Clouds simultaneously cool and warm the Earth’s surface, and the balance between those two effects depends on the time of day, the type of cloud, and where you are on the planet. Globally, the cooling side wins: measurements from the Earth Radiation Budget Experiment found that clouds reflect roughly 44.5 watts per square meter of incoming sunlight back to space while trapping about 31.3 watts per square meter of outgoing heat, producing a net cooling effect of around 13 watts per square meter. That single number, though, hides an enormous amount of complexity that matters for everything from your local weather forecast to how fast the planet warms in the coming decades.
How Clouds Cool the Surface During the Day
The most intuitive effect of clouds is the one you feel when a cloud drifts across the sun on a summer afternoon: the temperature drops. Clouds are made of tiny water droplets or ice crystals that are highly reflective. When sunlight hits a cloud layer, a significant fraction bounces back into space before it ever reaches the ground. That reflected energy never gets a chance to heat the soil, pavement, or ocean surface below. The global average shortwave (sunlight-reflecting) effect of clouds measured at about negative 44.5 watts per square meter during April 1985, a substantial cooling force that exceeds any single greenhouse gas in magnitude.1PubMed. Cloud-radiative forcing and climate: results from the Earth radiation budget experiment
Low-lying clouds like stratocumulus and stratus are the most effective daytime coolers. They sit close to the surface, are thick enough to block a lot of sunlight, and cover vast stretches of ocean, especially off the western coasts of continents. Because their tops are relatively warm, they don’t trap much outgoing heat compared to what they reflect. High, thin cirrus clouds are a different story, and we’ll get to those shortly.
How Clouds Warm the Surface at Night
After the sun sets, the cooling effect vanishes because there is no sunlight to reflect. What remains is the warming effect. The ground radiates heat upward as longwave (infrared) radiation, and cloud layers act like a blanket, absorbing some of that radiation and re-emitting it back toward the surface. On a clear winter night, temperatures can plummet dramatically, while an overcast night stays comparatively mild. This is why farmers historically worried about frost on clear nights but felt safer under cloud cover.
Research confirms this asymmetry directly: during the day, clouds cool the surface by blocking sunlight, while at night, clouds warm the surface by trapping longwave radiation.2PubMed Central. Diurnally asymmetric cloud cover trends amplify greenhouse warming The net result is that cloud cover narrows the gap between daytime highs and nighttime lows. An overcast day tends to be cooler than a sunny one, and an overcast night tends to be warmer than a clear one. The total daily temperature range shrinks.
Why Cloud Height Changes Everything
Not all clouds push temperatures in the same direction even during the same time of day. The height at which a cloud forms largely determines whether it has a net warming or cooling influence.
- Low clouds: Stratocumulus, stratus, and fog sit below about two kilometers. They are thick, bright, and strongly reflective. Because they’re close to the surface, their tops are relatively warm, so they don’t trap much additional heat. Their dominant effect is cooling.
- Mid-level clouds: Altostratus and altocumulus layers sit between roughly two and six kilometers. They reflect some sunlight and trap some heat; their net effect depends on thickness and coverage, and it is often close to neutral.
- High clouds: Cirrus and other ice-crystal clouds form above about six kilometers where temperatures are extremely cold. They are thin and semi-transparent, so they let most sunlight through. But because their tops are frigid, they are very efficient at trapping outgoing heat. Their dominant effect is warming.
This height dependence is one reason the net effect of clouds on temperature is so hard to pin down in climate models. A shift from low cloud cover to high cloud cover in a warming world could flip the sign of the cloud effect from cooling to warming in a given region, even without any change in total cloudiness.
Regional Differences That Flip the Script
The global average cooling effect of clouds is just that: an average. In some regions, clouds warm the surface more than they cool it. The Arctic is a striking example. During polar winter, there is little or no sunlight for months, so the reflective cooling effect is irrelevant. Clouds in the Arctic during winter act almost purely as insulators, trapping heat that would otherwise escape to space and keeping surface temperatures warmer than they would be under clear skies. Even during the Arctic summer, the situation is complicated: frequent temperature inversions and low sun angles mean the net radiative effect of clouds can flip from warming to cooling and back depending on the specifics.3Scientific Reports. High cloud coverage over melted areas dominates the impact of clouds on the albedo feedback in the Arctic
Over bright surfaces like sea ice and fresh snow, clouds actually have a harder time producing a cooling effect because the surface beneath them is already reflecting most incoming sunlight. Adding a cloud layer over fresh snow barely changes the amount of sunlight bouncing back to space, but the cloud still traps outgoing heat. The result is net warming, even during the daytime. This is one of the reasons the Arctic is warming faster than the rest of the planet: as sea ice melts and exposes darker ocean water, high cloud cover over those newly open areas strengthens the warming cycle.
In the tropics, by contrast, enormous towering cumulonimbus clouds dominate the scene. These deep convective clouds are extremely bright at the top and strongly reflective, but they also have very cold, high tops that trap outgoing heat efficiently. The two effects roughly cancel each other out in the deep tropics, leaving low-lying marine stratocumulus clouds off the western coasts of South America and Africa as the real heavy lifters of tropical cloud cooling.
Cloud Cover and City Temperatures
If you live in a city, cloud cover affects you in a way it doesn’t affect someone in a rural area. Urban heat islands form because concrete, asphalt, and buildings absorb and retain more heat than vegetation and soil. The intensity of that urban-rural temperature difference is sensitive to cloud cover. A study of Seoul found that each additional octa of cloud cover (one-eighth of the sky covered) reduced the urban heat island intensity by about 0.24°C.4Sustainable Cities and Society. Influences of wind speed, sky conditions, land use and land cover characteristics on the magnitude of the urban heat island in Seoul Under clear skies and calm winds, the temperature difference between the city center and surrounding countryside was at its peak. Cloud cover reduces that gap partly by lowering daytime surface heating and partly by reducing the nighttime radiative cooling that rural areas experience more strongly.
For practical purposes, this means the sweltering heat of a city summer evening is at its worst on clear, still nights. Cloud cover acts as a partial equalizer between urban and rural temperatures, though it doesn’t erase the heat island entirely.
When Clouds Show Up Matters as Much as Whether They Do
An underappreciated aspect of the cloud-temperature relationship is timing. A cloud that forms at noon has a very different effect from one that forms at midnight. Daytime clouds suppress the daily maximum temperature; nighttime clouds raise the daily minimum. If cloud cover shifts preferentially toward one part of the day, it can reshape temperature trends in ways that a simple average would miss.
This is exactly what has been happening across large swaths of the planet. Research shows that over twice the area of land has experienced nighttime warming that exceeds daytime warming by more than 0.25°C, and this asymmetry is driven primarily by increased cloud cover dampening daytime temperatures while raising nighttime ones.2PubMed Central. Diurnally asymmetric cloud cover trends amplify greenhouse warming In areas where night-time temperatures increased by more than 0.5°C above daytime temperature increases, cloud cover, humidity, and precipitation all went up. Where daytime warming outpaced nighttime warming, the opposite was true: cloud cover and precipitation declined.
The ecological consequences are real. Many organisms respond to nighttime minimum temperatures more than to daytime peaks. Warmer nights can extend growing seasons, alter insect life cycles, and shift the timing of plant flowering. Clouds are not just passive bystanders in these changes; they actively shape which part of the temperature cycle shifts.
Clear Skies and Heatwaves
The flip side of cloud cooling is that an absence of clouds allows temperatures to soar. The record-breaking heat that struck Eastern Europe in the summer of 2024 illustrates this vividly. Researchers found that a substantial decline in low cloud cover over the region created an exceptionally clear-sky environment. With fewer low clouds persisting, increased solar absorption at the surface intensified the anomalous high temperatures through a daytime positive feedback loop: clear skies heated the surface, which altered atmospheric conditions in ways that further discouraged cloud formation.5npj Natural Hazards. Breaking records under clear skies: the impact of sunshine duration and atmospheric dynamics on the 2024 Eastern European extreme summer temperatures
This kind of feedback is a recurring ingredient in extreme heat events worldwide. High-pressure systems that cause heatwaves also suppress cloud formation, and the resulting clear skies amplify the warming that the high-pressure system already encourages. It is worth thinking of cloud cover not just as an independent variable that affects temperature, but as something that temperature and atmospheric circulation in turn control, creating loops that can push conditions to extremes in either direction.
Clouds as Climate Amplifiers
For decades, the biggest source of uncertainty in climate projections has been how clouds will respond to a warming world. As the planet heats up, will clouds change in ways that amplify or dampen that warming? The evidence has grown considerably stronger that the net effect is amplification. Observational data constrains the global cloud feedback to roughly 0.43 watts per square meter per degree of warming, meaning that for every degree the planet warms, changes in cloud cover add additional heating on top of the greenhouse gas forcing that started the process.6PubMed Central. Observational evidence that cloud feedback amplifies global warming That finding implies less than a 1% chance that clouds could offset enough warming to keep equilibrium climate sensitivity below 2°C.
Climate models identify three main mechanisms through which clouds feed back on warming. First, high-altitude clouds tend to rise as the atmosphere warms, which makes them even colder at their tops and more effective at trapping outgoing heat. Second, tropical low cloud cover tends to decrease, removing some of the planet’s reflective shield. Third, low clouds at high latitudes tend to thicken and become more reflective, partially counteracting the first two effects.7WIREs Climate Change. Cloud feedback mechanisms and their representation in global climate models The first two effects are positive feedbacks (amplifying warming), and they outweigh the third, which is negative (dampening warming). Subtropical cloud changes appear to complement these positive feedbacks, further suggesting that future global cloud changes will amplify warming rather than counteract it.8Geophysical Research Letters. Reducing the uncertainty in subtropical cloud feedback
What makes this concerning is the scale. A cloud feedback of 0.43 watts per square meter per degree doesn’t sound like much, but over the entire surface of the Earth it adds up to a substantial additional energy input. It’s one of the main reasons that projections for end-of-century warming have not decreased as climate models have improved: as other uncertainties have been resolved, the cloud feedback has consistently pointed toward more warming rather than less.
Aerosols, Volcanoes, and Marine Cloud Brightening
Clouds don’t form out of nothing. Every cloud droplet condenses around a tiny particle called a cloud condensation nucleus, which can be a speck of sea salt, dust, soot, or sulfate aerosol. The number and type of these particles affect how many droplets a cloud has, how large those droplets grow, and consequently how reflective the cloud becomes. More particles generally mean more numerous but smaller droplets, which makes the cloud brighter and more reflective.
Volcanic eruptions provide natural experiments in this relationship. Satellite observations of eruptions in Hawaii showed a large enhancement in reflected sunlight from tropical marine clouds downwind of the eruption, mainly driven by an aerosol-induced increase in cloud cover. The finding implies a strong negative (cooling) forcing from aerosols, which means the current level of global warming is being driven by a weaker net radiative forcing than previously estimated, because aerosol cooling has been partially masking the full greenhouse warming.9Nature Geoscience. Substantial cooling effect from aerosol-induced increase in tropical marine cloud cover The practical worry here is that as air pollution declines (which is good for human health), some of that aerosol masking disappears, potentially unmasking additional warming that was always in the pipeline.
This same physics underpins the concept of marine cloud brightening, a proposed form of solar geoengineering in which sea salt particles are sprayed into the atmosphere over oceans to make low marine clouds more reflective. Simulations with advanced climate models suggest that seeding clouds over as little as 5% of the ocean area could achieve meaningful surface cooling.10Journal of Geophysical Research: Atmospheres. Climate Impact of Marine Cloud Brightening Solar Climate Intervention Under a Susceptibility‐Based Strategy Simulated by CESM2 The approach remains highly controversial and has never been tested at scale, with open questions about regional side effects on precipitation patterns, the durability of the cooling, and the governance challenges of one country or company intentionally altering the planet’s cloud cover.
Soil, Moisture, and the Feedback Loop Below the Clouds
The relationship between clouds and surface temperature runs in both directions. The ground doesn’t just passively receive whatever clouds deliver; it actively influences cloud formation. Wet soil evaporates more water into the atmosphere, increasing humidity and encouraging cloud development. Satellite observations have documented this feedback: positive anomalies in soil moisture and land-surface temperature enhance cloud reflectivity in the lower atmosphere, at altitudes of roughly one to three kilometers above the surface.11PubMed Central. Soil Moisture-Cloud-Precipitation Feedback in the Lower Atmosphere From Functional Decomposition of Satellite Observations
In a simplified version of this cycle: a wet surface heats up, evaporates moisture, feeds cloud formation, and the resulting clouds shade the surface and reduce heating, which slows further evaporation. In a dry cycle, the opposite occurs: dry soil doesn’t generate enough moisture for clouds, clear skies heat the surface further, and the soil dries out even more. These self-reinforcing loops help explain why droughts and heatwaves tend to intensify once they get going, and why wet periods can sustain themselves. They also explain why deforestation, which reduces both soil moisture and the evapotranspiration from trees, can lead to regional reductions in cloud cover and higher local temperatures.
What Clouds Mean for Plants and Crops
Clouds don’t just change the amount of light reaching the surface; they change its quality. Direct sunlight arrives in a strong beam from one direction, creating harsh shadows in a forest or crop canopy. Light that passes through or around clouds becomes diffuse, arriving from all directions at once. This diffuse light penetrates deeper into canopies, reaching lower leaves that would be in shadow under direct sun.
For many crops, this turns out to be surprisingly beneficial. The canopy photosynthesis of six major arable crops is enhanced under diffuse light, because more of the total leaf area contributes to carbon uptake rather than just the sun-facing top layer.12Agricultural and Forest Meteorology. Diffuse solar radiation and canopy photosynthesis in a changing environment Overcast conditions also tend to lower temperatures throughout the canopy and increase humidity, reducing water loss through leaf pores and keeping the leaves closer to their temperature optimum for photosynthesis. On extremely hot sunny days, leaves at the top of the canopy can overheat and actually shut down their carbon-fixing machinery, a problem that disappears under moderate cloud cover.
The picture isn’t universally positive, though. In tropical rainforests during the rainy season, persistent heavy cloud cover reduces the total amount of light so dramatically that it becomes the primary factor limiting carbon uptake and tree growth. Research on rainforest trees found that light, rather than water, temperature, or leaf nitrogen, was the main bottleneck during cloudy rainy seasons. When researchers artificially supplemented light, photosynthesis, vegetative growth, and reproduction all increased significantly.13PubMed Central. Cloud cover limits net CO2 uptake and growth of a rainforest tree during tropical rainy seasons So for agriculture in temperate climates, moderate cloud cover can boost yields, but in already light-limited tropical forests, more clouds mean less growth. The temperature moderation that clouds provide is only helpful when light isn’t the limiting resource.