A unit used in atmospheric physics to compare the "depth" or total volume of atmospheres, or components of atmospheres. The depth (in atmo-meters) is equal to the depth (in meters) the atmosphere, or one gas component of the atmosphere, would have if it formed a uniform layer at standard temperature (0oC) and pressure (1 atmosphere). One atmo-meter represents 2.686 99 x 1025 molecules of gas per square meter of planetary surface