From Glossary of Meteorology
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.