Clouds are the most visible part of the weather, constantly changing shape, color and height. They can be thin wisps high in the sky, fluffy white heaps on a summer afternoon or towering dark giants that bring thunder and hail. Yet a cloud is simply a collection of tiny water droplets or ice crystals floating in the air. How do they form, why do they look so different, and why do some produce rain while others drift by harmlessly? This explainer looks at the physics of clouds and precipitation, and what clouds can tell you about the weather to come.
Water vapor in the air
The air always contains some water vapor, an invisible gas. How much water vapor air can hold depends on its temperature: warm air can hold much more than cold air. When air contains as much water vapor as it can at a given temperature, it is saturated, and its relative humidity is 100 percent.
The temperature at which air becomes saturated is called the dew point. If air cools below its dew point, some of the water vapor condenses into liquid droplets. You see this when dew forms on grass on a cool morning, or when water droplets form on the outside of a cold drink.
Why rising air makes clouds
Most clouds form when air rises. As air rises, the pressure around it decreases, so the air expands. Expanding air cools, even without losing heat to its surroundings, a process called adiabatic cooling. Dry air cools by about 5.4 °F for every 1,000 feet it rises (9.8 °C per kilometer). When the rising air cools to its dew point, water vapor begins to condense and a cloud forms. The height at which this happens is visible as the flat bottom of many clouds, called the cloud base.
Air rises for several reasons:
- Convection: The Sun heats the ground unevenly, warming pockets of air that rise like bubbles. This creates the puffy cumulus clouds of summer days.
- Fronts: When a mass of warm air meets a mass of cold air, the lighter warm air is forced to rise over the denser cold air.
- Mountains: When wind blows air against a mountain range, the air is pushed upward. This is called orographic lifting. It often brings heavy rain to the windward side of mountains and dry conditions, called a rain shadow, to the other side.
- Convergence: When air flows together from different directions, it has nowhere to go but up.
The role of tiny particles
For water vapor to condense, it needs something to condense on. The air contains countless tiny particles of dust, sea salt, smoke, pollen and pollution, called condensation nuclei. Water vapor condenses onto these particles, forming cloud droplets. Without them, air would have to be extremely supersaturated before clouds could form.
Naming the clouds
In 1803, an English amateur meteorologist named Luke Howard proposed a system for naming clouds using Latin words. It is still the basis of cloud classification used by the World Meteorological Organization today.
The main Latin roots are:
- Cirrus, meaning "curl of hair": wispy clouds.
- Cumulus, meaning "heap": puffy clouds.
- Stratus, meaning "layer": flat, sheet-like clouds.
- Nimbus, meaning "rain": clouds that produce precipitation.
- Alto, meaning "high," used for mid-level clouds.
Combining these gives the ten main cloud types, grouped by height:
| Level | Cloud types | Description |
|---|---|---|
| High (above about 20,000 ft / 6 km) | cirrus, cirrostratus, cirrocumulus | thin, made of ice crystals; cirrostratus can create halos around the Sun or Moon |
| Middle (about 6,500 to 20,000 ft / 2 to 6 km) | altostratus, altocumulus | gray sheets or patches; altocumulus forms rows of puffy cloudlets |
| Low (below about 6,500 ft / 2 km) | stratus, stratocumulus, nimbostratus | gray layers; nimbostratus brings steady rain or snow |
| Vertical | cumulus, cumulonimbus | grow upward; cumulonimbus are thunderstorm clouds |
Why clouds float
Clouds can contain enormous amounts of water. A typical fair-weather cumulus cloud may contain hundreds of tons of water. So why does it not fall? Because the water is spread out in billions of extremely tiny droplets, each about 0.02 millimeters across, much smaller than a raindrop. These droplets fall very slowly, and the rising air that created the cloud easily keeps them aloft.
Clouds look white because their droplets scatter all colors of sunlight equally. Thick clouds look gray or dark at the bottom because less light passes through them.
From cloud droplets to raindrops
A typical raindrop is about a million times larger in volume than a cloud droplet. Turning cloud droplets into rain requires one of two processes.
Collision and coalescence
In warm clouds, especially in the tropics, droplets of different sizes fall at different speeds. Larger droplets fall faster, colliding with smaller droplets and merging with them. As they grow, they fall faster and collect even more droplets, until they are heavy enough to fall as rain.
The ice crystal process
In clouds where temperatures are below freezing, which includes most clouds outside the tropics at least in their upper parts, a different process dominates. Surprisingly, cloud droplets can remain liquid well below 32 °F (0 °C), a state called supercooled. When ice crystals form among these supercooled droplets, water vapor moves from the droplets to the ice crystals, which grow rapidly at the expense of the droplets. This is known as the Bergeron process. The growing ice crystals become snowflakes. If they fall through warmer air, they melt and reach the ground as rain.
This means that much of the rain falling in temperate regions actually began as snow high in the clouds.
Types of precipitation
- Rain: Liquid drops falling from clouds. Drizzle consists of very small drops, usually from low stratus clouds.
- Snow: Ice crystals that remain frozen all the way to the ground. Each snowflake's shape depends on the temperature and humidity in which it grew.
- Sleet (ice pellets): Snow that partly melts in a warm layer of air and refreezes into ice pellets before reaching the ground.
- Freezing rain: Rain that falls through a shallow layer of freezing air at the surface and freezes on contact with cold surfaces, coating trees and roads with ice.
- Hail: Lumps of ice formed in strong thunderstorms.
How hail forms
Hailstones form inside powerful cumulonimbus clouds with strong updrafts. An ice particle is carried upward by the updraft, collecting supercooled water droplets that freeze onto it. It may fall, then be carried up again, repeating the cycle several times and adding layers of ice, like the rings of an onion. When the hailstone becomes too heavy for the updraft to support, it falls to the ground. The stronger the updraft, the larger the hailstones can become. The largest recorded hailstones have been about 8 inches (20 centimeters) across.
Thunderstorms and lightning
Thunderstorms form when warm, moist air rises rapidly into cooler air, creating towering cumulonimbus clouds that can reach more than 40,000 feet (12 kilometers) high. At the top, the cloud often spreads out into an anvil shape.
Inside the cloud, collisions between ice crystals, graupel (soft hail) and water droplets separate electric charges. Typically, the upper part of the cloud becomes positively charged and the lower part negatively charged. When the charge difference becomes large enough, a giant spark jumps between parts of the cloud or between the cloud and the ground. This is lightning. A lightning bolt heats the air around it to about 50,000 °F (about 27,000 °C), hotter than the surface of the Sun. The air expands explosively, creating the sound we hear as thunder.
Because light travels much faster than sound, you see lightning before you hear thunder. Counting the seconds between them and dividing by five gives the approximate distance in miles; dividing by three gives kilometers. If you can hear thunder, you are close enough to be struck. Go indoors.
Reading the sky
Clouds can give clues about upcoming weather:
- Cirrus clouds increasing and thickening into cirrostratus often signal an approaching warm front and rain within a day or so.
- Cumulus clouds that stay small and flat on top usually mean fair weather. If they grow tall and dark in the afternoon, thunderstorms may develop.
- Altocumulus on a warm, humid morning can be a sign of thunderstorms later in the day.
- Lowering, thickening gray clouds often bring steady rain or snow.
These are rules of thumb, not guarantees, but they can make watching the sky more meaningful.
Questions readers ask
What are clouds made of? Tiny water droplets, ice crystals or both, so small and light that rising air keeps them floating.
Why are rain clouds dark? They are thick and contain many droplets, which absorb and scatter more light, so less sunlight passes through.
Is all rain the same size? No. Raindrops range from small drizzle droplets to drops about 0.2 inches (5 millimeters) across. Larger drops break apart as they fall.
Why do mountains get more rain on one side? Air rising over mountains cools and forms clouds and rain on the windward side. Descending air on the other side warms and dries, creating a rain shadow.
How far away is the lightning? Count the seconds between the flash and the thunder. Divide by five for miles or by three for kilometers.
In short
Clouds form when air rises, expands and cools to its dew point, and water vapor condenses on tiny particles into droplets or ice crystals. Their names describe their shapes and heights, from wispy cirrus to towering cumulonimbus. Rain forms when droplets merge or when ice crystals grow at the expense of supercooled droplets and fall, sometimes melting along the way. Strong updrafts in thunderstorms create hail and separate electric charges that produce lightning and thunder. Next time you look up, you will be watching physics in motion and reading clues about tomorrow's weather.