At this time of year, on the north side of the block, we get these clouds of very small flies. They fly in your nose, eyes, hair, etc and congregate in circular clouds. I have not been able to take a pic but this one represents them well: http://www.flickr.com/photos/foto_buff/147961711/ . People get caught in the clouds as they walk down the sidewalk. When they congregate in the front yard, I blow a cloud of cigarette smoke and they tend to go higher. People refer to them as "May Flies" as they come out in May but that is a very different bug. Any ideas?
CLOSED: Cloud of very small flies
Most likely non-biting midges in the family Chironomidae - at times, these can occur in astronomical numbers...