By Kathryn Prociv, Capital Weather Gang

Passengers with a window seat on the flight in the picture above were especially lucky on Monday, May 9 to observe one of Mother Nature’s most beautiful and unique cloud formations.

These are mammatus clouds. The term mammatus comes from the Latin word mamma meaning “udder” or “breast.” No surprise, as they are often referred to as “cow udder clouds” by those who see them for the first time. These types of clouds are some of the most ominous clouds in the sky and are often associated with severe thunderstorms.

Mammatus clouds can be found on the underside of the anvils of severe thunderstorms (especially supercell thunderstorms), resembling sagging pouches that are made up primarily of ice crystals. An individual “pouch” can range anywhere from ½ to 2 miles in diameter. A mammatus cloud field can stretch for dozens of miles across the sky.

Meteorologists aren’t completely certain how they form, but the most likely scenario is due to cold, dense air sinking toward the surface from aloft. This subsiding air “punches” through the anvil, resulting in the “pouch-like” appearance. The cloud droplets and ice crystals eventually evaporate, and the mammatus clouds dissipate.

Even though mammatus clouds themselves are harmless, they are often harbingers of a dangerous storm nearby, so if you see them in the sky, take caution. Since these distinct cloud types are associated with strong to severe and even tornadic storms, pilots are encouraged not to fly near them. Not only does it mean close proximity to dangerous storms, but the turbulence around them is likely extreme.

Severe thunderstorms produced up to egg-sized hail and 75 mph winds near Corinth, Texas, where this magnificent picture was taken. Corinth is a suburb of Dallas.

This article originally appeared on The Washington Post‘s Capital Weather Gang blog.