Water is the only substance that can exist as a solid (ice), a liquid (water), and gas (vapor) at the temperatures we find here on Earth. The amount of water vapor that can be present in the atmosphere greatly decreases when temperatures drop. Clouds develop when humid air is cooled so much that the excess vapor is converted to liquid water (or ice, if cold enough). Water vapor condenses onto tiny particles called aerosols to form liquid droplets or ice crystals. Grains of dust, pollution particles, sea salt, and even tiny organisms are examples of the kinds of aerosols that get blown into the sky and give water vapor something to cling to. The collection of millions of these tiny droplets or crystals is what we see as a cloud.
– Joel Norris, professor of climate and atmospheric sciences, Climate, Atmospheric Science, Physical Oceanography (CASPO) division