A good many works by Curtis are of the contre-jour kind, where the light source (the sun, in Curtis' images) is behind the subject. Normally, artists have the light source behind the painter or towards one side or another, illuminating the subject directly or from an angle. James Gurney discusses contre-jour painting here.
Needless to say, to be an effective contre-jour painter, one must have a very good color sense. This Curtis has. He also has a feeling for making strong, interesting compositions.