Richard A. Anthes, Hans A. Panofsky, John J. Cahir, and Albert Rango, The Atmosphere, Charles E. Merill Publishing Co., 1975
The equations and laws described thus far are very general. In a sense, they are too general in that they can be used to describe phenomena as large as cyclones or as small as the vortex of water draining out of a bathtub. Obviously, to avoid confusion, we do not want to study all sizes of motion at the same time. In practice, we select our principal time and space scales of motion. The time scale is determined by the lifetime, or period, of the phenomenon; the space scale is determined by the typical size or wavelength.
