Dry air undergoes greater temperature fluctuation than humid air, since water cools and heats more slowly than air. The temperature difference between midday and midnight can be as much as 50 degrees, and a small cloud obscuring the sun can make you pull on a jacket, even in summer. Two other local weather anomalies are inversions and orographic effects. Inversions happen as cold air drains down mountainsides to pool in the valleys. These cold-air pools can become trapped beneath a layer of warmer air.
Inversion is why West Yellowstone is often the coldest spot in the nation in winter.
Trapped beneath a warm air blanket that acts much like a ceiling, pollutants can't escape as they do in summer. Air climbing over a mountain range cools. Because cool air can't hold as much moisture as warmer air, suspended water vapor forms into droplets. This creates orographic effects such as the cottonball clouds that form over mountains. Valleys in the east side of mountain ranges can be dry and sunny while rain or snow falls just over yonder.