Links about these unusual clouds appearing all over the world and this has been going on for sometime but like many I did not know about this until someone posted here on FR about the one that happend Feb 22 over Mexico city - will find that link to that thread and will post it or you can do a search with “cloud” as a key word and you will find those threads.
http://www.youtube.com/watch?v=FXF9HSB627U
http://www.freerepublic.com/focus/f-chat/2456721/posts
Thread about the Mexico city cloud of early this week with photo...