I've flown diesels plenty of times with snow on the ground, but I won't pretend it's an easy or pleasant experience. Mind you, it's only ever the first start of a flying session that's been a problem in those conditions. You just have to be careful not to over-choke it, then flick as hard and fast as you can until you get a response. This can take some time with a stone-cold engine. If it kicks back (I once had my thumbnail ripped off!), you've over-choked it: back off the compression a quarter-turn, and keep flicking. Small diesels, up to 1cc., are the worst unless they're fitted with a spring starter: 2.5cc and 3.5cc, especially ball-race engines, aren't quite so bad.