Thanks for the link about gravity waves, Phil. I heard Steve Lyons mention that last night, too. The explanation on the web site was a wee bit over my head (by "wee" I mean "a whole lot"), but I think I've got the idea.

Am I correct in assuming that gravity waves happen all of the time, but it takes a combination of great power/energy and a very stable atmosphere to make it visible like in the satelite images yesterday?

As an aside, I've enjoyed learning from you all on this site for a for about a year now and am posting for the first time. Cane Watcher (the guy who asked the gravity waves question) clued me into this site during Isabel last year. Keep up the excellent work!

Brian