Wednesday, July 1, 2015

Monsoon Season: Dust Storms Start On Time In Arizona

A dust storm approaching Phoenix, Arizona this past
Saturday. Photo by Pat Smith Johnson  
In mid to late summer, thunderstorms develop over the deserts of Arizona.  These thunderstorms can create dust storms, and it's often quite a spectacle.

This combination is a creature of the annual monsoon season out in these deserts.

The unrelenting heat of summer causes air to rise, creating an area of low pressure in the deserts.

The low pressure is not a storm system, like normal lows. It's just the fact that the air is rising, and for complicated reasons, that leads to low atmospheric pressure, at least compared to surrounding areas.

When there's low pressure, air wants to come into the area of low pressure. Nature abhors a vacuum. Weak low pressure isn't a vacuum, of course, but still, things try to find an equilibrium. So air comes in to replace the air that has risen because it's so damn hot.

Some of this air comes from over or near the Gulf of Mexico. You know how gawd awful humid the Texas coast can get this time of year. Some of that humidity travels toward the deserts. The humidity is enough to help trigger thunderstorms.

The thunderstorms, by their nature, are hit and miss. There's still a lot of dust and sand out in the deserts that hasn't been drenched by a thunderstorm in the past couple days. That's pretty much true no matter how active the monsoon and thunderstorm activity has been.

Strong outflow winds from these storms often blow out into these dry areas, and kick up a ton of dust and sand. These outflow boundaries, and the dust they pick up, can travel many miles.

A thunderstorm a dozen or more miles from Phoenix can eventually envelope the city in dust.

The hottest time of year in the deserts is June, with that strong sun. By late June or early July, the process I just described has started the monsoons. The first ones, right on schedule, hit in the last few days.

ArizonaStormChase captured this awesome dust storm near Casa Grande, Arizona. Pretty cool!

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