In reply to nickinscottishmountains:
Sadly, won't keep you warm as a belay jacket...it's let's way too much air circulate.
Happily, a nano air will keep you comfortable whilst walking in or climbing when you're exerting yourself and it's colder.
Best jacket/mid layer I've ever bought...it does all the makers claim which is pretty much a first in my experience ...so good I bought a second.
Not bad looking either, brilliant hood under or over helmet and fits without a helmet too. Sleeves pull up well for dumping a bit of heat or to keep them out of the way. Repels rain and snow well and stays really comfy when pretty soaked.
Incidentally I have an arcteryx nuclei which is pretty good as a really lightweight belay jacket...and it was on sale at under £80 quid so even better!