Glass top = no-no. Your evap rate is likely to be higher in the winter because you are running your furnace and the inside humidity is lower. It might slow down in the summer, but then of course you'll be running fans over the tank, which most likely will increase the level of evaporation.
I embrace evaporation! It gives me a chance to add limewater which stabilizes my CA & Alk. In the hotter months, it's the cheapest and easiest way to cool your tank. A nice sized fan blowing over your sump will do wonders for water temp via the miracle of evaporative cooling.
Anyway, your level of evaporation is by no means abnormal.