Small sized organisms less than 2 mm, especially free-living microorganisms such as bacteria, fungi and algae tend to show world-wide distribution pattern. This suggests that many microorganisms travel to disperse long-distance in the air by wind. In fact, various microorganisms have been reported from the air at various altitudes in the troposphere, stratosphere and mesosphere (0–77 km a.s.l). Since dispersals to favorable habitats is important for microbes, it is possible that some microbes are adapted for long-distance dispersal in the air. For example, some bacteria and fungi living on leaf surface have been reported to produce ice-nucleation protains that function as condensation nuclei and ice nuclei. Hamilton and Lenton (1998) proposed a hypothesis that some microorganisms produce special substances that function as condensation nuclei and/or ice nuclei for cloud formation and migrate in the air safely and effectively in cloud droplets or in snowflakes. Recently, it has been reported that large part of the ice nuclei found in the new snow collected from various high-middle latitude areas were suggested to be microorganisms with ice nucleation protains. Therefore, microorganisms in the air could affect the cloud formation and precipitation in many regions of the world.