Mountains make weather

Filed in Expert Articles, General Information by on November 9, 2013

Tom Wiesen

I think it is fascinating to observe mountain weather because it is so variable. Have you ever noticed how some areas are piled deep with snow, while other areas have melted or blown dry? What factors affect this distribution of snow?

Considering the lay of the land helps us to understand why it snows here in the first place. To the west of Colorado lays vast arid lands. Relatively warm air moves in from the west and runs into the high mountains. The warm air is forced up and rapidly cools which results in condensation. Cold air is less capable of holding moisture than warm air and precipitation soon falls from the sky in the form of rain or snow.TWG_story_gen08img

The western slope of the mountains receives the bulk of the moisture from passing storms. As storms move over the highest ranges, a precipitation shadow forms to the east of these land masses and the climate is super-dry.

For instance, right now you could hike to the summit of 14,000 ft. Mount Princeton with just boots and gaiters. Eastern slope mountains are much drier, and are often windy and consequently hold little snow.

Meanwhile the snow depth at 10,600’ Vail Pass is over six feet deep. What causes this concentration of snow? In Colorado, as the weather comes from the west, not only is it forced up rapidly by the mountains, but it is also funneled up major river valleys which run east/ west.

Major rivers such as the Eagle, the Roaring Fork, the Yampa, and the San Juan all concentrate weather systems and then stall them out as they reach the heads of these valleys where high mountains jut up. The faster the weather is forced up, the more dramatic is the affect on precipitation.

Eagle County is blessed with two major mountain ranges on either side of the Eagle River that funnel our weather. The Gore Range borders the north with sharp and dramatic peaks rising above 13,000 feet. Meanwhile the lofty Sawatch Range runs east/ west along the southern edge of Eagle County eventually rising above 14,000 ft and boasting the highest peaks in the state in neighboring Lake County.

Snow depth typically increases as the elevation gets higher. However, once above tree line strong winds often scour the windward mountainsides, and then deposit snow in the leeward areas. This is why you can look up at high peaks and notice that some areas are bare rock while other areas form dreamy lines for backcountry skiers.

You may also notice that some areas melt completely dry when we have a sunny period without snow. These south-facing slopes often hold shrubby plant communities where wildlife such as elk, bighorn sheep and mule deer thrive.

Meanwhile just across the valley on the shady slope, the snow may be over four feet deep. This is an example of a local microclimate. Sunny slopes are far warmer than shady slopes and oftentimes a sun-crust will form on the sunny snow surfaces, while neighboring areas remain dry and powdery in the shade. Experienced skiers head for the shaded areas to find soft powder after days of sunshine.

Now consider the notion of a precipitation shadow. Examples of this are evident on the eastern side of the Sierra Nevada, on the eastern side of the Cascades, and to the east of the Rockies. Storms drop the bulk of their moisture, lose momentum and break up after they pass over the highest areas. While ski areas on the western slope start at around 8000 ft. elevation, east slope ski areas generally are higher in elevation to have ample snow.

Consider how precipitation amounts affect plant communities. In arid moderate elevations, junipers, pinyon, and sagebrush thrive. As elevation increases and precipitation increases, we get Douglas fir and aspen. At the highest and wettest elevations, thick subalpine forests are dominated by lodgepole pine, Engelmann spruce and subalpine fir.

On the drier eastern slope, some unique trees thrive that hardly occur on the western slope. For instance, high and dry windy ridges often support limber pine and bristlecone pine. East slope elevations below 10,500 ft. are often dominated by ponderosa pine. These trees require a dry climate and do not form a thick forest with a closed canopy, but are more spread out with lots of space between trees and form woodlands.

Mountain weather is complex and mysterious at times. While our region has been hammered with consistent snow, other areas in the state remain dry. Deep snow benefits skiers, whitewater enthusiasts, farmers and ranchers. Water is the source of life in this arid west, and we should be thankful that this season’s weather pattern has favored us.

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