Mammatus clouds forming over Squaw Valley ski resort, California, August 2010. Photographed by Matt Saal.
And over Monclova, Mexico, May 2004. Photographed by Raymundo Aguirre.
Other, equally curious examples can be found here. And yes, it even happens over Hertfordshire.
Mammatus, also known as mammatocumulus (meaning “mammary cloud” or “breast cloud”)
Someone call Dr Freud.
Because they are strongly reminiscent of breasts?
Who needs to call Dr. Freud are the French explorers who saw, then named, the Grand Tetons.
In energetic weather conditions producing thunderstorms, this type of cloud formation indicates a high risk of tornadoes.
Surreal Mammatus clouds over Hastings Nebraska. Time to take to the cellar.
http://www.dnr.state.ne.us/floodplain/mitigation/FEMA1590.html
Cumulo-Nimbus Mammatus
First stage of a tornado-generating cloud, BTW.