Dear Tom,

You often show a low pressure area over the border of California/Arizona. It is stationary, but don’t low pressure systems usually move?

—Richard Wall

Dear Richard,

You are referring to a heat low pressure area (or thermal low) that often develops in the border area of California and Arizona during the warm season. It is stationary. Heat lows are not associated with cold and warm fronts the way moving low pressure systems are. They occur over continents during the warm season as a result of intense heating compared to their surrounding environment. Because hot air is less dense than colder air, its air pressure is less. In the United States, a favored area for such development is the California/Arizona border area, a low region surrounded by much higher, colder elevations.