How Do Clouds Form, and Why Do They Float?

The air around you right now contains water, even on a clear day. You cannot see it, because it is in the form of an invisible gas called water vapor. A cloud is what happens when some of that vapor turns back into tiny bits of liquid water or ice. NOAA's JetStream weather school explains the three ingredients that make it happen, and why the result can weigh millions of tons and still float.

1. What a cloud actually is

NOAA defines clouds as the visible aggregate of minute particles of water and/or ice that form when water vapor condenses in the atmosphere. In plain words: a cloud is a huge crowd of droplets or ice crystals so small and so spread out that they hang in the air together.

2. Ingredient 1: a speck to condense on

You might expect water vapor to simply clump into droplets when the air gets cold enough. NOAA explains that it does not work that way on its own. Water molecules are too small to bond together into cloud droplets without help. They need a surface to condense on, an object with a radius of at least one micrometer, which is one millionth of a meter.

Those objects are called cloud condensation nuclei. NOAA describes them as tiny solid and liquid particles found in abundance in the atmosphere, such as:

• smoke from fires or volcanoes
• ocean spray
• tiny specks of wind-blown soil

They are hygroscopic, meaning they attract water molecules. NOAA notes that each nucleus is about 1/100th the size of the cloud droplet that forms around it. So every cloud droplet has a speck of dust, dirt, or salt at its core, even though the droplet is still made almost entirely of pure water.

3. Ingredient 2: air cooled to its saturation point

Nuclei alone are not enough. NOAA explains that the air temperature must also drop to the dew point, the point of saturation where evaporation equals condensation. At that point the air cannot hold more water as vapor, and vapor starts condensing onto the nuclei.

4. Ingredient 3: rising air

The most common way air reaches saturation is by rising. NOAA describes the process step by step:

1. A block of air, called a parcel, rises from near the ground.
2. Higher up, air pressure is lower, so the parcel expands.
3. Expanding makes the air cool, even though no heat has been taken away. Meteorologists call this cooling by expansion an adiabatic process.

NOAA gives the rate: for each 1,000 feet a dry parcel rises, its temperature drops by about 5.5 degrees Fahrenheit (9.8 degrees Celsius per kilometer). This is called the dry lapse rate. Once the parcel cools to its saturation temperature, at 100 percent relative humidity, water vapor condenses onto the nuclei and cloud droplets form.

5. A worked example

NOAA gives an example. Imagine air near the ground at 30 C (85 F) with a dew point of 18 C (65 F). In an ideal atmosphere, that parcel cools to its saturation point at about 4,000 feet.

You can roughly check this with the dry lapse rate. The air needs to cool by 30 minus 18, or 12 degrees Celsius. At 9.8 degrees per kilometer, that takes about 1.2 kilometers of rising, which is close to 4,000 feet. (The real calculation is slightly more involved, because the dew point itself shifts a little as air rises, but the estimate lands in the right place.)

That height is roughly where you would expect the flat bottoms of puffy clouds on such a day. The flat base marks the level where rising air first reaches saturation.

6. Why clouds change shape and disappear

NOAA explains that the atmosphere is always moving. As air rises, drier air mixes into the parcel, and both condensation and evaporation keep happening at once. When more water condenses than evaporates, a cloud grows. When evaporation wins, the cloud shrinks and vanishes. That constant tug-of-war is why clouds shift shape as you watch them.

7. How can something that heavy float?

NOAA notes that clouds can weigh tens of millions of tons yet float in the atmosphere. The trick is that the weight is spread across an enormous volume of air, divided among countless tiny droplets. Each droplet is far too small and light to fall quickly on its own.

Clouds are not stuck in place, either. NOAA says they can be carried along by winds of up to 150 mph (240 km/h), or stay put while the wind passes through them.

8. Try it at home

On a cold day, breathe out and watch a small cloud appear. Your breath is warm and full of water vapor. When it meets cold air, it cools to its saturation point and condenses into tiny droplets, the same basic process as a cloud in the sky.

9. Key takeaways

• A cloud is a huge collection of tiny water droplets or ice crystals formed when water vapor condenses.
• Water vapor needs tiny particles called cloud condensation nuclei, such as dust, smoke, or sea salt, to condense on.
• Air must cool to its dew point, most often by rising, expanding, and cooling at about 5.5 F per 1,000 feet.
• The flat base of a puffy cloud marks the height where rising air first reaches saturation.
• Clouds can weigh millions of tons but float because their weight is spread across tiny, slow-falling droplets in a vast volume of air.