On July 5th, 2011, a powerful dust storm swept through Phoenix, Arizona. Many residents said it was the largest they'd seen in decades--if ever. EarthSky spoke with Ken Waters, the warning coordination meteorologist for the National Weather Service in Phoenix. He explained that the July 5 Arizona dust storm started with rapidly descending air moving outward from area thunderstorms--combined with a lot of available dust.
These dust storms are common in Arizona. They're known as haboobs (derived from Arabic word for "strong wind"). But not a storm of this size. "[It] lasted for a good two hours, and covered at least 100 miles in horizontal distance, maybe more," says Waters. "It was truly massive. It's kind of like a once-in-a-lifetime event, like when you see a tsunami or tornado. We were just in awe. The video is just amazing, to watch it"
Waters explained that how dust storms form, and why it is that yesterday's storm in Phoenix got so incredibly big:
"Dust storms form from monsoon thunderstorms that form up in the afternoon. When the severe storms reach their full maturity, they produce what we call a microburst, which is like a very rapidly descending burst of air that comes through the thunderstorm that actually puffs out, hits the ground, and then disperses away from the storm.
"You can picture it like throwing a pebble into a lake--you get a wake that goes out in all directions. So it's basically a very strong wind that goes out. You couple that with the fact many of these areas [around Phoenix] have a lot of dust on them and they haven't had rain in 3 to 4 months. So there's a lot of available dust. And it doesn't take much wind to pick that up. And that's what happened yesterday."
There were particularly strong thunderstorms in the Phoenix vicinity on July 5, with a lot of lightning, but no rain. Those conditions made for a very strong downburst of air onto the ground and picked up dust from ground that had been parched for months. In other words, as with all weather events, multiple factors contributed to the size of the July 5 Arizona dust storm.
Waters described what it's like inside a dust storm, should you happen to get caught in one:
We're estimating the height of the dust storm to be about 5,000 to 8,000 feet above ground level. Picture this: this huge wall [of dust] just basically overtaking everything, traveling at probably 40-50 mph. And the remarkable thing is that it's just like…lights out. You can see the sun, you can see the sky, and then, within seconds, this humongous cloud of dust overtakes you, and you get very strong winds. If you're around and about outside, you definitely need to take cover, the dust is blinding, not just in visibility, but it's physically damaging to the eyes. One of the biggest dangers that we see are for the folks who are driving. Because it's like a light switch. It's so sudden. You're cloaked in darkness."
Some have speculated that the severity of the dust storm might be attributable to shifts in larger climate patterns such as El Nino and La Nina--or even the longer term pattern of global warming. But at this time, says Waters, it's not possible for him to make a direct connection.
Phoenix was still feeling the after-effects of the dust storm two days later. The city was powdered with dust: "I looked outside after sunrise, and there was a thick cloud of dust, that was still hanging in the atmosphere, and you could see it, maybe 1,000 to 2,000 feet above the surface. There was so much that some of it got suspended in the atmosphere, and created a cloud. And there hasn't been enough wind to disperse it."
Bottom line: The huge dust storm that moved through Phoenix on the evening of July 5, 2011 was initiated by rapidly descending air moving outward from area thunderstorms. The air hit the ground, then dispersed away from the storm in all directions. There had been no rain in parts of Arizona for 3 to 4 months, so there was a lot of available dust. And as it moved outward from area storms, the wind picked up parched dust that had been lying on the ground. The result was a dust storm that lasted a least two hours, covering at least 100 miles in horizontal distance, maybe more.
Written by Earthsky.org