photo #6825148

When clouds form over land in preference to over the water, it is generally because the land heats up through the day more rapidly than the water does. The warmer land causes the air above it to expand and lift in columns, known as thermals, on top of which clouds develop. 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. This is like a plant version of sweating, known as ‘transpiration’, and the process can release enough moisture into the air to encourage cloud formation like this extensive array of fair-weather Cumulus. Studies of satellite data suggest that within 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. The collective atmospheric effect from the trees means that, year round, the flow of water across the skies of the Amazon is greater than through the enormous river system below.Cumulus clouds spotted over the north-western Amazon basin by the MODIS instrument aboard NASA’s Aqua satellite. An example of research into the effect of trees on atmospheric moisture is ‘Observations of increased tropical rainfall preceded by air passage over forests’ (2012) by Spracklen, Arnold & Taylor, published in Nature, Vol. 489, Issue 7415.