In order to do this, you must work very fast as the sun is shifting constantly and it seems as if the most dramatic lighting is also accompanied by looming dark clouds and rising winds. Working from photographs would seem to make more sense. But I have come to realize, even while not experiencing the bugs, sun, wind, dirt and discomfort of working out of doors, that there is something completely different in the translation by the camera, computer and printer of what my brain and eyes are seeing. No matter what I've tried (so far), the photos I'm taking do not look what what I saw. The colors are different, the field of focus is different.
That's not to say I haven't gotten some really cool reference photos. A few weeks ago I took a walk down one of the local nature preserve conservation walks and came upon these scenes (among many others). And despite it being a fairly long walk down the gradually descending path, it may be worth hauling pastels and the easel down there to paint.