How are clouds formed? The short answer
Clouds form when air carrying invisible water vapor rises and cools. Higher up there is less air pressing down, so a rising bubble of air expands, and expanding air gets cooler.12 Cool air can't hold as much vapor as warm air. When the bubble cools to its dew point, the extra vapor condenses onto tiny floating specks of dust, sea salt, smoke or pollen, making droplets or ice crystals so small and light that they stay up in the air.12 Billions of them together make a cloud.
water vapor + rising air + cooling + specks = cloud
How are clouds formed, step by step?
- Water evaporates. The Sun warms oceans, lakes and rivers, and some water turns into water vapor, the gas form of water, which mixes into the air.2
- Air rises. Something pushes the air upward: warm ground, a mountain, a weather front, or winds meeting in an area of low pressure.2
- The air expands and cools. Dry rising air cools by about 9.8 °C for every kilometer it climbs (5.5 °F per 1,000 feet). Meteorologists call this the dry lapse rate.1
- It reaches its dew point. At some height the air is saturated: the relative humidity reaches 100%.1
- Vapor condenses on specks. Water vapor clings to tiny particles called cloud condensation nuclei and becomes droplets, or ice crystals if it's cold enough.1
- A cloud appears. The droplets are only about 0.01 to 0.02 mm across, and there are a few hundred of them in every cubic centimeter of cloud.68
Clouds are never finished. Air keeps mixing in at the edges, so droplets are always forming in some places and evaporating in others. When more water condenses than evaporates, the cloud grows. When evaporation wins, it shrinks. That's why clouds change shape and can seem to appear out of nowhere.1
What are clouds made of?
Clouds are made of liquid water droplets, ice crystals, or a mix of both.26 Not water vapor: vapor is a gas and you can't see it.12 You can check this at home with a boiling kettle. Right at the spout there's a small clear gap. That's pure vapor. A little farther out, the vapor cools into tiny droplets, and that's the white puff you see. A cloud works the same way.
Every cloud droplet also has something solid at its heart. On its own, water vapor has a hard time turning into droplets. It needs a speck with a radius of at least one micrometer (a thousandth of a millimeter) to condense on.1 The air is full of such specks: smoke from fires and volcanoes, sea salt from ocean spray, wind-blown soil and dust, and pollen.12 A droplet ends up about a hundred times wider than its speck, but it's still almost pure water.1 Without these "dirty air" particles, the National Weather Service notes, there would likely be hardly any clouds at all.8
Why are clouds flat on the bottom?
Look at a field of fair-weather cumulus clouds and you'll notice their bottoms are flat and all at roughly the same height. That flat base is the height where rising air cools to its dew point and the vapor starts to condense.58 Below it, the air is clear. Above it, it's cloud. Neighboring bubbles of air start out with about the same temperature and humidity, so they all reach that point at about the same height.
Pilots and forecasters use a rule of thumb to estimate it: take the difference between the air temperature and the dew point at the ground, in °C, and multiply by about 125 meters (or about 400 feet per °C).15 On a day with 25 °C air and a 15 °C dew point, that gives a cloud base around 1,250 meters. This is only an estimate, and it's the rule used in the "Up, up, and cooler" toy above. Once the cloud forms, condensation releases heat, so the rising air cools more slowly than 9.8 °C per km; the toy uses a simplified 6 °C per km inside the cloud.8 More humid air (a higher dew point) means a lower cloud base.
What makes air rise? Three main ways
Sources: UCAR Center for Science Education2 and the National Weather Service.8
How much does a cloud weigh?
A lot more than you'd think. The U.S. Geological Survey walks through a classic estimate, based on work by cloud scientists at the National Center for Atmospheric Research (NCAR):3
- A typical fair-weather cumulus holds about 0.5 grams of liquid water in each cubic meter of cloud.
- Picture a cloud about 1 kilometer wide, 1 kilometer long and 1 kilometer tall. That's 1 billion cubic meters.
- 1,000,000,000 × 0.5 g = 500,000,000 g = 500,000 kg, about 500 metric tons, or roughly 1.1 million pounds.
1,000,000,000 m³ × 0.5 g/m³ ≈ 500 metric tons
That's about as heavy as 300 mid-sized cars.4 Remember it's an estimate: real clouds come in every size, and the amount of water in each cubic meter varies. Big storm clouds are far heavier. NOAA says clouds can weigh tens of millions of tons.16 Try your own numbers in the weigh-a-cloud toy at the end of the explainer (it uses the same simple multiplication, so treat its answers as illustrations, not measurements).
How do clouds float if they're so heavy?
Here's the surprise: clouds don't really float. They fall, just incredibly slowly, and rising air keeps pushing them back up.
- The droplets are tiny. A cloud droplet falls only about 1 centimeter (0.4 inch) per second.7
- Rising air holds them up. The same upward air currents that built the cloud keep it aloft.3 Updrafts range from a few centimeters per second to several meters per second, easily faster than the droplets fall.7
- The weight is spread out. Those 500 tons are spread through a whole cubic kilometer of air. Each cubic meter holds only about half a gram of water, and the drier air below the cloud is denser than the cloud itself.3
- Condensation releases heat. When vapor condenses into droplets, it gives off heat. That warms the air inside the cloud, making it a bit lighter than the air around it and helping it keep rising.48
When droplets bump together and grow big enough, they fall faster than the air can lift them. A raindrop 2 millimeters wide holds as much water as about a million cloud droplets.8 Follow that story in why does it rain?
What are the different types of clouds?
In 1803, Luke Howard sorted clouds into three basic shapes plus a rain cloud, and we still use his Latin names today.5 Scientists also group clouds by the height of their base.517
Heights from NOAA and UCAR; clouds sit lower near the poles and higher in the tropics.51113 A cumulus with tall "cauliflower" tops is a sign that showers may be on the way.11 And fog? Fog is simply a cloud that forms at ground level.6 See what causes fog. Storm clouds that grow tall enough can also make hail and lightning.
Why are clouds white, and why do some turn gray?
Sunlight is a mix of all colors. The tiny molecules of the air scatter blue light more than other colors, which makes the sky blue. Cloud droplets are much bigger, and they scatter all colors about equally, so the light stays white.9
Much of that scattered light bounces back up and out of the sides of the cloud, so less of it reaches the bottom. That's why cloud bases look gray. In a thick rain cloud, even less light gets through, so the base looks dark and stormy. Seen from above, from an airplane, the tops of clouds are always brilliant white.9
Make a cloud in a jar (with an adult)
NASA's "Cloud Detectives" activity uses a jar, hot water and ice to show all three cloud ingredients at once.10 Here's a no-flame version:
- Ask an adult to help with the hot water. Pour about 3 cm (an inch) of hot tap water into a clear glass jar. Swirl it so the sides get warm. This adds water vapor.
- Turn the jar's metal lid upside down, put a few ice cubes on it, and set it on top of the jar. The ice cools the air at the top, like air rising into the cold sky.
- Lift the lid for a moment and give one very short spray of air freshener or hairspray into the jar, then quickly put the lid back. The spray adds tiny particles: your condensation nuclei.
- Watch a swirling gray-white cloud form. Take the lid off and the cloud drifts out.
NASA's version uses a lit match for the specks. Only do that with an adult; the spray works without a flame. Try it once with the spray and once without, and compare the clouds you get.
Common misconceptions
"Clouds are made of water vapor (or steam)." Water vapor is an invisible gas. A cloud is liquid droplets or ice crystals, which is why you can see it.
1210
"Clouds are weightless." A typical cumulus holds an estimated 500 metric tons of water.
3
"Clouds float because they're lighter than air." Each droplet is heavier than air and slowly falls, about 1 cm per second. Rising air keeps pushing it back up.
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"Cold air squeezes water out like a sponge." Air isn't a sponge. Cooling brings the vapor to its dew point, and it still needs specks to condense on.
1
"Every cloud brings rain." Most clouds never produce rain or snow that reaches the ground. A raindrop needs the water of about a million cloud droplets first.
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Clouds and the big picture
At any moment, about two-thirds of Earth's surface is covered by clouds.14 Every one of them follows the same recipe: water vapor, rising air, cooling, and specks of dust. Clouds are a key step in the water cycle, carrying water evaporated from the sea over the land, where it can fall as rain or snow.2
FAQ
How are clouds formed, step by step?
1) Water evaporates from oceans, lakes and rivers into the air as invisible water vapor. 2) Something makes the air rise: warm ground, a mountain or a weather front. 3) The rising air expands and cools. 4) It cools to its dew point, where it can't hold all its vapor. 5) The extra vapor condenses onto tiny specks of dust, salt or smoke, forming droplets or ice crystals. 6) Billions of those droplets make a cloud.
How do clouds float if they're so heavy?
Cloud droplets are so tiny that they fall only about 1 centimeter per second, and the rising air that made the cloud keeps pushing them back up. The weight is also spread through a huge volume: about half a gram of water in each cubic meter. When the droplets grow big enough to fall faster than the air rises, they fall as rain.
Is a cloud a liquid or a gas?
A cloud is made of liquid water droplets, ice crystals, or both, floating in air. It is not a gas: water vapor is invisible. You can see a cloud precisely because the vapor has condensed into droplets.
Why do clouds suddenly appear?
A cloud appears the moment rising air cools to its dew point and vapor starts condensing into droplets. On a sunny day, warm bubbles of air rise one after another, so small cumulus clouds can pop up quickly. Clouds also evaporate at their edges all the time, so they can vanish just as quickly.
How much does a cloud weigh?
A classic estimate from the USGS, based on NCAR cloud research: a cumulus cloud holds about 0.5 grams of water per cubic meter, and a cloud 1 kilometer on each side has 1 billion cubic meters. That's about 500,000 kilograms (500 metric tons, or 1.1 million pounds) of water. Big storm clouds weigh far more.
What are clouds made of?
Clouds are made of tiny water droplets (about 0.01 to 0.02 mm across) or ice crystals. Each droplet formed around a microscopic speck of dust, sea salt, smoke or pollen, called a cloud condensation nucleus. High, cold clouds like cirrus are made entirely of ice.
Why are clouds white?
Cloud droplets are much bigger than air molecules, so they scatter all colors of sunlight about equally, and white light stays white. The bottoms of thick clouds look gray because less light makes it through to the base.
Sources
- NOAA JetStream, How Clouds Form
- UCAR Center for Science Education, Clouds and How They Form (and its pages on surface heating, mountains and weather fronts)
- USGS Water Science School, How Much Does a Cloud Weigh?
- Encyclopaedia Britannica (MinuteEarth), How Do Clouds Float?
- NOAA JetStream, The Four Core Types of Clouds
- Encyclopaedia Britannica, Cloud
- Encyclopaedia Britannica, Mechanisms of precipitation release
- National Weather Service, Cloud Development and Precipitation
- Met Office, Why Are Clouds White?
- NASA eClips, Cloud Detectives (Cloud in a Jar)
- UCAR Center for Science Education, Cumulus clouds
- National Geographic Education, Fog
- UCAR Center for Science Education, Cloud Types
- NASA Earth Observatory, A Celebration of Clouds
- Mukherjee & Wu (NASA Goddard), Estimating Cloud Base Height via Shadow-Based Remote Sensing, Remote Sensing (2026)
- NOAA JetStream, Introduction to Clouds
- NOAA SciJinks, Types of Clouds