While you may think of a rainforest as being perpetually wet and rainy, the world’s largest rainforest, the Amazon, actually has a dry season when the clouds clear and sunlight drenches the trees. It is during this period, the time without rain, that the forest grows the most. For much of the Amazon Rainforest, the dry season occurs in June, July, and August. During this period, the thick blanket of clouds brought in by large-scale patterns in the atmosphere disappear, and smaller-scale processes that influence the weather become apparent. This image, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite on August 19, 2009, reveals how the forest and the atmosphere interacted to create a uniform layer of “popcorn” clouds one afternoon. The clouds formed from water vapor released from trees and other plants throughout the day. Plants convert light, carbon dioxide, and water into sugar and oxygen through photosynthesis. The excess oxygen, a waste product of photosynthesis, exits the leaves through tiny pores. As the plants exhale oxygen, water vapor also escapes, a process called transpiration. During the dry season, the rainforest gets more sunlight. The plants thrive, putting out extra leaves and increasing photosynthesis. The photosynthesizing plants release water vapor into the atmosphere. Water vapor is more buoyant than dry air, so it rises and eventually condenses into clouds like the popcorn clouds shown in this image.


These clouds are almost certainly a result of transpiration. The clouds are distributed evenly across the forest, but no clouds formed over the Amazon River and its floodplain, where there is no tall canopy of trees. While water may evaporate from the Amazon River itself, the air near the ground is too warm for clouds to form. Trees, on the other hand, release water vapor at higher levels of the atmosphere, so the water vapor more quickly reaches an altitude where the air is cool enough for clouds to form. When water vapor condenses, it releases heat into the atmosphere. The heat makes the air even more buoyant, and it rises. The higher it rises, the more the air expands and cools, which allows more water vapor to condense. Eventually, thunderstorms can form. The more concentrated clusters of clouds in the image are likely thunderstorms.

In the Amazon, transpiration may play a significant role in transitioning between the rainy and the dry seasons. Westward-blowing trade winds carry moisture from the Atlantic Ocean over South America year round. Once over the continent, regional winds channel the moist air north or south. When winds blow north, Atlantic moisture goes with it, and the part of the rainforest south of the equator experiences a dry season. When winds shift to the south, the seasons reverse. Tropical climatologist Rong Fu, of Georgia Institute of Technology, believes that the shift in wind direction toward the southern Amazon may be triggered by late dry-season thunderstorms originating from transpiring plants. The widespread thunderstorms carry heat high into the atmosphere. The heated air rises, and air from the north replaces it. This movement of air creates the winds that channel monsoon moisture back to the southern part of the Amazon Rainforest. (NASA image courtesy Jeff Schmaltz, MODIS Rapid Response at NASA GSFC. Caption by Holli Riebeek.)