Most of the time I am not lucky enough to have a camera on hand when rainbows appear.
To the left you will find a picture I took of the Skyventure Arizona tunnel yesterday afternoon, after a rainstrom passed thorough the area. In order to track my progress in shooting this phenomena, I have added another image to this post which I took back in March of 2009 at Skydive Elsinore.

I shot the image of airplane with a Canon Rebel XTi set on RAW, focal length 25, FNumber 11, exposure time 1/800. Whereas the photo of the tunnel was shot with my Canon 7d set on L, focal length 21, FNumber 22, exposure time 1/80. Both images received some post work in Adobe Photoshop Elements.