have storm windows made for each pane. They can be installed in the cooler months with simple turning of a wing nut ( or maybe fancier hardware) and stored in the summer. Using storm windows can increase your windows efficency to AS equal quality to new double pane vinyls because it is the spacing between the glass that is the insulator. I did this research and try looking that up before removing an architectural detail from your home and overspending on new windows you probadly don't need.

Step 1: Wait for a breezy night. Moving a candle around the windows the flame will indicate ONLY approximately where the drafts are located at.

Step 2: Install exterior storm windows as mmonika suggests above. This will protect the wood windows from receiving the brute force of the outside weather elements (like rain for example)

Step 3: Repair leaks (if any) that were found in step #1