A massive dust storm moved through Phoenix, Arizona on Monday night, and by some accounts, the city looked like the set of a Martian horror movie. Experts say a rare Category 5 haboob moved through the region and is only the fifth time that Category 5 has been used in the last sixteen years. Let’s break down the storm and the scale being used to classify it.
What Is A Haboob?
The haboob that moved through Phoenix met the criteria for the rare Category 5 rating. What is a haboob? “An intense sandstorm or dust storm caused by strong winds, with sand and/or dust often lofted to heights as high as 1500 m (~5000 ft),” according to the Glossary of the American Meteorological Society. The “wall of dust” causes, “A rapid and significant reduction in visibility and an increase in wind speed following the passage of the leading edge of a haboob, which can last for tens of minutes to a few hours,” the AMS Glossary went on to say. The term “haboob” has its origins in the Arabic word habb, which means “to blow.” It was used to describe storms in Sudan but is used universally to describe wind-driven, dust storms in arid regions around the world.
According to AMS experts, haboobs are caused by density or gravity current often associated with thunderstorm outflows, but sharp atmospheric boundaries like fronts or drylines can also be culprits. “When a haboob is caused by a strong density current, the leading edge wall of dust roughly conforms to the shape of the associated density current head,” noted the AMS Glossary. The stunning haboob captured by Trey Grenwood in Phoenix seems to be a good example.
How Was The Haboob Rated?
“Our consortium of central Arizona meteorologists (Arizona State, National Weather Service, air quality, power, media, etc.) are using the PHX-DUST ranking index that we published last winter,” wrote Randall Cerveny by email. “Definitely a huge dust event! And the bizarre thing is that it is still (technically) on-going! We have horrible air quality still at 1 PM LST here in Phoenix,” the Arizona State University professor of geographical sciences went on to say Tuesday afternoon.
“The PHX-DUST Scale is the first scientific post-event classification tool designed to measure and rank the severity of dust storms in the Phoenix metropolitan region,” according to the Arizona State University website. “It evaluates storms using multiple indicators, including particulate matter (PM10), wind speed, duration and geographic extent,” said the experts who developed the scale along with regional partners. Cerveny told me that the particulate matter numbers associated with the haboob and its duration were particularly extreme. That certainly explains why it hit the Category 5 level.
Many hazardous weather phenomena area associated with risk communication scales. The most popular ones include the Enhanced Fujita Scale (tornadoes) and the Saffir-Simpson Scale (hurricanes). “The PHX-Dust Scale provides a standardized way to communicate how severe a dust storm was and what it means for public safety, infrastructure and daily life,” said scale-affiliated researchers. While a publicly-facing scale, it has been vetted in the peer-reviewed literature and involved 22 researchers, agencies, and collaborating partners.
Tom Gill is an expert on dust storms at University of Texas – El Paso. “Dust storms are a very underappreciated weather hazard and have wide-ranging impacts,” wrote Gill by email. “They, unfortunately, play a role in way too many fatalities and injuries through highway crashes and their economic impact on desert cities is underappreciated, due to things such as their health impact triggering respiratory, cardiovascular, and other diseases, to other costs that really start adding up which you might not think about,” Gill warned. Those impacts include degrading electrical power delivery and infrastructure, increased street cleaning costs, soiling solar panels, disrupting agriculture, and other impacts.
Gill noted that basing the scale on particle concentration in micrograms per cubic meter may not be as accessible to the public, but Cerveny reminded him that the scale was designed for the agencies that have a stake in it such as air quality management, aviation, utilities, and city health organizations.
So what was Gill’s take on this haboob event? This email from him after seeing the numbers tells the story. “Whoa, that is a big one! We’ve never had haboobs exceed 5000 in El Paso before,” he proclaimed. Gill has studied more than a few of these in his career, so that’s impressive.