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The atmosphere is an inhospitable place for germs compared with soil and water. Dryness, cool temperatures, and excessive exposure to ultraviolet light are hard on a microbial cell. For decades,microbiologists assumed that the air is the last place a cell wants to spend time. Other than studying how germs that cause illness (pathogens) can spread through the air to infect other people, science did not spend much time on the microbes that populate the atmosphere. But aeromicrobiologists (scientists who study the microbes in the atmosphere) are now showing that hundreds of trillions of bacteria and fungi probably live in clouds. Because these cells must absorb nutrients from the clouds to stay alive, they directly affect the chemistry of the atmosphere. The cells also act as tiny particles onto which moisture or ice crystals accumulate. When the accumulation grows large enough, it falls to Earth as precipitation, carrying the microbes with it. In this way microbes in the air affect our weather and can even make it rain.

This agar plate contains several different types of bacteria captured from the air. [Ginger Krieg Dosier, NIH Image Gallery, 2009]