Dress for cooler temperatures and be prepared for sudden weather changes. Temperature t's colder at higher altitudes. Although in summer you may be quite warm in the middle of a sunny, still-air day. Layers of clothing are a good idea. The exact change is a bit difficult to calculate as it's affected by such things at the time as humidity, the sun's radiation, condensation and convection processes.
Dry air loses temperature twice as rapidly as damp.
Actual temperature loss ranges from 2.5 to 5 F per 1,000 feet. If you're in the San Juans the top of Imogene Pass might be 10-15 F cooler, and usually much more windy, than it was in Ouray. Just M rren on June 10, 1990. Bear in mind that summer means little atop a tall peak. M rren has a climate closer to Trondheim, Norway than Bern, Switzerland.