Weather Whys; Thermal Low

This is weather-wise. If you see a low-pressure system depicted on your TV weather program, it will probably be shown as a huge circular or comas-shaped mass of clouds, extending perhaps from the US-Canadian border to the Gulf of Mexico. But lows can come in many shapes and sizes. Severe thunderstorms have their own kind of miniature pressure system, with highs, lows, and fronts. And there are other sorts of small-scale systems, too. For instance, if you watch the TV weather maps in the summer, it's very common to see an area of low-pressure centered somewhere in the desert southwest. And yet, you know the desert isn't exactly known for its summer rainfall. So how can there be such a tendency toward low-pressure, but so little precipitation? The reason is, that low is what's called a thermal low, and is driven solely by the heating of the ground. Since warm air tends to rise, the heating process results in air molecules being taken away from the area nearest to the earth.

And that relative lack of air molecules creates a shallow area of low-pressure that shows up on the surface weather maps. But if there's low-pressure, why isn't there rain? Well, occasionally, one of those thermal lows might kick up a shower, but that's not usually the case. And one reason for that is, there's just not much moisture available. In most areas of the country, rising warm air often carries a fair amount of water vapor with it. That water can then condense into puffy cumulus clouds and sometimes thunderstorms. But the air rising from the desert floor is very dry. Eventually, if it were allowed to rise far enough, it might be able to produce a cloud or two. But the thermal low is usually a very shallow thing, and the air stops rising long before it might cool off enough to produce a cloud. Weather-wise is produced with a grant from the National Science Foundation. For Weather-wise, I'm Drew Barla.