Shelf cloud and accompanying dust storm approaches downtown Phoenix. (Chopperguy photographer Jerry Ferguson and Pilot Andrew Park)

It was a haboob. The word is Arabic and means “blowing or drifting,”  and to meteorologists it is the term used to describe intense dust storms inherent to arid regions.

Haboobs are caused when the strong winds blasting out of a thunderstorm hit the ground and kick up the loose sand and dust covering the arid landscapes. Just as a shelf cloud marks the leading edge of a thunderstorm from above, a thick dust cloud marks the leading edge of this same thunderstorm from below.

Arcing across the sky landscape stretching dozens of miles from end-to-end, these dust storms can reach up thousands of feet in the air, and move across the landscape at highway speeds.

While these monstrous haboobs with their menacing shelf clouds hold astonishing beauty, they can be incredibly dangerous. Often accompanied by 60 mph winds or higher, they can pack a serious punch as they steam-roll across the landscape. In addition to the strong winds, the dust can cause visibility to drop to zero in heart beat, blocking out the sun turning day to night, and making it nearly impossible to see until the winds die down and the dust settles.

The thunderstorms that often spark these haboobs have a tremendous flash flood potential thanks to a desert landscape that is not efficient in absorbing the sudden heavy rains produced by these types of storms.

Haboobs are most common across the desert southwest during the monsoon thunderstorm season, which typically lasts from July to September. The monsoon season for thunderstorm occurs when dew points and humidity across the desert southwest increases thanks to a shift in wind direction, from the dry north winds intrinsic to the region most of the year to moist southerly winds that draw in moisture from both the Gulf of California and the Gulf of Mexico.

This added moisture provides fuel for daytime thunderstorms, that usually ignite over the mountains. Like clockwork during the summer months, these afternoon thunderstorms bring needed rainfall to a very thirsty desert, provide stunning lightning displays for weather enthusiasts and storm chasers alike, and spark the iconic dust storms that have become a signature of monsoon season.

Phoenix is the haboob capital of the United States. Last week’s thunderstorms in the area were not caused by a monsoon set-up, but rather a spinning upper low that parked itself over the region for a couple days.  But while the monsoon season may be over until next summer, Mother Nature proved she doesn’t need an excuse to show off during all seasons.

Here are a couple other photos of last week’s haboob as it engulfed downtown Phoenix.