Why does a low-pressure symbol appear almost daily near the SW border of Arizona and California?

Ask Tom Why
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Dear Tom,

I’ve noticed on the Tribune weather page daily maps that a low-pressure symbol appears almost daily near the southwest border of AZ and CA. Why is that?

Thanks,
Bob Hansen
Lindenhurst

Dear Bob,

The low pressure that shows up over the southwestern deserts in the summer is called a “thermal” low or “heat” low. It is not associated with storms, but, rather, results from intense heating of the lowest levels of the atmosphere. In summer, the nearly cloudless deserts are characterized by very dry air that rapidly heats up in the strong summer sun. This causes air near the surface to expand and rise, lowering the atmospheric pressure in the process. Heat lows are stationary and weak and, as with all northern hemisphere lows, produce a counterclockwise wind circulation. Similar heat lows are found in the Sahara and other desert areas worldwide.

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