Why do clouds float in the sky?
Short answer
Clouds are collections of extremely small water droplets or ice crystals. Each particle falls very slowly, and rising air inside and around many clouds can help keep the particles suspended. That makes the whole cloud look as if it is floating in the sky.
In 30 seconds
Air containing water vapor rises, expands, and cools. As it cools, the vapor changes into tiny droplets or ice crystals around small particles in the air. These particles are so small that they do not quickly fall to the ground. When they grow larger, gravity wins more easily and they can fall as rain or snow.
What is a cloud?
A cloud is not simply a mass of invisible water vapor. Water vapor is a gas and cannot be seen directly. The visible cloud is made of tiny liquid droplets, ice crystals, or both. Low clouds often contain liquid droplets, while higher and colder clouds can contain ice crystals.
When moist air moves upward, the surrounding air pressure becomes lower and the air expands. Expanding air cools, making it easier for water vapor to condense into droplets or form ice crystals. This is one way clouds form.
Why do the tiny particles not fall right away?
Gravity pulls every droplet and crystal downward. But when a particle is extremely small, the resistance from the surrounding air has a large effect compared with its weight. It therefore falls very slowly and can remain in the air for a long time.
Rising air helps too
Moist air is often rising where clouds form. An upward flow of air makes it harder for tiny droplets and ice crystals to fall. So clouds appear to stay in the sky because of both the small size of their particles and the movement of the air.
A cloud is not a single solid object hanging in the air. Particles form, evaporate, and are carried by wind while the cloud changes shape.
When does a cloud become rain?
Droplets can collide and join together, gradually becoming larger. Once they become heavy enough, air resistance and rising air can no longer keep them suspended. They begin to fall from the cloud, and if they grow or remain liquid on the way down, they reach the ground as rain. Ice particles can fall as snow.
A common misunderstanding
It is not quite correct to say that clouds float simply because they are made of light water vapor. Water vapor is invisible. The cloud we see is visible because droplets and ice crystals scatter light.
A familiar example
Your breath can look white on a cold day for a similar reason: warm, moist air cools and forms tiny droplets. When a related process happens higher in the atmosphere, we see a cloud. When similar droplets form close to the ground, we call it fog.