Joseph, Probably from here you'd have to resort to the torch to get much better results. The iron you're using should be fine if it weren't for the cold and wind. I've tried to think of some good ways to counteract this factor, but nothing really strikes as being a reliably good option. If you wished to try it, depending on just how cold and windy it is there, you MIGHT be able to get the edge you need by wrapping the gutter near the joints with rags soaked in hot water, but I doubt the heat from this would last long enough to make it feasible. The other option I considered was having a second person running a heat-gun in the area while you worked the joint. Have used this method recently soldering stainless bar/restaurant type sinks and it worked well.