WRAL WeatherCenter Blog

I know (or at least I think I know) that "humid air" ( for lack of a better descriptor) can only get so hot explaining why NC rarely gets into the 100s ala AZ or NV where air is drier. Why is this so? Why does a high moisture content buffer us from extreme temps in the 100s? If it does that?

Posted June 27, 2007

MIKE MOSS SAYS:       Tripp,     There's a cause and effect issue here that is more complex than just humid air versus dry air. The extremes of temperature that occur in the desert southwest for example and the fact that the air is often quite dry there are related to surrounding topography, geography and prevailing weather circulations, and the long term effects those features have on the vegetation and soil characteristics of the area. The same can be said about North Carolina, as we are subject to the effects of nearby mountains, oceans, and prevailing weather patterns in such a way as to produce a more humid climate and relatively lush vegetation in terms of tree canopy, meadows, grasslands and farmland. These differences lead to differneces in humidity and temperature, rather than the humidity itself moderating the temperature extremes.

The source for our humid airmasses, for example, is often the Atlantic and Gulf of Mexico, which rarely warm to much more very much more than the low 80s over a large area. This warm water results in humid air that is not extremely hot when it flows into the southeast and across our state. In addition, this humid air supports the formation of clouds and occasional showers, which can block some solar radiation and produce some evaporative cooling, acting against temperature extremes. Importantly, this also results in ground that is usually vegetated as I mentioned above, and ground that rarely dries completely out in the upper few inches. Trees and some shrubbery and grasses, tapping into moisture well below the surface, are aboe to absorb a significant amount of solar radiation and remain relatively cool by way of evapotranspiration, which diverts some of the solar energy into latent heat (in the form of more humid air) rather than sensible heat (in the form of higher temperatures).

By contrast, in a desert region the topography and/or weather patterns result in verty little rainfall and very little widespread, leafy vegetation, and the upper few inches of the desert floor are very dry as well. This means that solar radiation that is absorbed is converted much more directly to sensible heat, rapidly driving up the temperature of the ground and thereby heating up, by conduction and convection, the low layers of air above it. Even if the air above the desert floor were somehow made very humid, as long as it contained no liquid water droplets it could heat up just as much as the "dry" air.

One way to illustrate the importance of surface vegetation in all this is to think back on some sunny summer day that you've been at a beach or a playground, and walked barefoot from hot beach sand or a hot sandy baseball diamond onto a grassy area nearby. The difference is dramatic and is even more pronounced when you scale that up and compare widespread areas of desert floor versus a forested region.

On the rare occasions that we do manage to top 100 degrees here, one usually finds that we have a westerly breeze blowing downslope fromt he mountains, that there is a large high pressure area in the region producing strong subsidence (both of which act to "compress" air from above, making it warmer than it otherwise would be. In addition, those episodes often follow lengthy dry periods so that our ground is about as parched as it ever gets and grasses and trees are dormant or are at least resisting the release of much moisture in the form of evapotranspiration.


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