Camping on the shores of Llyn Idwal one night. Clear apart from cloud on the horizon somewhere. All still and silent, then suddenly there’s a roaring sound approaching fast. In a second it slammed into the tent shaking it a lot, then roared off down the hillside. From that point onwards there were a few more gusts of wind which gradually built into howling gales and rain that blew us off the mountain at first light after having had no sleep 🙂
It was quite scary since it sounded like a solid object about to crash into us – rockfall, crashing plane etc – certainly had no time to move, only had just enough to think ‘am I a goner here?’.
I did find out what it probably was later though – a kind of rolling vortex that comes down steep mountain sides in certain conditions – but I’ve forgotten it’s name!