Clouds are formed when moist, warm air rises and expands in the atmosphere. The rising water vapor condenses and forms small water droplets which make up the clouds. When the water vapor cools, the low temperature of air lowers its capacity to hold water vapor.
Cloud formation involves a series of processes that cause water droplets or ice crystals in the atmosphere to form into clouds. Cooling of air is a part of these processes. Warm air is capable of holding more water vapor than cool air, which means that extra water vapor starts to condense into liquid water droplets when air starts to cool.
Water vapor typically needs a condensation nuclei, such as dust and pollen. Eventually, water vapor condenses upon these condensation nuclei to form a cloud. The droplets that comprise a cloud are extremely tiny, and they are light enough to float in the air. When the cloud becomes too heavy, water droplets fall down to the ground in the form of precipitation such as rain, snow, sleet and hail.
Clouds have various shapes and sizes, from thin wispy clouds to large and dark clouds. The factors that generally influence the size of clouds include heat, bodies of water, mountain ranges, seasons and volcanic eruptions. Clouds rarely form in the desert, because there is not enough water to evaporate and form clouds. On the other hand, clouds frequently form in coastal regions, because there is adequate moisture from surrounding waters.