dewpoints; orographic lifting

charlie roswell

During forecst discussions of the past week, someone stated that dewpoints in the 40's were high enough for severe weather in Colorado. But in most of the plains dewpoints in the 60's are more desireable. Why is this?

Also, what exactly is orographic lifting, and does it have anything to do with the first question?

Second thing first - orographic lifting occurs whenever air is lifted from the surface by a change in elevation. This is the primary mechanism responsible for supercell storms in Colorado and Wyoming as there are no drylines to trigger convection like here in the Southern Plains.

I cannot speak as well on the dewpoints but I imagine that lower temperatures at higher elevations allow less moisture (in terms of mixing ratio etc.) to saturate the air, leading to lower dewpoints.
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
Bill, I'm afraid that your definitions of potential temperature and equivalent potential temperature are incorrect. Both temperatures are adjusted to 1000 mb following a dry adiabat from the pressure at their actual location - but the equivalent potential temperature includes the latent heating owing to any moisture present as well. So, for dry air the potential temp and eqivalent potential temp are the same.