Rainforests have more average annual rainfall than any other biome: Their yearly precipitation totals may be three times or more those of the next-wettest biome, the temperate deciduous forest. General defining features of rainforest ecosystems -- which include various types of both tropical and temperate rainforests -- are high annual precipitation, high humidity and relatively modest temperature variation throughout the year. The rainforest cannot exist without its trees, on which it partly depends for the very element that most defines it -- rain.
Rainforests are characterized by heavy annual rainfall, thick vegetation and diverse plant and animal species. At a minimum, a true tropical rainforest receives 80 inches of rainfall annually. Most receive much more. The state of Kentucky, by comparison, receives an average of 40 to 50 inches of rain per year. This means even the driest of rainforests get twice the rainfall Kentucky does. Rainfall amounts in the rainforest are relatively equal throughout the year, though some have one slightly wetter month per year. In the rainforest, it rains more than half the days of the year, every year.
Function of Trees
Rainforests exist as nearly closed systems, with much of their rainfall coming from water that is recycled throughout each day through the rainforest water cycle and transpired from the lush growth of trees. Deforestation, therefore, can affect rainfall amounts in some rainforest ecosystems -- especially tropical rainforests. Because they are located near the equator, their temperatures remain consistently warm. The thick tree growth combined with warm temperatures and high humidity in the air from evaporation provide a cycle that produces as much as 50 to 75 percent of its own rain daily. Trees play an important part with transpiration and by shading the rain-soaked ground, which keeps humidity high. Without the right amount of trees, rain lessens, vegetation dies, and the rainforest's existence is threatened.
Tropical rainforests grow near the equator, where the warmth of the sun and the length of daylight are constant. Temperatures fluctuate less than 20 degrees, and 12-hour days provide equal amounts of sunlight and darkness. Tropical rainforests, as opposed to temperate rainforests, always exist in a warm environment. Because of the limited fluctuation in temperature during daylight hours, rainfall remains constant and heavy. True tropical rainforest -- equatorial evergreen rainforest -- typically receives at least 2,000 millimeters (80 in.) of annual rainfall, but the wettest may get 10,000 millimeters (304 in.) or more. The rain falls year-round, because there are no warm and cool seasonal changes to affect rainfall amounts. The Amazon rainforest is the largest, and gets rain 130 to 250 days per year. Humidity stays at a constant 80 percent, and it is rare that rainfall dips below 152 millimeters (6 in.) per month.
Tropical Rainforest Types
Equatorial evergreen rainforest is the “classic” form of tropical rainforest, composing the two largest tracts in the world: the Amazon and Congo Basin rainforests. Other ecosystems in the tropics, however, are sometimes classified as distinct types of tropical rainforest, mainly defined by greater seasonal fluctuations in precipitation. For example, monsoon rainforest, often broadly classed as a variety of tropical moist forest, occupies the wettest regions of monsoon climates. These forests may actually receive heavier rainfall -- 10,795 millimeters (425 in.) or more in extreme areas -- than the tropical-wet climates of equatorial evergreen forests, but on a much more seasonal basis.
Temperate rainforests are less common than tropical rainforests. They are found along coastal areas, most notably in marine west-coast climate zones. The largest and most famous temperate rainforest is along the west coast of the United States, from northern California extending up to Alaska. Japan, Norway, New Zealand, Great Britain, Chile and Australia have temperate rainforests, as well. The rainfall and humidity remain high, just as in a tropical rainforest, but the temperature does fluctuate more. The fluctuations are not great, however, and frosts and temperatures over 80 degrees are rare. Rainfall reaches an average of 100 inches annually. Temperate rainforests generally are found at higher elevations. Some of the rainfall amounts come from fog, which can be very heavy in a temperate rainforest.