Airplane Rainbow Circle
Credit: Getty Images/iStockphoto

And some fliers have noticed another phenomenon that happens on cloudy days outside of the airplane.

When you're flying above the clouds, catching the plane's shadow on the floor of clouds below you is not an uncommon sight; you just need to be sitting in the right place at the right time. But that's not all you'll get, if the situation is right: Sometimes you'll also spot a faint rainbow halo surrounding the shadow.

So why does that happen?

The halos are called “glories,” and they occur when the sun is directly behind your head and there's a cloud directly in front of you.

EarthSky explains that the glory must intersect with an anti-solar point, or a spot that “faces the opposite direction to the sun,” according to

The sun's light scatters among droplets in the cloud instead of on falling rain (which is what causes the arched rainbow shapes we're used to seeing from the ground).

You can also spot a glory from tall buildings, hot-air balloons, mountains, or any other spot that can get you above a veil of clouds.