The Phoenix Dust Storm Scale (PHX-DUST) ranks the severity of dust storms (haboobs) in the Phoenix metropolitan area of Arizona, United States, on a five-category scale using post-event data. A coalition including Arizona State University researchers, the Phoenix office of the National Weather Service, regional air-quality and flood-control agencies, a utility, and local broadcast meteorologists convened to build a classification system for dust storms. The scale assigns categories based on peak hourly concentrations of particulate matter (PM₁₀) measured by a network of air-quality monitors. Three secondary classifications record how far a given storm spread, its duration, and its wind strength (Krahenbuhl et al., 2026).
Dust storm enveloping North Phoenix, Arizona, August 8, 2025.
A January 2026 paper in the Bulletin of the American Meteorological Society introduced the scale and applied it retrospectively to 189 Phoenix-area dust storms recorded from 2010 through 2023 (Krahenbuhl et al., 2026). Forecasters and researchers piloted the scale during the 2025 monsoon season. Media outlets reported storm classifications using the scale during the 2026 monsoon season. In early August of 2026, two storms were classified as Category 5 within five days of each other (Shepherd, 2026; Petersheim & Sarabia, 2026).
Its widespread-versus-isolated designation depends on the layout of the specific Phoenix area sensor network. Any storm’s actual physical footprint could extend beyond the sensor network. Creating a similar scale for other locations would require building a comparable sensor network (Krahenbuhl et al., 2026).
Concept development
Need for a severity scale
In Arizona, only flooding and extreme heat or cold kill more people than dust storms do (Lader et al., 2016). Exposure to PM₁₀ during dust events has been associated with cardiovascular and non-accidental mortality (Crooks et al., 2016; Reed & Nugent, 2018). Dust storms have also been associated with outbreaks of Valley fever, or coccidioidomycosis (Pappagianis & Einstein, 1978; Tong et al., 2017).
One 2023 study put the U.S. death toll from windblown dust at a minimum of 232 people between 2007 and 2017 (Tong et al., 2023). The authors believe the actual total is higher because official hazard statistics tend to miss dust-related deaths.
Windblown dust in arid regions of the western United States, including dust storms, has also been linked to increased hantavirus exposure risk (Gorris et al., 2025).
Phoenix-area physicians have also linked dust storms to increased emergency-room visits for asthma and chronic obstructive pulmonary disease (Christiansen, 2026).
Dust storms also affect transportation and electrical infrastructure. A 2016 dust storm delayed 16 flights at Phoenix Sky Harbor International Airport (Cervantes, 2016). Sudden losses of roadway visibility during dust storms have contributed to fatal crashes (Mohebbi et al., 2019). Blowing dust can contribute to power outages by fouling electrical equipment (Maliszewski et al., 2012).
A 10-mile stretch of Interstate 10 between Eloy and Picacho Peak, on the Phoenix-Tucson corridor, is equipped with an automated dust-detection and warning system that lowers the posted speed limit, in 10 mph increments down to as low as 35 mph, when sensors detect low-visibility conditions (Arizona Department of Transportation). The system activated during a Category 3 storm on 12 July 2026 (Wirtz & Christiansen, 2026).
Despite the scale of the hazard, no standardized, publicly available way to describe or compare the severity of individual dust storms existed before 2026.
Origins and design
The PHX-DUST scale grew out of a working group of Phoenix-area stakeholders that first convened in 2021 to address these needs. Government participants included the National Weather Service’s Phoenix forecast office (NWS-PHX), the Arizona Department of Environmental Quality (ADEQ), the Flood Control District of Maricopa County (FCDMC), the Maricopa and Pinal County Air Quality Department, and the U.S. Forest Service. Arizona State University (ASU) researchers, Salt River Project (SRP), local television meteorologists, and forensic meteorologists also participated (Krahenbuhl et al., 2026).
The consortium’s stated design goal was to keep it simple. The working group considered proposals to add variables such as vegetation indices or drought indices. They decided to build the scale from only parameters already available in near-real time from existing sensor networks (Krahenbuhl et al., 2026).
Because the hurricane wind scale (1-5), Saffir-Simpson, and the tornado strength scale (EF0-EF5), Enhanced Fujita, are widely recognized, the developers selected a five-level scale with its highest rating of Category 5. They decided to classify storms after the fact because the hourly-averaged air-quality data cannot be assembled in real time. This post-event classification is similar to how tornadoes are classified using the Enhanced Fujita scale (Krahenbuhl et al., 2026).
The scale’s authors describe it as initially applicable only to the Phoenix metropolitan area. The Phoenix area has the dense combination of air-quality and meteorological monitoring needed for storm classification. They suggest the general approach could serve as a model for other dust-prone urban areas (Krahenbuhl et al., 2026). ASU has described the PHX-DUST scale as the first scientific post-event classification system designed specifically to measure and rank dust storm severity for a U.S. metropolitan area (Arizona State University).
System design
Data network
PM₁₀ concentrations are measured by 22 monitors operated by the Maricopa County Air Quality Department. This data is reported through the EPA’s Air Quality System database (Krahenbuhl et al., 2026).
Wind readings come from a separate system. Eleven stations from Maricopa County’s Flood Control District ALERT (Automated Local Evaluation in Real Time) network contribute wind data. These stations are part of a late-1970s National Weather Service design whose sensors sit lower to the ground, at 1.9 to 2.4 meters. This is significantly closer to the ground than the 10-meter standard used at NWS airports. The scale’s developers considered them to be suitable for identifying dust-driving wind events (Krahenbuhl et al., 2026).
Since the air-quality figures are hourly averages required under EPA rules, a storm’s rating usually cannot be finalized until about a day after it passes, which rules out real-time or forecast use (Krahenbuhl et al., 2026).
Classification
Primary category
A storm’s primary category is based on the single highest hourly PM₁₀ reading recorded anywhere in the network over the course of the event. Any event peaking under 1,000 micrograms per cubic meter is not ranked (Krahenbuhl et al., 2026).
The developers selected maximum hourly PM₁₀ concentration as the primary measurement because it is available in near-real time and corresponds to the dense walls of dust characteristic of haboobs. The category thresholds themselves are evenly spaced 1,000 microgram-per-cubic-meter increments, consistent with the developers’ goal of keeping the scale simple (Krahenbuhl et al., 2026).
- Category 1: 1,000 µg/m³ minimum
- Category 2: 2,000 µg/m³ minimum
- Category 3: 3,000 µg/m³ minimum
- Category 4: 4,000 µg/m³ minimum
- Category 5: 5,000 µg/m³ and above
Source: Krahenbuhl et al., 2026.
Secondary classifications
Each storm also receives secondary classifications for areal coverage, duration, and wind-gust strength (Krahenbuhl et al., 2026).
- Areal coverage is reported as the share of stations that logged more than 500 µg/m³ in a given hour, a cutoff set above Phoenix’s typical background haze (the local “brown cloud”) so ordinary pollution is not mistaken for a dust event. Half or more of the stations crossing that line makes a storm widespread (W); anything less is isolated (I).
- Duration is measured as the total hours during which at least one station stayed above that same 500 µg/m³ mark. Three hours or more counts as long-duration (L); under three hours is short-duration (S).
- Wind-gust strength is classified as high (H) if any network station recorded a gust greater than 25 mph (11.2 m/s). This threshold comes from the U.S. Environmental Protection Agency (EPA)’s Exceptional Events Rule and the National Weather Service’s threshold for issuing blowing-dust advisories.
A storm’s full rating combines the scale number with its three letters, for example Category 5, Widespread, Long-Duration, High-Gust, shortened to 5-WLH (Krahenbuhl et al., 2026).
Scope and limitations
The geographic restriction is intentional and follows directly from how the scale calculates its secondary classifications (Krahenbuhl et al., 2026). The widespread-versus-isolated designation is calculated as a fraction of monitoring stations logging hourly PM₁₀ concentrations above 500 µg/m³. It is not a measurement of the storm’s areal extent.
PHX-DUST relies on beta attenuation monitoring (BAM) instruments to measure PM₁₀ concentrations. Some nearby monitoring networks use tapered element oscillating microbalance (TEOM) instruments, which tend to produce higher readings than BAM instruments during high-wind and heavy-dust conditions. Measurements from networks using the two instrument types are not directly comparable. The developers state that another region would need a dense monitoring network using internally consistent instrumentation before adopting a comparable scale (Krahenbuhl et al., 2026).
Applications
Participating organizations have identified several applications for the PHX-DUST scale. ADEQ treats the classifications as supporting evidence for exceptional-event petitions to the EPA when dust storms cause air-quality measurements to exceed federal limits. Phoenix’s emergency-management office uses the scale in after-action reviews associated with the city’s 2021 Climate Action Plan (Krahenbuhl et al., 2026).
The National Weather Service expects the scale to improve dust-storm warnings. Salt River Project is examining possible relationships between storm severity, power outages, and reduced solar-energy production. Researchers at Arizona State University and the Flood Control District of Maricopa County use the classified storm record to test dust-transport models and examine changes in storm frequency and intensity (Krahenbuhl et al., 2026).
Broadcast meteorologists and storm chasers can use the classifications to compare storms. The paper also identifies applications in forensic meteorology and in efforts to prevent dust-related shutdowns at southern Arizona observatories (Krahenbuhl et al., 2026).
Notable events
5 July 2011
When the scale was applied retroactively to the 2010-2023 dataset, the dust storm on 5 July 2011 achieved the highest rating. It was classified as Category 5, widespread, long-duration, high-gust (5-WLH). The storm produced a peak PM₁₀ reading of 6,348 µg/m³. Every network station exceeded the 500 µg/m³ mark and concentrations remained elevated for six straight hours. Wind gusts reached 49 mph, or 21.9 m/s (Krahenbuhl et al., 2026).
The National Weather Service Storm Data report attributed the event to collapsing thunderstorms near Tucson. Downburst winds exceeded 70 mph (31.3 m/s) and pushed a wall of dust more than 100 miles (160 km) long and over a mile (1.6 km) high into the metro area. The report documented approximately $30,000 in damage and 13 indirect injuries (National Weather Service Storm Data). Across the full dataset, three of the 189 classified events received Category 5 and 116 received Category 1 (Krahenbuhl et al., 2026).
2025 experimental rollout and 2026 season
During a trial run of the scale in the 2025 monsoon season, ten Phoenix-area dust events were classified. Of these, five reached Category 1, three reached Category 2, one reached Category 4, and one reached Category 5 (Krahenbuhl et al., 2026).
During the 2026 monsoon season, two Phoenix-area storms were classified as Category 5 within five days. The first occurred during the night of 3-4 August and was the fifth Category 5 event identified in the historical record (Shepherd, 2026). It also had the longest recorded duration, with elevated dust concentrations continuing for approximately 16 hours. By early August, eight haboobs had occurred in the Phoenix area during the 2026 monsoon season (KTAR News, 2026).
A second storm was classified as Category 5 early on 6 August, becoming the sixth such event in the record. Outflow winds from decaying thunderstorms near Tucson produced the storm. A monitor in west Chandler recorded the highest PM₁₀ concentration at 5,841 µg/m³. Elevated concentrations continued for three hours, and several stations recorded gusts of at least 25 mph (Petersheim & Sarabia, 2026).
The 3-4 August storm affected a broader area of the Phoenix metropolitan region. The 6 August storm was concentrated around Chandler, with lower concentrations recorded in Tempe, Scottsdale, and northern Phoenix (Petersheim & Sarabia, 2026).
Future development
The scale’s developers describe it as a preliminary framework that can be revised with additional data and experience. The goal, as the paper states, was to get something on the boards for later refinement. The participating agencies plan to review the scale’s performance after each monsoon season (Krahenbuhl et al., 2026).
Future refinements proposed in the paper include tying categories to synoptic weather patterns, testing the index in an operational forecasting context, applying more advanced spatial techniques such as kriging to map storm extent, and incorporating artificial intelligence into dust storm forecasting and monitoring (Krahenbuhl et al., 2026).
See also
Saffir-Simpson scale, Enhanced Fujita scale, Beaufort scale, Haboob, Dust storm, Air quality index
Sources
Krahenbuhl, D.S., Pace, M.B., Heintzman, R., et al. “Phoenix Dust Storm (PHX-DUST) Scale: A Cooperatively Developed Dust Storm Scale for Phoenix, Arizona.” Bulletin of the American Meteorological Society, 107(1), E9-E25, January 2026.
Arizona State University School of Geographical Sciences and Urban Planning, “Phoenix Dust Storm (PHX-DUST) Scale.”
Shepherd, Marshall. “A Rare Category 5 Dust Storm Hit Phoenix Monday Night.” Forbes, August 5, 2026.
Petersheim Jr., Brian; Sarabia, Steven. “Phoenix hit by second Category 5 dust storm of the season days apart.” KOLD News 13, August 6, 2026.
National Weather Service Storm Data, “5 July 2011 dust storm,” NOAA.
Lader, G., Raman, A., Davis, J.T., Waters, K. “Blowing Dust and Dust Storms: One of Arizona’s Most Underrated Weather Hazards.” NOAA Technical Memorandum NWS-WR 290, 2016.
Gorris, M.E., et al. “Hantavirus Is Associated With Open Developed Areas and Arid Climates, Highlighting Increased Risk in the Western United States.” Transboundary and Emerging Diseases, 2025.
KTAR News, “Phoenix hit by 2 rare Category 5 dust storms in one week,” August 2026.
Crooks, James L.; Cascio, Wayne E.; Percy, Madelyn S.; Reyes, Jeanette; Neas, Lucas M.; Hilborn, Elizabeth D. “The Association between Dust Storms and Daily Non-Accidental Mortality in the United States, 1993-2005.” Environmental Health Perspectives, 124(11), 1735-1743, 2016.
Reed, L.K.; Nugent, K. “The Health Effects of Dust Storms in the Southwest United States.” The Southwest Respiratory and Critical Care Chronicles, 6(22), 42-46, 2018.
Pappagianis, D.; Einstein, H. “Tempest from Tehachapi Takes Toll or Coccidioides Conveyed Aloft and Afar.” Western Journal of Medicine, 129(6), 527-530, 1978.
Tong, Daniel Q.; Wang, Julian X.L.; Gill, Thomas E.; Lei, Hang; Wang, Binyu. “Intensified Dust Storm Activity and Valley Fever Infection in the Southwestern United States.” Geophysical Research Letters, 44(9), 4304-4312, 2017.
Tong, Daniel; Feng, I.; Gill, Thomas E.; Schepanski, Kerstin; Wang, Jianzhong. “How Many People Were Killed by Windblown Dust Events in the United States?” Bulletin of the American Meteorological Society, 104(5), E1067-E1084, 2023.
Christiansen, Andrew. “Phoenix dust storms prompt health warnings as Category 5 dust lingers.” FOX 10 Phoenix, August 9, 2026.
Cervantes, R. “Dust storm causes flight delays at Phoenix Sky Harbor International Airport.” ABC15 (Phoenix), August 22, 2016.
Mohebbi, Amin; Green, Gabriel T.; Akbariyeh, Simin; Yu, Fan; Russo, Brendan J.; Smaglik, Edward J. “Development of Dust Storm Modeling for Use in Freeway Safety and Operations Management: An Arizona Case Study.” Transportation Research Record, 2673(5), 175-187, 2019.
Maliszewski, P.J.; Larson, E.K.; Perrings, C. “Environmental Determinants of Unscheduled Residential Outages in the Electrical Power Distribution of Phoenix, Arizona.” Reliability Engineering & System Safety, 99, 161-171, 2012.
Arizona Department of Transportation. “New for Monsoon 2020: When dust kicks up, speed limit goes down.” ADOT Blog.
Wirtz, Taylor; Christiansen, Andrew. “Arizona monsoon season brings Category 3 dust storm to Phoenix.” FOX 10 Phoenix, July 13, 2026.