These 13 lines are from a prologue, you say. How far along is the work? Since this is a prologue, it may not have much to do with the plot or characterization.

One thing we look for in the first 13 is an indication the genre--fantasy, hard sci-fi, space opera, etc, but not necessarily in a prologue. Editors and agents depend more on the first chapter. This fragment mentions a character by a name that I take to be Indian (Native American) which gives a slight hint of fantasy in that tradition.

What are you looking for in the way of critique? Sense and feel of the opening? Does it hook a reader? Mechanics, wording, grammar and nits? Looking for readers of a larger sample?

The mechanics are good and I have no nits to complain about. You may hear about the passive voice in the second sentence, but I'm with you about that one. John hasn't been introduced yet, so there is no viewer to see the clouds. The prevalent to be verbs of the first three sentences are there for painting the scene and work well. Active verbs come into play soon enough.

Without a better indication of genre or the conflict John faces, I can't say that I'm hooked. A prologue doesn't need to do that, though.

The sense and feel of these lines came over well--so well that it brought back a memory. I was driving a jeep into a stream rather than riding a horse, but the effect of cold water was memorable. The rest of the imagery is vivid and well presented. I could easily picture the time of day, weather and the reluctant horse.