Mammatus clouds
© Karen Winship/ York Weather StationAwesome Mammatus clouds over Clifford’s tower in York.
These spectacular mammatus clouds engulfed the sunset sky of York, England on January 7, 2015.

Watch some amazing pictures and learn more about their formation below.

Mammatus are pouch-like clouds that protrude down from the bottom of a thunderstorm's anvil cloud.

Mammatus clouds
© Russ @Alien_OrificeAnd with the light of the sun setting, these weird clouds are become magical.
The storm's anvil consists of ice crystals but, as it spreads out at the top of a thunderstorm, some of the ice begins falling and cools the clear air below.

Despite popular misconception, mammatus clouds usually are an indication severe weather has passed and thunderstorms are weakening.

They are not a sign that a tornado is about to form.

Mammatus clouds
© Hilary HoweThe mammatus cloud is generally poorly understood.
However, mammatus clouds sometimes form on the upwind, or backside, of severe thunderstorms that at the same time are dropping large hail and unleashing 60-70 mph bursts of wind.

Mammatus clouds
© Craig BloodworthLike big udders or breasts forming in the sky.
Storms have to have wicked updrafts of rising air, probably moving at 75-100 mph, to spread an anvil into the steering winds in the first place as a thunderstorm's anvil cloud naturally and easily spreads with the wind.

But, in most cases, mammatus can be found beneath the anvils of dying thunderstorms.

Here a good scientific article about the mysteries of mammatus clouds.