My dad, God rest his soul, used to take pleasure in pointing out these cloud formations when I was a teen, calling them alto-cumulus mammatus, and then asking me to see if I could guess why they had that name. Ha, ha, adolescent fun for all.

Anyway, if you see them, they predict rain, and form when larger, heavier droplets begin to fall down from the layer of cloud above.

These dramatic examples were photographed over the Olympic Valley, California.