In general, fly lines suited to warm climates are made with stiffer cores and harder coatings. Such specialty lines nearly always identify themselves by name–Tropic Plus, Bermuda Triangle, Flats Taper, to name only a few. These lines don’t really come into their own until you turn up the heat and they limber up a bit. I’ve yet to fish a tropical line that performs well in the colder waters of the Northeast—even in the dead of summer.