For any moist surface, the ratio of heat energy used for sensible heating (conduction and convection) to the heat energy used for latent heating (evaporation of water or sublimation of snow). The Bowen ratio ranges from about 0.1 for the ocean surface to more than 2.0 for deserts; negative values are also possible. It is named for Ira S. Bowen (1898-1978), an American astrophysicist.

The ratio of sensible to latent heat fluxes from the earth's surface up into the air. This is equal to the psychrometric constant times the ratio of kinematic temperature flux to kinematic moisture flux. It can be estimated as the psychrometric constant times the ratio of potential temperature difference to mixing ratio difference, where the differences are measured between the same two heights in the atmospheric surface layer. Typical values are 5 over semiarid regions, 0.5 over grasslands and forests, 0.2 over irrigated orchards or grass, 0.1 over the sea, and negative in some advective situations such as over oases where sensible heat flux can be downward while latent heat flux is upward.

A Bowen ratio is the ratio of energy fluxes from one medium to another by sensible and latent heating respectively.