©Stefano Padovan

Mammatus clouds,something very strange but easy to see in summer.It  happens when there is cold air and small ice crystals from the top of huge cumulus coming vertically down to the ground,Close to the surface they find hot and humid conditions so from ice they change into vapour.A very heavy condition that usally brings heavy rain and winds.Airplane pilots totally avoid this kind of clouds.

Here some explanations:

• http://en.wikipedia.org/wiki/Mammatus_cloud
• http://es.wikipedia.org/wiki/Nube_mastod%C3%B3ntica