Bacteria are the smallest living things with a cellular structure; each individual bacterium consisting of one single colorless cell, which is usually either spherical or rod-shaped. Individual bacteria measure from 0.0001 inches to 0.00001 inches in length, so they can be seen only with the help of a high-power microscope. They are so small that they can float in the atmosphere, usually as 'passengers' on dust particles, up to a height of several thousand feet, except immediately after a heavy downpour, when the air is washed clean.