zero is usually my temp at which i quit... (won't see a daytime high of 20 until march!) sun shining, no wind, and zero isn't actually too bad. trick is you do not want the engine getting very cold, have it fueled up before you get to the field if possible, start it as soon as you can, then fly until your fingers are numb. I have gloves that have the index and thumb tips cut off, I hate flying in full gloves but having the tips exposed at least gives me some tactile feel and keeps a little more warmth in the hand. (having one of those chemical hotpacks in the gloves helps!)