The ratings don't factor in weather, or hills, or tight fairways, or anything else. Ratings just look at the current ratings of all the propagators and the scores of all the propagators, and new ratings are calculated off the averages.
Your rating is a numerical measure of how well you played compared to everyone else. If the rain hurt your score more than it did other players' scores, that will make your rating go down. If the rain had less of an effect on you than other players, you'll get a higher rating.
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