It may be tempting to take a heat gun to glass during the winter time to remove lettering or heat up the surface for application.  **CAUTION** DON’T, just like taking a warm glass and filling it with cold water will cause it to crack the same thing happens with a window!  Odds are you will crack it and make for a bad day.  Our advice, wait for a nice sunny day, even though the outside temperature may be low the surface temperature of the glass will likely be much higher than the ambient temperature, if this is the case you can probably apply your lettering without worry.