Liquid water path  (LWP) is one of the most important variables for modeling the atmosphere.  LWP is measure of the weight of the liquid water droplets in the atmosphere above a unit surface area on the earth, given in units of kg m−2 (AMS Glossary). It is critical for understanding radiative transfer. The liquid water path can be approximately retrieved from passive and active remote sensing such as microwave radiometer instruments, for example SSM/I.