Solid particles suspended in the air. Larger particles (>100 μm approximately) have terminal velocities greater than about 0. 5 m s−1 and fall out quickly. These include hail, snow, graupel, insect debris, room dust, soot aggregates, coarse sand, gravel, and sea spray. Medium-size particles (1 to 100 μm approximately) have sedimentation velocities greater than 0. 2 m h−1 and settle out slowly. These include fine ice crystals, pollen, hair, large bacteria, windblown dust, fly ash, coal dust, silt, fine sand, and small dust. Small particles (<1 μm, approximately) fall so slowly that they can take days to years to settle out of a quiescent atmosphere. For a turbulent atmosphere they may never fall out; however, they can be washed out by rain in a process called rainout or washout, leading to wet deposition onto the earth's surface. Examples of these particles include viruses, small bacteria, metallurgical fumes, soot, oil smoke, tobacco smoke, clay, and fumes. Oil and tobacco smoke are sticky, and are removed from the atmosphere when they happen to touch and stick to an object such as plant or house furnishings, in a process called dry deposition. See criteria pollutants, precipitation.