How clouds form Clouds form from water that has evaporated from lakes, oceans, and rivers, or from moist soil and plants. The evaporated water, called water vapor, mixes with the air.
Air can hold only a certain amount of water vapor at a given temperature .
Warm air can hold more water vapor than cool air. The level of the dew point depends upon the amount of water vapor in the air the more vapor, the higher the dew point . For water vapor to condense in clouds, particles so small they can be seen only through a microscope must be present. These particles, called cloud condensation nuclei, become the centers of the droplets. Many of the nuclei form from chemical reactions involving sulfuric acid and ammonia.