Who doesn't like to look at a rainbow?  Usually they are formed by the the sun refracting in tiny water droplets in the air, typically after an afternoon rain shower.  But there are other and rarer rainbows. 

One very unusual one is formed when the sunlight is refracted by the tiny ice crystals in cirrus clouds, always above about 20,000 feet.  This is a very unusual phenomenon of weather not seen by many.  Note the contrail from a high flying jet, probably about 39,000 feet.