A cloud is a visible mass of tiny water droplets, ice crystals, or a mix of both, floating in the atmosphere. Those droplets are extraordinarily small: a typical cloud droplet measures about 20 microns in diameter, roughly 100 times smaller than a raindrop. Despite looking solid and heavy from the ground, clouds are mostly empty air with these microscopic particles suspended throughout.
How Clouds Form
Cloud formation starts with something you can’t see: water vapor, the invisible gaseous form of water that’s always present in the air around you. When air rises, it cools. As it cools enough, that water vapor needs to condense back into liquid, but it can’t do so on its own. It needs a surface to cling to.
This is where tiny airborne particles called aerosols come in. Specks of dust, sea salt, soot, pollen, and even sulfate particles from volcanic eruptions all serve as condensation nuclei. Water molecules are drawn to these particles like magnets, collecting on their surfaces to form droplets. Billions of these droplets packed together become visible as a cloud. The altitude where this condensation begins is called the lifting condensation level, and it corresponds to the flat bottom you often see on cumulus clouds.
The type and number of aerosol particles in the air actually changes how a cloud looks. In clean environments, like over the open ocean, naturally occurring salt particles act as nuclei, producing fewer but larger droplets. In polluted air, the large influx of soot particles creates more centers of attraction, so the water spreads across many more droplets that are individually smaller.
Water Droplets, Ice Crystals, and Supercooled Water
Not all clouds are made of the same material. Low, warm clouds consist mainly of liquid water droplets. High, cold clouds are composed of ice crystals, typically about a tenth of a millimeter long. And many clouds contain both at the same time.
One of the stranger facts about clouds is that water droplets can remain liquid at temperatures far below freezing. Because the droplets are so small, they can stay in liquid form at temperatures as cold as -30°C (-22°F). These are called supercooled droplets, and they’re common in the middle layers of the atmosphere. When supercooled droplets and ice crystals exist near each other, water molecules migrate from the droplet to the crystal, causing the ice to grow at the droplet’s expense. This process is one of the key ways precipitation begins inside a cloud.
How Clouds Produce Rain
A cloud droplet is about 20 microns wide. A raindrop is about 2,000 microns (2 millimeters). That means a raindrop is roughly 100 times the diameter of a cloud droplet, and it would take about a million cloud droplets to make a single raindrop. Getting from one to the other requires a process called collision and coalescence.
Because condensation nuclei come in different sizes, the droplets that form on them vary in size too. Larger droplets fall faster than smaller ones. As they fall through the cloud, they collide with smaller droplets and often stick together, growing larger with each collision. This cycle repeats until the drops are heavy enough to fall out of the cloud as rain. Raindrops max out at about 10 millimeters in diameter. Beyond that, they break apart into smaller drops from air resistance.
The 10 Main Cloud Types
The World Meteorological Organization recognizes ten fundamental cloud types, called genera. They’re grouped by altitude into three layers.
High clouds form above about 6,000 meters (20,000 feet) and are made almost entirely of ice crystals:
- Cirrus: thin, wispy streaks that often look like white brushstrokes across the sky
- Cirrocumulus: small, white patches arranged in rows or ripples
- Cirrostratus: a thin, pale sheet that covers the sky and often produces a halo around the sun or moon
Middle clouds sit between roughly 2,000 and 7,000 meters (6,500 to 23,000 feet):
- Altocumulus: white or gray patches, often in rounded clumps or waves
- Altostratus: a gray or bluish sheet covering the sky, thin enough that the sun appears as a dim, hazy disc
- Nimbostratus: a thick, dark, featureless layer that produces steady rain or snow
Low clouds form from the surface up to about 2,000 meters (6,500 feet):
- Stratus: a flat, gray blanket that may produce drizzle
- Stratocumulus: low, lumpy gray or white masses, the most common cloud type worldwide
- Cumulus: the classic puffy, white “fair weather” clouds with flat bases and rounded tops
- Cumulonimbus: towering storm clouds that can stretch from near the surface to the top of the troposphere, producing heavy rain, lightning, and hail
Why Clouds Matter for Earth’s Temperature
Clouds play an outsized role in regulating the planet’s energy balance. On average, clouds reflect about 50% of the solar energy that hits them back into space. Across the entire planet, clouds bounce roughly 23% of all incoming sunlight away before it can warm the surface. This reflective power is one of the most significant cooling forces in the climate system.
But clouds also work in the opposite direction. They absorb heat radiating up from Earth’s surface and re-emit some of it back downward, acting like a blanket. Whether a particular cloud warms or cools depends largely on its altitude and thickness. Low, thick clouds tend to cool by reflecting a lot of sunlight. High, thin cirrus clouds tend to warm because they let sunlight through but trap outgoing heat. The net effect across all cloud types is one of the most complex and consequential variables in climate science.
Clouds Beyond the Ordinary
Most clouds form in the troposphere, the lowest layer of the atmosphere where weather happens. But some exist far higher. Noctilucent clouds form in the mesosphere at altitudes just over 80 kilometers (50 miles), where the air is almost unimaginably thin and dry. Instead of condensing on dust or salt, water vapor at that altitude freezes around tiny specks of meteor dust. These clouds are visible only around twilight, when the sun has dipped below the horizon but still illuminates the upper atmosphere, giving them an eerie, electric blue glow. They’re most commonly seen at high latitudes during summer months.
Clouds also exist on other planets. Jupiter’s bands are ammonia ice clouds. Venus is permanently shrouded in thick clouds of sulfuric acid droplets. Saturn’s moon Titan has clouds of liquid methane. On Earth, though, our clouds are made of the same water we drink, just lifted high enough and cooled enough to become visible.