reservoir. The surface waters are consistently warm due to intense solar radiation, which lowers the energy barrier for water molecules to break free.
The oceans are the source of approximately 86% of global evaporation, and their sheer volume ensures a constant supply for the atmosphere. Beyond the oceans, the tropics contain extensive terrestrial water sources, including the world’s largest rainforests. These environments contribute a massive volume of water vapor through evapotranspiration—the combined effect of evaporation from the soil and transpiration from plants.
This widespread availability of liquid water means that solar energy can be continuously utilized to drive the phase change. The evaporation process is not restricted by dry surfaces, unlike in arid or polar regions where water is scarce. The combination of warm water and abundant coverage provides the substrate for perpetual, high-volume vapor generation.
Atmospheric Convection and Circulation
While intense solar energy drives evaporation, high rates require the continuous removal of humid air above the liquid surface, replaced by drier air. This is accomplished by dynamic atmospheric circulation patterns specific to the tropical zone. Intense heating of the ground and sea surface causes the air to become buoyant and rise rapidly, a process known as vertical convection.
This rising air efficiently carries the water vapor high into the troposphere, preventing the air layer above the water from becoming saturated. This continuous removal maintains a strong concentration gradient, allowing more water molecules to escape into the surrounding air. This upward movement forms the core of the global atmospheric flow system called the Hadley Cell.
The Hadley Cell circulation features air rising near the equator and descending in the subtropics around 30 degrees latitude. The return flow near the surface manifests as the trade winds, which blow toward the equator and are relatively dry initially. As these winds travel across the warm tropical oceans, they accumulate moisture, transporting drier air to the surface and carrying the resulting vapor away.