Sitting here in the house, I just heard a pair of very, VERY loud helicopters going by. As I ran to the door to seen them, they were very low and clearly identifiable as US Military Chinook helicopters. I got this shot of the second one, and interestingly enough, there is something visible that I have never seen before. Just above the aircraft in this shot are vapor trails coming from the rotors as they spin and compress the air. The lower white arc extends from the rotor blade, while there is another just behind it (above it in the photo) from the pass of the previous blade's pass. How cool is that?
To see this phenomenon more clearly, click on the photo for a larger version.