You're right about the "warm nose" you sometimes hear us refer to. That's when the warm air overruns the cold air at the surface. This creates a problem in the winter because tall clouds will spit out precipitation beginning as snow high up. Then, it will melt as it descends through the warmer air mass (the warm nose), and depending on temperatures below that layer it may be able to refreeze before hitting the ground (sleet) or refreeze upon contact of the ground (freezing rain).
Why do we call it a nose? Well in diagrams that we use to gauge the temperature profile of the atmosphere, it just LOOKS like a nose. Check it out here: http://okfirst.mesonet.org/train/materials/Winter/zr-sounding.jpg
The cold air is at the surface, then the "nose" looking area is the warm air, with constantly falling temperature above that like we would typically expect.
Thanks for the question!
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