Is This The Angriest Sky You've Ever Seen?
I've shown this to a number of people only to be met with the same question - is this photoshopped? So to answer your question before you ask, it's a single image more or less straight out of camera.
I've never seen a storm come across the horizon so quick - it took less than four minutes. After I took this it was a mad dash to safety.
The ironic thing is that while southern Spain where this was taken was experiencing very unseasonal weather, Ireland was bathing in glorious sunshine.