No idea how this was done but it's done well… This post has been reshared 1 times on Google+ View this post on Google+ « 2 flash flood warnings An even better rain weather app….. Checkout raindar….. »
Petapixel describes the process of that here, very interesting and it seems pretty time consuming as well! http://www.petapixel.com/2012/12/01/five-hours-of-airplane-landings-captured-in-thirty-seconds/#more-91275
Painfully time consuming… I do not think I am patient enough