 In (5.12), we understand that John's looking at the sky temporally overlaps the presence of the dark clouds in the sky (i.e., the dark-cloud situation at least contains the end of the looking event). At a deeper level, we also understand that John perceived the sky to be dark with thunderclouds, and naturally assume that John took the clouds to be harbingers of an impending storm, as we ourselves would. In (5.13), the two clauses appear to report successive events and furthermore, the first event is understood to have led causally to the second—John's decision was driven by whatever he saw upon looking at the sky; and based on our knowledge of weather and the function of umbrellas, and the fact that “everyone” possesses that knowledge, we further infer that John perceived potential rainclouds, and intended to fend off any rain with his umbrella in an imminent excursion.