"We looked for isolated rainshowers with just the right sized cloud and the sun had to be low enough in the sky with enough sunlight reaching the shower. We also had to position the plane so that the sun was on one side and the shower on the other. You can see the window reflections on the wing due to the sun being on the opposite side."


http://ceravoloimages.com/gallery3/atmospheric_optics/rainbows/chasing_rainbows.html