I've seen a few people on here asking how the Phoenix dust storm happened, and what causes dust storms in general. I'm going to give a brief, simplified overview of how dust storms form (there is a way more complicated explanation involving the electromagnetic properties of sand/dust/particles and how static helps them to lift off the ground, but I don't understand it enough to present that in this diary), and how the Phoenix storm formed.
Jump the squiggle of doom and give the guard the secret handshake to continue.
Dust storms (also called sand storms, haboobs, fuckupyourlung-a-thons, and AHHHH in OCDese) are caused when winds pick up large amounts of loose soil and spread them over large distances, sometimes hundreds or even thousands of miles. They mainly form in dry places with loose soil (think of the sand in the desert or soil on a poorly farmed land), and can grow several miles tall and hundreds of miles wide if conditions are perfect. The winds that cause dust storms can come from different types of weather events, but the main ones are dry frontal passages (usually a cold front with lots of wind but no precipitation… this happened often during the Dust Bowl in the 1930s) or from thunderstorms over the desert.
The most common cause for dust storms here in the United States are from thunderstorm-generated winds, which can exceed 60 MPH when they turn severe. These winds can either be contained within the thunderstorm, or they can “blast out” from the storm as an outflow boundary, going possibly a hundred or two miles from the storm. Think of these winds as the front of a shoe, or a wedge-shaped bubble extending out from the storm:
Source: My own, beautiful creation.
When these winds go over dry, loose soil (like one would find in the American southwest), it can easily pick it up and keep it aloft within that wedge of air flowing out from the storm. If the winds are strong enough, it can pick up a whole layer of this loose dust/sand/soil and turn it into what we know as a dust storm:
Dust storm approaching the NWS Office in Tempe, AZ. Picture: NWS Phoenix
The one in Phoenix was caused by intense storms off to the city’s southeast (see the National Weather Service radar below), which caused a downburst with winds of 70 MPH. These winds rippled out of the thunderstorm downhill towards the Phoenix metro area, and picked up a ton of soil and sand as it went. The storm grew to about a mile tall (6000 feet, estimated by the NWS), and almost 200 miles wide at its strongest.
Image: NWS Phoenix
These types of storms aren’t uncommon in this part of the country (and elsewhere in the world), especially during thunderstorm season. Just a few weeks ago, there was a big dust storm that hit Amarillo due to a thunderstorm dying out and sending off strong winds as it decayed. There are 4 reasons why the one in Phoenix was such a big deal: 1) it hit a large metro area, 2) we live in the camera/social media age, so there are literally thousands of pictures and videos of the event, 3) the news loves stuff like this, and 4) this dust storm was also notable in its size. Southern Arizona is in a moderate drought (see drought monitor picture below), and this combined with the outflow from the strong storms to Phoenix’s southeast created good conditions for a pretty big dust storm to form. So, to answer the “is this unusual?” questions…not really, no. The size was unusual for this area due to the drought and strength of the thunderstorm winds, but it was just a big storm that hit a big city and got a good bit of press because of it.
Hope that answers any questions you might have. If I screwed something up or left anything out, don’t hesitate to correct me. I know how shy you guys are in pointing out when someone is wrong. ;)
10:15 AM PT: Forgot to put this in the tip jar before I posted...helicopter footage of the dust storm as it moved through Phoenix.