Precipitation is one of the factors that define a rainforest. This handout shows the average rainfall in Manaus, Brazil.
Oceans and atmosphere currents, wind currents, and mountains play a role in the amount of rain that falls on tropical forests. The air over oceans is generally warmer than the air above the land just offshore. If winds move the moisture-laden air from the ocean to the cooler air above land, a rain-shower results. Even more rain will fall if the warm ocean air moves over the mountains, since air cools when it rises.