When clouds form over land in preference to over the water, like in this satellite view of the Amazon Delta in Brazil, it is generally because the land heats up through the day more rapidly than the water does. The warmer land causes thermals, which create clouds. But in the case of an enormous tropical rainforest like the Amazon, there is another reason: the trees.

Under the heat of the Sun, the trees release moisture into the air through their leaves – like a plant version of sweating, known as transpiration. They also release volatile organic chemicals, which form into condensation nuclei: microscopic airborne particles that encourage cloud droplets to form. In the flow of air over the Amazon from the Atlantic to the Andes, over half of the rain is generated by the forest itself, in a repeating cycle of transpiration and precipitation. Thanks to its trees, the annual flow of water through the skies of the Amazon is greater than through the enormous river system below.