1. Why Rising Air Is Where Clouds Begin
Air always carries some invisible water vapor. Much of it gets there by evaporation from oceans, lakes, and rivers [2]. Nothing looks like a cloud until that air moves upward.
When air rises, the air pressure around it drops, so it spreads out and its temperature falls. The Met Office describes it simply: it is like climbing a mountain, where the air tends to get colder as you go up [1]. Cold air cannot hold as much water vapor as warm air. So as the air cools it gets closer to saturation, and the vapor can condense into tiny liquid droplets, much as water forms on a cold window [1].
Air can be pushed upward in several ways. Sunshine can warm the ground, and the warm, lighter air above it rises. Wind can be forced up the side of a hill or mountain. Air can also be lifted where two large masses of air meet at a weather front, or near areas of low pressure [2][1].
The amount of water does not grow. Invisible vapor turns into visible droplets. But cooling alone is not the whole story, because something else has to be there to catch the water.
2. Why Water Vapor Needs Tiny Specks of Dust or Salt to Form Droplets
You might expect water droplets to appear as soon as the air is cold enough. They do not. It is much easier for water vapor to condense when it has a particle to condense on. Dust and pollen are examples, and such particles are called condensation nuclei [2]. The Met Office says the new droplets cling to little bits of dust [1].
There is a reason for this, explained in a Japanese encyclopedia entry on condensation nuclei. In real air, the amount of moisture above the point of saturation (called supersaturation) is only about 1 percent at most. A droplet that starts from nothing would need much more than that. So in natural clouds and fog, the particles that work as seeds are mostly those with a radius of at least about 0.1 micrometer (one micrometer is a thousandth of a millimeter) that also soak up water easily. Sea salt and ammonium sulfate are named as important examples [5].
So the first answer to the title is a three-part recipe: air is lifted, it cools toward saturation, and the vapor condenses onto tiny particles. Together these make a cloud of droplets. (High, cold clouds can be made of ice crystals instead, which follow a somewhat different path [1].) One way to picture it is that a cloud is air and dust working as a team.
3. How Tiny Cloud Droplets Grow Into Raindrops
Cloud droplets are very small. The Met Office says they are only about a hundredth of a millimeter across, and a cloud is a huge collection of them [1]. Rising air keeps them aloft.
Inside the cloud, some droplets collide with others and merge, while others grow as more water vapor condenses onto them [3]. Rising air can also draw in moist air from near the ground, which creates new droplets and makes the cloud larger [4].
When droplets grow so large that the updraft can no longer hold them up, they fall as rain [3]. They keep colliding with droplets on the way down, so raindrops tend to be larger near the bottom of the cloud [4]. (Cold rain that starts as ice is a longer story that this page leaves out.)
That is the growing part. Now for the ending, which is less obvious.
4. How Rain Can Make Its Own Cloud Fade Away
A falling raindrop drags the surrounding air downward, which creates a downdraft. The Japan Meteorological Agency explains that this pulls dry air from higher up into the cloud. The small droplets there evaporate, new ones stop forming, and when the small droplets are gone, the cloud disappears [4].
In other words, rising moist air builds the cloud, and the rain it produces creates a downdraft that takes away the supply of rising air. For the explanation, the agency pictures a puffy cumulus cloud that lives about an hour. It stresses that this is an assumption used to tell the story, not the lifetime of every cloud, and that real clouds vary a lot [4].
5. Try It: Watch One Small Cloud Appear and Fade
On a clear day with a few puffy clouds, pick one small cloud and watch it from a window or a safe spot. Never look at or near the sun. Choose a cloud well away from it.
Draw its outline every ten minutes. Does it swell and get a sharp edge, as if it is still being fed by rising air? Or do the edges fray and thin out, as if it is fading? You may not be able to tell for sure, and that is fine. If you lose track of the cloud, you can stop.
Compare what you see with the hour-long story above. How long did your cloud last? Weather agencies such as the Met Office and the Japan Meteorological Agency have pages for young readers, and they are a good place to look next.