Lanzfeld
Here's a method that works fairly well under various circumstances (rain, fog, darkness, etc.).
http://www.subsim.com/radioroom/showthread.php?t=110619
It might look hard at first but if you play around with it long enough you'll find out that the concept is actually quite easy.
I'll look around for the old bink video for this post if you want. Somebody else may have it already uploaded on a file sharing site.
cheers