Haboobs and Monsoons; Arizona Summer Weather Phenomenons

A massive dust storm moved through Phoenix, Arizona last Tuesday night (July 5, 2011).  It had hurricane force winds gusting up to 81 mph in some places.  The storm left some Phoenix residents temporarily without power.  The term “haboob” is from the Arabic language and it means “strong wind or ‘phenomenon’ “.

During thunderstorm formation, winds generally move in a direction opposite to the storm’s travel and they will move from all directions into the thunderstorm. When the storm collapses and begins to release precipitation, wind directions reverse, gusting downward and outward from the storm and generally gusting the strongest in the direction of the storm’s travel.

When this downdraft reaches the ground, dry, loose sand from the desert floor is essentially blown up, creating a wall of sediment preceding the storm cloud. This wall of sand can be up to 100 km (60 miles) wide and several kilometers in elevation. At their strongest, haboob winds can often travel at 35–50 km/h (20–30 mph), and they may approach with little to no warning. In the July 5th haboob in the Phoenix area there were sustained winds of 69 mph.

Phoenix experiences various degrees of dust storms, but the haboob is the largest and most dangerous. According to the National Weather Service, Phoenix experiences on average about 3 haboobs per year during the months of June through September.

Often the precursor to the haboob is the phenomenon called the “monsoon” which is the season of summer thunderstorms usually occuring between the end of June through the middle of September.  The thunderstorms are also called “chubascos” in Spanish and typically come from a weather formation beginning with moisture traveling northward from Mexico and the Gulf of California (Sea of Cortez).  This moisture gets pump into the warming deserts and the combination of heat rising from the desert floor and the humid air typically rises over mountainous areas first and then begins to travel in direction of the most prevalent high pressure winds.  As the cumulonimbus clouds or “thunderheads” build vertically, they begin to create huge displays of lightning and thunder and then downbursts of heavy rains.  As most southwesterners know, these are the times when desert arroyos (normally dry sandy washes or dry creeks) can become deathtraps for those who dare to cross them on foot or in vehicles.  There are now “stupid motorists laws” which levy heavy fines for those who become trapped and need to be rescued by local police and firefighters.

There is something refreshing and renewing about the monsoon season.  Is is especially so this year because of the near record days (81) here in Tucson without rain.  The deserts and mountains had become dangerously dry and the low humidity and high winds led to the largest wildland fires ever in Arizona.  With the increasing rains, the desert is “greening up” and the forests’ fire danger is going down.  And there is also something aromatic in the air, once the monsoon begins to rain down upon the many creosote bushes found here in our Sonoran desert.