The Amazon watershed lies in the tropics, the band around the equator from 30 degrees north latitude to 30 degrees south, which receives more solar radiation than any other place on the planet. The lushness of the forest is due partly to sunlight and partly to wind and rainfall patterns whose effects reach beyond the forest.

Tropical vegetation absorbs moisture, then releases it through the leaves in a process called transpiration. As the sun’s energy warms the surface of the earth near the equator, the moisture-laden warm air rises and cumulus clouds form, eventually releasing the moisture again as rain. This strong convection occurs in a band called the Inter-Tropical Convergence Zone (ITCZ), which shifts slightly north of the equator during the northern hemisphere summer and slightly south of the equator during the southern hemisphere summer. 

Clouds travel westward with the east-west trade winds, which also carry moisture from the Atlantic Ocean over the South American continent. At the western edge of the Amazon basin, however, they are blocked by the Andes Mountains. Some of the air rises up the slope, cooling and dumping moisture as rain in the piedmont and on the foothills. Some turns south, forming a current called the South American low-level jet, which skirts the mountain range, then circles back eastward across the continent, carrying precipitation to southern Brazil and northern Argentina.