The Amazon rainforest is one of the most humid places on Earth, with relative humidity routinely sitting between 80 and 100 percent during the wet season and rarely dipping below 60 percent even in the driest months. But the raw numbers only tell part of the story. The Amazon is not simply a passive recipient of moisture from the Atlantic Ocean; the forest actively manufactures much of its own humidity through a self-reinforcing water cycle that scientists are still working to fully understand.
Where the Moisture Actually Comes From
Most of the water vapor hanging in the Amazonian air traces back to two sources: the tropical Atlantic Ocean and the forest itself. Moisture-laden trade winds blow westward off the Atlantic and hit the basin, delivering rain as they go. But as those air masses move inland, they would normally dry out. The reason they don’t is that the trees recycle water back into the atmosphere through transpiration, the process by which roots pull water from the soil and leaves release it as vapor. Previous studies have estimated that roughly 25 to 35 percent of the basin’s rainfall originated as evapotranspiration from within the Amazon itself, with strong diurnal and seasonal patterns in how that recycled moisture behaves.1Journal of Geophysical Research: Atmospheres. Amazonian Moisture Recycling Revisited Using WRF With Water Vapor Tracers In some agricultural regions near the forest edge, the contribution is even higher. One moisture-tracking study found that forests account for about 48 percent of annual rainfall on average, and more than half of that forest-sourced moisture comes from protected areas.2Geophysical Research Letters. Forests Mitigate Drought in an Agricultural Region of the Brazilian Amazon: Atmospheric Moisture Tracking to Identify Critical Source Areas
This means the Amazon’s extraordinary humidity is not just a feature of its geography or latitude. It is an emergent property of the ecosystem itself. Remove the trees and you remove a significant chunk of the water supply, a reality that has serious implications for deforestation.
Wet Season Versus Dry Season
The Amazon is not uniformly steamy year-round. The basin experiences a pronounced wet season (roughly December through May in most areas) and a dry season (June through November), though the exact timing shifts depending on whether you are in the northern or southern part. During the wet season, daily rainfall can easily exceed 10 millimeters, and the air is so saturated that dense morning fog blankets the canopy. That fog layer is not just a visual spectacle; it increases cloud reflectivity enough to reduce evapotranspiration during those hours by cutting the solar energy that reaches the canopy.3PubMed Central. Fog and rain in the Amazon The fog is largely absent during the dry season, meaning the canopy receives more direct sunlight and the atmospheric demand for water increases.
In the dry season, relative humidity can fall into the 60 to 70 percent range during the afternoon, which may not sound low by temperate standards but represents a significant dip for a tropical forest. The air’s “thirst” for water, measured by scientists as vapor pressure deficit (VPD), rises sharply. A higher VPD means the gap between how much moisture the air could hold and how much it actually holds is wider, which stresses plants and accelerates water loss from leaves. Despite this seasonal shift, the Amazon remains far more humid than most other ecosystems on the planet, even at its driest.
How Trees Keep Pumping Water in the Dry Season
One of the more remarkable findings about the Amazon is that many of its trees don’t simply endure the dry season; they continue transpiring at impressively high rates through it. This keeps humidity elevated even when rain slows. The secret lies underground. Amazonian trees have deep root systems that can reach many meters into the soil, tapping water that accumulated during the previous wet season. At night, some trees engage in hydraulic redistribution: their roots move water from deeper, wetter soil layers to shallower, drier ones, essentially irrigating the upper root zone while everyone sleeps. Modeling work has estimated that this process increases dry-season transpiration by roughly 40 percent across the Amazon, with direct consequences for regional temperature through changes in how much heat the surface radiates versus how much it puts into evaporating water.4PubMed Central. Root functioning modifies seasonal climate
More recent modeling that accounts for dynamic root water uptake from very deep soil layers confirms this picture, showing that mature forests in upland regions can avoid water stress during dry periods by pulling moisture from deep in the vadose zone. That work found a domain-averaged increase in transpiration of about 29 percent during dry months when deep-root access was included in the model, compared to a simulation without it.5Geoscientific Model Development. Implementing deep soil and dynamic root uptake in Noah-MP (v4.5): impact on Amazon dry-season transpiration Forests in valleys, meanwhile, can tap directly into groundwater. The seasonal shift in which soil layers roots draw from, shallower in the wet months, deeper in the dry months, helps explain why the Amazon stays so much greener and more humid than you might expect during its driest period.
Flying Rivers and Moisture Export
The Amazon’s humidity doesn’t stay in the Amazon. Vast atmospheric corridors of moisture, sometimes called “flying rivers,” carry water vapor from the basin southward and southeastward across the continent. These airborne moisture streams are a primary reason that southeastern Brazil, including its massive agricultural belt, receives reliable rainfall. The Amazon basin functions as a continental-scale moisture pump, with its evapotranspiration feeding precipitation patterns thousands of kilometers away.6Forest Research: Open Access. Spatio-temporal dynamics of flying rivers: assessment of precipitation model effectiveness and its influence on the SACZ
The recycled moisture also exhibits interesting spatial patterns within the basin. The contribution of Amazonian evapotranspiration to local atmospheric moisture has a strong diurnal cycle: it builds through the morning and afternoon as the sun drives transpiration, some of it rains out through convective storms in the early evening, and then nighttime winds associated with the South American Low-Level Jet push whatever remains downwind.1Journal of Geophysical Research: Atmospheres. Amazonian Moisture Recycling Revisited Using WRF With Water Vapor Tracers The northern and southern portions of the basin show contrasting seasonal behavior in how much recycled moisture they produce and receive, reflecting differences in when each hemisphere’s dry season hits.
What Happens When You Remove the Trees
If the Amazon generates a large share of its own humidity through transpiration, then removing forest cover should reduce humidity. That is exactly what researchers observe. Deforestation lowers evapotranspiration, which dries the atmosphere, increases atmospheric stability (making it harder for convective storms to form), and extends the distance that remaining moisture travels before falling as rain. Some of that moisture ends up leaving the basin entirely instead of recycling locally. These combined effects result in measurably less rainfall in deforested regions.7PubMed Central. Historical deforestation drives strong rainfall decline across the southern Amazon basin
Deforestation also reduces surface roughness. A forest canopy creates aerodynamic drag that slows wind and encourages moisture to linger and precipitate locally. Replace that canopy with pasture or cropland, and wind speeds increase, pushing moisture farther before it drops as rain. The result is a double hit: less water enters the atmosphere in the first place, and what does enter travels farther before coming back down. For communities and farms near deforested zones, this can mean noticeably drier conditions even if they themselves haven’t cleared any trees.
During major droughts, the contrast becomes even starker. Moisture tracking has shown that during the severe Amazonian droughts of 2005 and 2010, moisture supply from oceans and non-forested areas decreased, but supply from standing forests remained stable and partially compensated for the shortfall.2Geophysical Research Letters. Forests Mitigate Drought in an Agricultural Region of the Brazilian Amazon: Atmospheric Moisture Tracking to Identify Critical Source Areas In other words, the forests acted as a buffer against drought, a service that disappears with deforestation.
The Drying Trend
Beyond individual drought events, there is a longer-term trend toward drier conditions in parts of the Amazon, especially during the dry season. The key metric scientists track is vapor pressure deficit, which captures the gap between how much moisture the air can hold and how much it actually holds. When VPD rises, the atmosphere is demanding more water from every surface, plant, and soil patch it touches. Over the southeastern Amazon, VPD increased by roughly 6 millibars over the period 1987 to 2016, driven by about 2°C of warming (which raises the air’s moisture capacity) and a roughly 2.5°C decrease in dew point temperature (meaning less actual moisture in the air).8Environmental Research Communications. Increasing dry-season vapor pressure deficit in the Amazon Basin: historical trends, future projections, and links to deforestation
The pattern is not uniform. Over the northwestern Amazon, the drying is less of a steady linear trend and more a series of jumps associated with major drought years. The 2005 mega-drought brought VPD to a higher baseline, and the 2015 drought pushed atmospheric drying to the highest levels recorded since 1979.8Environmental Research Communications. Increasing dry-season vapor pressure deficit in the Amazon Basin: historical trends, future projections, and links to deforestation Each major drought seems to ratchet the baseline upward rather than returning to previous conditions, which worries scientists who study tipping-point dynamics in tropical forests.
Deforestation amplifies this drying locally. Attribution analyses show that VPD increases are strongest near recently deforested regions, not just because of the global warming signal but because fewer trees means less evapotranspiration cooling the surface and moistening the air. Warming raises the saturation vapor pressure by about 7 percent per degree Celsius of temperature increase, a basic thermodynamic relationship, so even if the same amount of water were present in the air, higher temperatures alone would widen the VPD gap. When you combine warming with reduced moisture supply from deforestation, the drying effect compounds.
How Dry-Season Moisture Recycling Is Shifting
A recent analysis of how the Amazon’s moisture recycling changes between wet and dry seasons found a small but statistically significant increase in the dry season’s reliance on forest-recycled moisture for rainfall, rising from about 21 percent of rainfall to 22 percent. That may sound like a rounding error, but the flip side is more concerning: a lower fraction of the moisture that the forest puts into the air during the dry season actually comes back down as rain within the forest itself. That fraction dropped by about 21 percent, from 41 percent recycling efficiency to 32 percent.9Water Resources Research. Enhanced Dry Season Moisture Recycling in the Congo and Amazon Rainforests In plain terms, the forest is working harder to keep itself moist but getting less of its own moisture back, because more of it is being exported downwind. And that leakiness increases as conditions get drier.
What Rising VPD Means for the Trees Themselves
Humidity is not just a number that describes the forest’s atmosphere; it directly shapes how the trees function. When the air gets drier (VPD rises), trees close the tiny pores on their leaves called stomata to conserve water. This reduces water loss but also cuts off the supply of carbon dioxide the tree needs for photosynthesis. Research on tropical trees has shown that stomatal conductance and photosynthesis consistently decline as VPD increases.10PubMed. The stomatal response to vapor pressure deficit drives the apparent temperature response of photosynthesis in tropical forests When scientists statistically accounted for VPD, they found that the optimum temperature for photosynthesis in Amazonian trees is actually higher than previously thought, around 33 to 36°C rather than the apparent optimum of 30 to 31°C that showed up in raw measurements. The earlier, lower estimates were confounded by the fact that hotter days also tend to be drier days, and it was the dryness, not the heat alone, that was shutting down photosynthesis.
This distinction matters for understanding the forest’s future. If rising VPD, rather than rising temperature per se, is the main brake on tree productivity, then the trajectory of humidity becomes at least as important as the trajectory of temperature in predicting how the Amazon will fare under climate change. A forest that can access deep soil water and keep transpiring might tolerate higher temperatures than one that has been stripped of its root-zone moisture buffer by drought or land-use change.
Variation Across the Basin
It’s worth remembering that “the Amazon” covers roughly 5.5 million square kilometers, and humidity conditions vary enormously across it. The western Amazon, closer to the Andes, tends to be wetter and more consistently humid year-round. The eastern edges, particularly the southeastern arc of deforestation, experience a longer and more pronounced dry season. Satellite-based analyses of integrated specific humidity across the basin have detected a slight declining trend in overall moisture during the years studied, along with large-scale oscillation patterns in humidity that stretch across the basin.11Journal of Geophysical Research: Atmospheres. Oscillation modes of humidity over the Amazon basin derived from GPS RO profiles
Within any given patch of forest, humidity also varies vertically. The canopy top is exposed to sun and wind, and relative humidity there can swing sharply over the course of a day. Descend to the understory, shaded and sheltered, and the air is cooler, stiller, and far more consistently saturated. This vertical gradient creates a stack of microclimates that support different communities of plants, insects, and fungi at each level. Epiphytes like bromeliads and orchids thrive in the humid mid-canopy, while the forest floor remains damp enough for fungi and mosses to carpet every available surface. It is this layered, self-sustained humidity that makes the Amazon’s biodiversity possible, and makes the prospect of its drying so consequential for millions of species that have evolved to live in air that is almost always close to saturation.
Measuring Humidity in a Place That Fights Back
Getting reliable humidity data from the Amazon is harder than it sounds. The forest is vast, remote, and tough on instruments. Traditional weather stations are sparse, and those that exist are often at forest edges or clearings that don’t represent conditions under the canopy. Eddy covariance towers, which measure exchanges of water, heat, and carbon dioxide between the forest and the atmosphere, provide detailed local data but cover only a handful of sites. Researchers have developed methods using energy-balance approaches and maximum entropy production theory to estimate evapotranspiration across larger areas, and these show good agreement with tower measurements at hourly, daily, and monthly scales.12Geophysical Research Letters. Estimation of Evapotranspiration of Amazon Rainforest Using the Maximum Entropy Production Method Satellite-based observations of atmospheric moisture, including GPS radio occultation data, fill in some of the spatial gaps but have their own limitations in temporal resolution and vertical detail.
More recent field campaigns have combined vertical profiling of temperature, humidity, VPD, wind speed, and light from below the canopy to above it, building a more complete picture of how the forest’s internal atmosphere works on an hour-by-hour basis.13Agricultural and Forest Meteorology. Daytime water and CO2 exchange within and above the Amazon rainforest These efforts are critical because the Amazon’s humidity is not a single number you can look up on a chart. It is a dynamic, spatially variable, vertically stratified quantity that shifts with the time of day, the season, the local topography, the age and density of the forest, and increasingly, with the pace of deforestation and the warming of the global climate. Getting the measurement right is not just an academic exercise. Models that project the Amazon’s future, and with it the future of South American rainfall, global carbon storage, and biodiversity, depend on understanding exactly how much water this forest puts into the air, and how that amount is changing.