Clouds are fascinating no matter what their shape, and it’s always easy to lose track of time while watching them from the ground. They always seem to be making whimsical shapes and shifting throughout the day.

But some atmospheric happenings are rarer than others. Take these amazing clouds, for example.

These are mammatus clouds.

They look like little cotton balls hanging out in the sky!

The name comes from the Latin word for “breast” or “udder.”

So yes, these are boob clouds.

They give the sky a bubbly, popcorn-like texture.

Bubbles, cotton balls, and breasts are all friendly, round things, but mammatus clouds are usually associated with severe thunderstorms, and even tornadoes. They form on the bottom of massive cumulonimbus clouds.

There are several types of mammatus clouds.

They can look smooth, jagged, translucent, or opaque. It all depends on atmospheric conditions.

Right now, there are several theories about how these clouds form.

Though they vary in the mechanical details, most of the theories have to do with the meeting of warm and cool air. Different conditions produce different mammatus clouds, ranging from dense clusters of bubbles to single bubbles spaced out over the open sky.

Conditions must be just right to get this kind of formation.

Typically, aviators avoid flying if they see mammatus clouds because of their association with storms, but it looks like this plane had no choice but to brave them.

The clouds can be dense and fluffy, or light and sparse.

This looks almost like a cozy quilt.

But they’re always impressive.

(via Twisted Sifter, Wikipedia)

If you ever see clouds like this, take a picture and marvel at the many strange forms that nature can take — and then get inside, because it’s probably going to rain.