What causes rainbows around planes’ shadows?

Dear Tom,
I was flying back into Chicago and while landing I saw the shadow of my plane completely surrounded by a round rainbow. What causes this type of action?

Christina Mocha
Grand Beach, Michigan

Dear Christina,
The circular rainbow that you saw is called a “glory”. Glories are visible when sunlight interacts with small cloud droplets that reflect and refract the sun’s rays directly back toward the sun. They are only visible around the shadow of an object in the direct path of both the sun’s incoming and reflected rays, in your case, the plane’s shadow. Before air travel, glories were only visible to people on sunny mountain peaks, viewing them around the shadow of their head on a cloud or fog bank below. A glory’s colors are the same as a primary rainbow’s: red on the outside and violet on the inside.