**RE: Dewpoints and elevation**

Yes, lower dewpoints are generally necessary for convection at higher elevations. The easiest way to explain why is to look to the definitions of potential temperature (theta) and equivalent potential temperature (theta-E). First, theta: Theta, measured in degrees (usually Kelvin, K = degrees C + 273), is the temperature that a parcel at a given altitude would have if the latent heat of condensation and freezing were released. Theta is a function of both temperature and dewpoint, and it is always higher then the air temperature. Higher dewpoints = higher theta. Specifically, theta is calculated by lifting a parcel dry-adiabatically to the condensation level (LCL), and then moist-adiabatically up to a high elevation, thereby releasing virtually all of the latent heat. It is then brought dry-adiabatically back down to the original level.

Now, theta-E, also measured in degrees K. This is calculated similarly to theta, however the parcel is now brought down to 1000 mb instead of to the original elevation. In this way, we have a measure of temperature that takes into account both moisture AND elevation. To get to the punch line, less moisture is required at higher elevations for a given amount of instability because the pressure from which the parcel originates from is lower therefore more latent heat can be released. Higher dewpoints and/or higher elevation = higher theta-E = more heat available for instability.

Theta-E is available on model output charts (Eta/NAM, GFS, RUC, SPC meso-analysis, etc).

- bill