When air is cooled below its dew point, the water vapor it contains tends to condense as droplets of water or tiny ice crystals. Condensation may not occur, however, in the absence of tiny particles on which the water or ice can form. These particles are known as condensation nuclei. The most common types of condensation nuclei are crystals of salt, particulate matter formed by the combustion of fossil fuels, and dust blown up from the earth's surface. In the process of cloud-seeding, scientists add tiny crystals of dry ice or silver iodide as condensation nuclei to the atmosphere to promote cloud formation and precipitation.