Warm air can hold much more water vapor than cold air. Consequently, if warm air containing a lot of water vapor rises up into the atmosphere, where it is cooled, it is forced to release all of the water vapor that it cannot hold in the form of gas. This is called condensation. As the water vapor in the atmosphere changes its state, water droplets and ice particles are formed. Then they become visible as little droplets of water, which we call cloud.
Clouds are continuously in the process of evolving in the atmosphere.
Sometimes we see white clouds high in the sky, sometimes low gray or dark clouds cover large parts of the heavens. Our weather depends greatly on the types of clouds which form in the sky. If more and more air rises up and is cooled, then more and more drops of water appear in the air, and eventually the drops join together and become too heavy to remain in the atmosphere. They fall down as rain. In fact, cloud droplets are extremely small, about 0.01 mm in diameter.