The Tropics - The Engine of the Global Water Cycle

Precipitation is highest around the equator and drops as latitude increases.

The Tropical Band
Source - Earthobservatory. nasa.gov

Rain Forests
Rain forests is our name for places that have the highest rainfall. 
As the atmosphere over the Tropics have some of the highest water vapor concentrations, the most rainfall occurs in the latitude band created by the Tropic of Cancer and the Tropic of Capricorn.


Latitude and Precipitation

Precipitation & Latitude
Source - roperld.com

As expected, the precipitation (rainfall) is highest between the bands created by the two Tropics.
The amount of water vapor available in the tropical atmosphere is orders of magnitude greater than that available elsewhere. In other words, the water cycle in the tropics circulates greater volumes of water than the water cycle does in other parts of the Earth, recognizing the existence of Temperate Rain forests that are the result of local variations in atmospheric moisture content, local temperature and other factors.
The tropics are the engine of the global water cycle (aka, the Global Hydraulic Cycle).