The reason the rainbow is typically a better image is because it is usually a symbol of good luck and if you are christian, a reminder of God's promise never to flood the earth so devastatingly again. When a rainbow apears, there is a lot more sunlight, because that is what is needed for it to be created. The components of a rainbow are direct sunlight and water droplets (i would like to say the water has to be going at a certain angle)
