One of the instrument flyers will break cloud in a minute to tell you about the impervious horrors of icing in cloud, but the freezing level does not endanger your flight when clear of cloud and precipitation, even if you fly (visually) above it. Your calculations of true air speed will, of course, be affected by temperature. On a cold day in clear conditions, the aircraft leaps into the air, as its engine feasts on the scrummy air molecules and its wings squish them down onto Sir Isaac's head.
Edit: Tony came out of the clouds whilst I was still going around in circles in the sunshine.