I believe the "storm window" dates back the time of DC3 and such. When flying through stormy or icy conditions, the front windows of said aeroplane would become very hard to see though due to build up of ice or rain. The pilots could then open the side window in order to see though when the vis got really bad (The side window on a DC3 is much larger that that of a light GA aircraft). Hence the side opening window became know as the "storm window".