As it moves through the water cycle, water often changes from a liquid, to a solid (ice), to a gas (water vapor). Water in oceans and lakes is typically liquid; but it is solid ice in glaciers, and often invisible water vapor in the atmosphere. Clouds are tiny droplets of liquid water or small ice crystals.
Water at the surface of the ocean, rivers, and lakes can become water vapor and move into the atmosphere with a little added energy from the Sun through a process called evaporation. Snow and ice can also become water vapor through a process called sublimation. And water vapor gets into the atmosphere from plants by a process called transpiration.
Because air is cooler at higher altitude in the troposphere, water vapor cools as it rises high in the atmosphere and transforms into water droplets by a process called condensation. The water droplets that form make up clouds. Water vapor can also condense into droplets near the ground, forming fog when the ground is cold. If the temperature is cold enough, ice crystals form instead of liquid water droplets.
If the droplets or ice crystals within clouds grow in size, they eventually become too heavy to stay in the air, falling to the ground as rain, snow, and other types of precipitation.