King Tor, Dartmoor
Using a polariser can help to bring contrast in skies like this but when shooting wideangle you have to be careful that the effect doesn't become uneven. In this case I darkened the polarised part of the sky and brightened the polarised part in order to reduce the effect.
This was shot handheld at ISO400 because I knew for this shot image quality wasn't an issue and I had to get a shot before the sun entered a hazier part of the sky!
These clouds preceded a front coming in from the west. It is often the case that just before and just after weather fronts you get high altitude clouds and if the timing is right you get amazing sunset skies.