Cumulonimbus clouds also produce heavy rain and lightning. Sometimes hail or, on rare occasions, a deadly tornado comes from a cumulonimbus cloud. 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.
How clouds form 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. These nuclei include particles of ammonium sulfate and magnesium sulfate. Other condensation nuclei include tiny particles of dust and particles from smoke.