Just the other day we had this same question.(Must be getting cold everywhere.)
Install a sliding storm door. They work great and won't spoil your view.
Coastal Industries in Haverhill, MA makes them for one but, I'm sure you can get them all over the country.
I can't recall thier web site this minute so , if you go to the top of the page and enter "Jason" in the search box and click on "user name", it will bring you to all the posts I've made. Scroll down and you will see one called "Get a sliding storm door". It has all the info there.