The physical environment of tropical rain forests
Tropical rainforests often have 160 to 400 inches of rain a year (some forests receive as much as 10,000 mm). 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 quickly dispels the
misconception that this is a changeless Eden, where Natuer¡¦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.
The rain forest is one of the Earth's best recyclers, because around 75% of the
water that falls as rain will evaporate and be recycled back into the
atmosphere. Then it will fall down as rain again. This is almost a closed water
cycle and unlike any other on Earth. "The rainy season" is between mid July
and mid September.
On most days in the Tropics the rainforests receive 12 hours of sunshine
providing plenty of energy for plant growth. But they grow in very poor soil as
millions of years of rain has washed away the nutrients. Conveniently, the
rainforest has adapted to these conditions and gets its nutrients by recycling
the dead plants that fall to the forest floor. Micro-organisms rapidly break
down this matter into usable nutrients like calcium, nitrogen and phosphorus.
Rainfall: It is almost always raining in a rainforest. Rainforests get over 80 inches (2 m) of rain each year. This is about 1 1/2 inches (3.8 cm) of rain each week.
The rain is more evenly distributed throughout the year in a tropical rainforest (even though there is a little seasonality). In a temperate rainforest, there
are wet and dry seasons. During the "dry" season, coastal fog supplies abundant moisture to the forest.
Temperature: The temperature in a rainforest never freezes and never gets very hot.
The range of temperature in a tropical rainforest is usually between 75¢X F and 80¢X F (24-27¢XC). Temperate rainforests rarely freeze or get over 80¢X F
(27¢X C).There are only two seasons in a tropical rainforest, the wet season and the dry season.
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Effects of Altitude on Climate and Vegetation
For each 1,000 foot rise in altitude there is a 4¢X F drop in temperature. For example, if at sea level the average temperature is 75¢X F, at
10,000 feet the average temperature would be only 35¢X F. This has a dramatic effect on plant and animal distribution.
In tropical mountainous areas several types of forests occur. At sea level there are lowland rainforests and mangroves. Montane forests are
cooler and may contain deciduous trees. At cloud level moist, dripping cloud forests may occur. They are cooler than lowland forests. High
mountains may also have alpine, tundra and snow covered peaks.
