Earth Science: Atmosphere – Tropical Climates
The tropics are known as humid, muggy areas with evergreen rain forests rich in animal and plant species. But tropical habitats can also include the grasslands of the savannas and even dry deserts.
The term “tropics” comes from the Greek tropos, which means “to turn.” It describes the climate zone on either side of the Equator between the Tropic of Cancer and the Tropic of Capricom-between 23.5° north and south latitude. This zone forms a belt around the circumference of the Earth, incorporating about two-thirds of the total landmass. The key characteristics of the tropical belt are continuously high solar radiation and consistently high temperatures. At sea level the average monthly temperatures never fall below 68°F (20°C). Temperature fluctuations between day and night are greater than those over the course of the year.
Temperatures in the tropics decline with rising altitudes. The lowland tropics have a warm tropical climate and the tropical mountains have a cold tropical climate. Lowland tropical areas may be humid, with a climate typical of tropical rain forests; these are the inner tropics. On the other hand, the outer tropics have a subhumid climate typical of savannas. The transition point occurs where the annual temperature fluctuations are greater than 18°F (10°C).
From the jungle to the desert
Only the rain forests experience constantly humid conditions. They are located within the inner tropical convergence (ITC) zone. This zone covers the area between 10° north and south latitude. The annual rainfall decreases the farther away from the Equator an area is located, while the number of dry months increases.
Rain- fall occurs during one or several rainy seasons. The rainy seasons of south Asia are often accompanied by heavy winds called monsoons blowing in from the ocean. A long dry season follows, as dry and cold air moves in the opposite direction from the mainland toward the ocean.
The subhumid tropics are divided into vegetation zones according to the number of rainy months: the semihumid savanna has 7 to 9.5 rainy months, the dry savanna has 4.5 to 7, and the arid shrublands have 2 to 4.5. The desert or semidesert areas of the dry tropics receive less than two months of rain per year.
COASTAL CLIMATES
On the coast, the water moderates the temperature by acting as an insulator; the oceans may take longer to warm up but they also take longer to cool down. The marine air also carries plenty of water vapor.
The very dry coastal deserts of Atacama and Namlb are exceptions. Cold ocean currents cause short-term fog, but they simultaneously prevent the formation of rain clouds.