So far, the largest one I've seen was at El Mirage Dry Lake, in Southern CA.  As it approached, it really increased in size and had me pretty freaked, but I decided to let it pass over me anyway.  I'd say the wind speed was around 30-40 mph, not too bad.  Here's the sequence of events:

 

 

d-1.jpg (22283 bytes)

d-2-1.jpg (16150 bytes)

d2-11.jpg (21079 bytes)

d2-2.jpg (24816 bytes)

d2-3.jpg (26672 bytes)

d2-4.jpg (27093 bytes)

d2-4-2.jpg (16906 bytes)

d2-4-3.jpg (7447 bytes)

d2-6.jpg (8441 bytes)

d2-aa.jpg (17397 bytes)

d2-b.jpg (13124 bytes)

d2-c.jpg (13520 bytes)

 

return.jpg (5888 bytes)