Clouds form from water that has evaporated from lakes, oceans, and rivers, or from moist soil and plants. This evaporated water, called water vapor, expands and cools as it rises into the air. Air can hold only a certain amount of water vapor at any given temperature. Warm air can hold more water vapor than cool air can. When the temperature drops, some of the water vapor begins to condense (change to a liquid) into tiny water droplets.
For water vapor to condense, particles so small they can be seen only through a microscope must be present. These particles, called condensation nuclei, become the centers of the droplets. Many condensation nuclei are tiny salt particles or small particles present in smoke. Most droplets measure from 1/2,500 to 1/250 inch (0.01 to 0.1 millimeter) in diameter.
If the temperature is cold enough, and other conditions are right, water vapor does not condense and form a liquid droplet. Instead, the water vapor turns directly to ice through a process called sublimation. For sublimation to occur at temperatures above -40 °F. (-40 °C), small particles similar to condensation nuclei, with a shape somewhat like an ice crystal, must be present. These particles are called freezing nuclei.