PHOENIX (AP) - A thick cloud of dust has swallowed Phoenix at sundown three times this summer, covering the city with grit and baffling even longtime residents who can't remember seeing so many dramatic "haboobs" during a monsoon season.
A 1,000-feet-high wall of dust traveled at least 50 miles into metro Phoenix and neighboring Pinal County on Thursday evening before dissipating.
It turned the skies brown, reduced visibility, created dangerous driving conditions and caused some airline flights to be delayed. The storm, also known by the Arabic word haboob, also coated anything left outside in a thin layer of fine dirt and left some people who walked outside for a minute or two with grit between their teeth.
Danny Shepherd, a deejay who was driving to a gig at a coffee house during the storm, has lived in metropolitan Phoenix since 1989 and can't recall so many big dust storms in one year. "They're typical but I also think there's been a lot more this year - big ones and small ones and the haboob, the grand-daddy of them all," Shepherd said.
National Weather Service meteorologist Ken Waters said the area is experiencing a typical number of dust storms this year, but what sets this season apart from others is the size and power of three of the storms.
"Each year, you are going to get some variety of the dust storms," Waters said. "We don't see a single causative factor for why they seem to be stronger this year."
Waters said thunderstorms moving through southern Arizona supplied winds of up to 60 mph that stirred up fine dust in the agricultural fields and sent Thursday's storm to the nation's sixth-largest city.
"It was a strong dust storm, but nothing on the order of the big one in July," he said.
This season's most powerful dust storm came on July 5 when a mile-high wall of dust halted airline flights, knocked out power for 10,000 people and covered everything in its path with a thick sheet of dust. Another dust storm hit July 18 reaching heights of 3,000 to 4,000 feet, delaying flights and cutting off power for more than 2,000 people in the Phoenix metro area.
Weather experts said such massive dust storms only happen in Arizona, Africa's Sahara desert and parts of the Middle East because of dry conditions and large amounts of sand.
At a Honda car dealership in Phoenix, a crew of four men used a portable pressure washer to hose down vehicles covered in a thick coat of dust that made the car windows opaque.
Erik Fisher, a new car director at the dealership, estimated his crew will have to wash about 350 cars Friday thanks to the dust storm. "They'll probably be out there half a day," Fisher said. "It's a lot of extra man power."
Pollution levels skyrocket during dust storms and create even more breathing problems for people with asthma and other conditions.
Dennis Dickerson, a compliance manager at the Maricopa County Air Quality Department, said Thursday's dust storm dramatically raised air pollution levels. Dickerson said even people without health issues should stay out of severe dusts storms.
Dickerson said the normal level of dust in the air is anywhere from 20 to 70 micrograms per cubic meter. Thursday's 24-hour average for Phoenix reached nearly 160 micrograms per cubic meter. That figure was just slightly above what federal standards deem healthy.
Dr. Art Mollen, who operates a family medicine practice in Scottsdale, Ariz., said there were twice as many patients as usual in his waiting room at the start of work Friday. People came in with asthma flare-ups, sinus inflammation, opthalmological migraines and other problems.
"We have been inordinately busy today because of the dust storm," Mollen said, noting also that since early July he has seen more patients with allergy problems, even though Arizona's allergy season happens in the spring and fall.
Mollen also said he expects to see more cases of valley fever, a fungal pneumonia that thrives in the hot, arid Southwest in dirt found just a few feet beneath the earth's surface. It can be stirred up by construction, wind or other activities.
Meteorologists also were trying to determine whether microbursts caused storm damage in metro Phoenix and neighboring Pinal County.