My meteorologist friend, Micheal Erb, says:
"It's probably virga. Virga is rain or snow that's falling from the cloud but not reaching the ground. In this case it would be snow (it's very cold up there). It's not too rare, but still very neat to look at."
If you google "virga" you can find lots of cool photos, like this one here:
http://www.allthesky.com/atmosphere/virga-e.html.