i think bobmactx has it right
we get some crazy sand/windstorms here.... might not get such a solid wall of sand... but we get some that turn the sky dark... its crazy...
ive got some pics of a pretty bad one... but not the worst ive seen on my web page...
heres some pics of it from my webcam... it was in 03...
http://www.jschrodt.net/dust/index.htm