Generally speaking, a rainforest is an environment that receives high rainfall and is dominated by tall trees. A wide range of ecosystems fall into this category, of course, including the old-growth temperate forests of the Pacific Northwest. But most of the time when people talk about rainforests, they mean the tropical rainforests located near the equator.
These forests, concentrated in Africa, Australia, Asia, and Central and South America, receive between 160 and 400 inches (406.4 to 1016 cm) of rain per year. Unlike the rainforests farther to the north and south, tropical rainforests don't really have a "dry season." In fact, they don't have distinct seasons at all. The total annual rainfall is spread pretty evenly throughout the year, and the temperature rarely dips below 60 degrees Fahrenheit (16 degrees Celsius).
This steady climate is due to the position of rainforests on the globe. Because of the orientation of the Earth's axis, the Northern and Southern hemispheres each spend part of the year tilted away from the sun. Since rainforests are at the middle of the globe, located near the equator, they are not especially affected by this change. They receive nearly the same amount of sunlight, and therefore heat, all year. Consequently, the weather in these regions remains fairly constant.
The consistently wet, warm weather and ample sunlight give plant life everything it needs to thrive. Trees have the resources to grow to tremendous heights, and they live for hundreds, even thousands, of years. These giants, which reach 60 to 150 ft (18 to 46 m) in the air, form the basic structure of the rainforest. Their top branches spread wide in order to capture maximum sunlight. This creates a thick canopy level at the top of the forest, with thinner greenery levels underneath. Some large trees, called emergents, grow so tall (up to 250 ft / 76 m) that they even tower over the canopy layer.
As you go lower, down into the rainforest, you find less and less greenery. The forest floor is made up of moss, fungi, and decaying plant matter that has fallen from the upper layers. The reason for this decrease in greenery is very simple: The overabundance of plants gathering sunlight at the top of the forest blocks most sunlight from reaching the bottom of the forest. The lowest levels of the rainforest are extremely dark, making it difficult for robust plants to thrive. As little as 1 percent of the light shining onto the forest reaches the lowest levels.
This makes for a fascinating biological community in which plant life is striving to reach 100 ft (30.5 m) into the air, and most food for animals comes from above. In the next couple of sections, we'll look at some of the plants and animals of the rainforests to see how they live and interact in this sumptuous, yet highly competitive, world.