Floating Shapes in the Sky
Clouds drift across the sky every day, in countless shapes and sizes, from thin wispy streaks to towering white mountains. They bring rain, shade, and dramatic skies. But what exactly are these floating shapes, and where do they come from? A cloud is not solid and it is not smoke. It is made of an enormous number of tiny water droplets or ice crystals, and it forms through a process that begins invisibly in the air around us.
Invisible Water in the Air
The air all around us contains water in an invisible gas form called water vapor. This water vapor gets into the air mainly through evaporation, when the Sun's heat turns liquid water from oceans, lakes, rivers, and soil into gas. Warm air is able to hold a good deal of this invisible water vapor. As long as the water stays as vapor, we cannot see it, and the sky stays clear. The first step toward a cloud is getting this moist air to rise.
Rising and Cooling Air
Air can be lifted upward in several ways: the Sun heats the ground, which warms the air above it and makes it rise; wind pushes air up the side of a mountain; or a mass of cold air slides under warm air and forces it upward. Whatever the cause, as the moist air rises higher, it cools down, because the higher atmosphere is colder. This cooling is the key, because cool air cannot hold as much water vapor as warm air can.
Condensation Into a Cloud
When the rising air cools enough, it can no longer keep all its water as vapor. The water vapor then condenses, meaning it changes from gas back into tiny liquid droplets. This condensation happens around microscopic particles floating in the air, such as bits of dust, salt, or smoke, which give the water something to gather onto. When a vast number of these tiny droplets or ice crystals collect together, they become visible as a cloud. If the droplets keep growing larger and heavier, they may eventually fall as rain.
Source
This article was written using information from Wikipedia.