CNN CNN Newsroom July 13, 2013 15:35:00
You need dusty terrain. we have that in the southwest. you also need thunderstorms and with a thunderstorm within it, theres an updraft component. but theres also a downdraft component, and with the downdraft component, the winds press down and out, and when they do, they form a gust front, and that kicks up all the dirt. thats what creates that wall of sand which really you could barely see through at all. quite scary and ominous looking at it. today around phoenix no threat for thunderstorms. ...
Source: archive.org