Did you notice yet that rainforests only exist in equator proxiwity between the Tropic of Cancer (23°27’ N) and the Tropic of Capricorn (23°27’ S)? This observation is relatively easy to explain.
In equator proxiwithy the sun stands almost vertical and transfers thus distinct more enegry than away from the equator. On the one hand this causes constant high temperatures between 20 and 30°C throughout the year. Likewise the strong solar radiation causes a distinct evaporation and a distinct warming of the air masses.
At this point, let me take a little excursion in the world of physics and shine a light on the conceptualities absolute and relative humidity. The capacity of air to absorb water vapour is temperature-dependent. The warmer air is the more water can be taken up. The absolute humidity is indicated in g/m³, describes therefore how much gramme water in one cubic metre air are currently contained. By contrast, the relative humidity is a percentage indication, which refers the current water vapour content to the maximal water vapour content at this temperature. The absorption capacity of air for water vapour is limited, the air is saturated at a certain value (which is temperature-dependent), further water can not be absorbed any more. If maximal water vapour saturated air cools down, so condenses surplus water. A phenomenon, which each of us could observe during a hot bath at his bathroom mirror.
After this excursion back again to the tropics. The warm, maximal water vapour saturated air rises and cools down at the same time. In addition the air masses encounter heavily water vapour saturated trade winds, which fly from the sea inland. Surplus water vapour condenses, huge clouds develope, from which abundant precipitations fall. The annual precipitations amounts are considerable. In Germany amounts the annual precipitation circa 700 to 1.000 mm/square metre. In the tropics are minimum quantities of 2.000 mm/square metre, regional maximum quantities of up to 10.000 mm/square metre reached.