FRISCO — Slowly, almost imperceptibly, summer is giving way — but not without a fight. Some years, a sudden cold snap at the end of August, often accompanied by a dusting of snow on the peaks, brings a sharp reminder of the fact that we live at 3,000 meters elevation. But not this year. Instead, the flow of moist, subtropical air out of the southwest has persisted, bringing moist relief to the forests of the Colorado high country. You can almost see the young trees drinking up the water. When the monsoon eventually ends, it’s going to be a rude awakening, at least for photographers who have been enjoying the rich play of clouds in the sky nearly every day.