Moisture Metrics

[JamesCaruso]

My understanding is that dewpoints are not a completely objective measure of moisture in the atmosphere because they are also a function of altitude/pressure. For example, while we usually want dewpoints in the 60s for significant severe and tornados, dewpoints in the 50s can be adequate in eastern Colorado because those dewpoints represent a similar amount of moisture in absolute terms, but the dewpoint metric is lower because of the altitude and resulting lower pressure. My understanding is that mixing ratio is a more objective measurement of the absolute moisture content - i.e., if the absolute moisture content is the same in central KS and in eastern Colorado, both locations will have the same mixing ratio but Colorado will have a lower dewpoint.

Assuming the above is correct, my question is how does precipitable water (PW) fit into this? My assumption is that it, too, is an objective measure of absolute moisture content, and that a PW value of X in two different locations would translate to the same mixing ratio in both those locations, but that the dewpoints can be different because of altitude/pressure. Is this correct?

Thanks,
Jim

 

[MClarkson]

PW is typically the water content of the entire column, from the surface to the top. So you could have one location that is very moist at the surface, and very moist aloft, with a huge PW total. Or that same surface moisture, combined with dry air aloft, could produce a modest PW total, for example.

 

[Mike Johnston]

Well, it's a bit of an apples and oranges comparison. The PWAT measures the amount of moisture through the depth of the troposphere, so it's a measure with a vertical component. But, it can affect severe weather as a higher PWAT could influence storm mode toward HP. Also, a higher PWAT could mean reduced updraft strength because it generally implies a higher water loading factor.