In the southern Amazon, the rainy season begins in October, but the signal for this change does not come from the sky. A 2017 study published in PNAS found that the transpiration of the forest—water released by plants into the air—triggers shallow convection, which prepares the atmosphere for deep thunderstorms and the arrival of rains, two to three months before the Intertropical Convergence Zone, a region of heavy rainfall near the equator. Researchers measured the arrival of rains between 2005 and 2013 and found that the earliest rains began between October 4 and 8, while the latest started between November 2 and 6. This one-month difference spans an area several times the size of France. The transpiration of the Amazonian forest plays a crucial role in initiating the rainy season. A shallow convection moisture pump, or SCMP, humidifies the atmosphere and prepares the arrival of deep thunderstorms. However, deforestation and fires reduce this critical transpiration, extending the dry season. Evapotranspiration from the forest—water released through plant leaves and soil—accounts for about 30 to 50 percent of the region’s rainfall. While no one disputed this fact, a key question remained: does the water returned to the air drive the seasonal cycle of rainfall, or does it merely adapt to it? To answer this, a team led by Jonathon Wright and Rong Fu, along with researchers from NASA’s Jet Propulsion Laboratory, combined several independent satellite data sets. One key dataset measured the presence of deuterium, a heavy form of hydrogen, in water vapor using the TES instrument. This helps identify vapor released by leaves. At the end of the dry season, transpiration feeds shallow convection, which humidifies and destabilizes the atmosphere. The researchers named this process the "shallow convection moisture pump." This process does not directly activate deep convection but first goes through an intermediate stage. Isotopic signatures show that transpiration is the main source of moisture for this convection. Once the atmosphere is prepared on a regional scale, deep thunderstorms multiply rapidly, attracting surrounding moisture and triggering the rainy season. This mechanism operates two to three months before the arrival of the Intertropical Convergence Zone, meaning the forest starts the rainy season, rather than the rain starting the forest. This pattern has direct consequences for deforestation. The researchers suggest it provides a framework for understanding how the retreat of the forest extends the dry season and increases vulnerability to droughts. Fewer trees mean less vapor at the precise moment the atmosphere needs it to shift. Fires add another risk: aerosols from end-of-dry-season burns could impair the efficiency of the moisture pump. Smoke spreads during the period when the pump should begin, suggesting that deforestation and fires can delay the start of the rains. The dry season in the southern Amazon has already lengthened in recent decades, especially due to a later start of the rains. Simulations even suggest a sudden shift of the forest to savanna if this trend continues. The study focuses on the southern Amazon and data from 2005 to 2013. It shows that the forest triggers the start of the rains, not that it produces all its rain. The mist rising from the canopy at dawn is a visible sign of this transpiration, which satellites have tracked.