Rainforests are defined—as you would expect—by rainfall, and in fact they are literally created by it. (“Rain forest”—two words—is the older usage: both are accepted, but most modern authors and researchers combine the two, as does PTK, as in “Rainforest.”) They can be found where rain exceeds 80 inches per year, and can appear in temperate as well as tropical zones, so long as the rainfall is sufficiently plentiful.

Tropical rainforests often have from 160 to 400 inches of rain a year. But they aren’t the wettest or even the hottest places on Earth. (The wettest is Mount Waialeale, in Hawaii, USA, and the hottest is Libya in North Africa.) But just as important as the amount of rain in shaping the unique character of rainforests is the constant humidity and high average temperature. In the Amazon basin you can expect at least 130 days of rain a year and, in many places, up to 250 days. The relative humidity never falls below 80%, and temperatures vary little between daytime averages of 31 degrees Centigrade (88 Fahrenheit) and night-time lows of 22 degrees C (72 F).

Sometimes this constancy of temperature and humidity leads people to argue that rainforests have no seasons, but in the tropics this is only partially correct. There may not be a cold winter and a hot summer, but there are DRY seasons and WET seasons. Plants and trees flower at these different times of year, profoundly influencing the lives of the creatures who inhabit them. And our contemporary understanding of rainforests (see ECOsystem) quickly dispels the misconception that this is a changeless Eden, where Nature’s endless bounty means things are always the same. In fact there’s a constant fight for light, water and nutrients, one of the reasons natural selection has had such a powerful effect in creating the great numbers of species which make tropical rainforests the richest places for biodiversity on Earth.