From Glossary of Meteorology


An idealized condition in which compressibility vanishes.

Most liquids are nearly incompressible. For shallow layers of the atmosphere with velocity fluctuations much less than the speed of sound, incompressibility is a good approximation and is often made for mathematical convenience. For a fluid with a density depending on temperature, incompressibility does not imply non-divergence.
See anelastic approximation, Boussinesq approximation.