# Poisson constant

From AMS Glossary

## Poisson constant

For dry air, κ = 0.2854. For moist air, where

*R*_{d}and*c*_{pd}are the gas constant and specific heat of dry air, ε is the ratio of the gas constants of water vapor and dry air,*c*_{pv}is the specific heat of water vapor, and*r*_{v}is the water vapor mixing ratio. The Poisson constant appears in the Poisson equation and in the definition of potential temperature.