Clouds and Fog

In Rocky Mountain National Park, a change of weather can be just a short walk away – which is why it’s so important to be prepared for any kind of weather. Just as boulders in a stream direct the flow of water over and around, so mountains direct air masses.

When air is forced up and over a mountain ridge, it cools. Cool air holds less moisture than warm air, resulting in cloud formation. If the air is unstable, we’ll see large, puffy cumulus clouds form as the air rises – or even a thunderstorm. If the air is stable and the wind flow is strong, we’ll see lenticular clouds – also called lens clouds, cap clouds, or “flying saucer” clouds. These wonderfully unique clouds are common along the eastern slopes of the Rockies, especially in the wintertime when high winds aloft crest the continental divide and then sweep outwards towards the plains. Unlike other clouds that are blown along by the wind, lenticular clouds do not move because they form as the air rises and dissolve as it descends in the "bounce."

Differences in the heating and cooling of mountain slopes also affect cloud formation. Solar radiation increases with altitude (which is why sunscreen is so important when you hike in the mountains). Surfaces exposed to the sun rapidly heat up, and those not exposed stay cool. This can result in dramatic difference between sun and shade. In some areas the difference may be as great as 40 -50°F. On sunny days, upslope air flows develop due to this differential heating. These rising plumes of air are especially noticeable in the summertime and are responsible for our afternoon thundershowers. At night, the mountains cool quickly because of the thin, dry air. The cool air descends the slopes forming drainage flows that accumulate in valleys. If moisture is present, this cool, sinking air can cause local night-time valley fog.