Weber Clouds

Weber: Clouds form on horizons, but what follows?
By William A. Weber Sunday, October 21, 2007

Chris Brown Photography
Chris Brown Photography

Cloud formations are one of Boulder’s most interesting and beautiful environmental features. When the clouds come down below the summits of Eldorado Mountain, Green Mountain and Bear Peak and cover the upper half of the Flatirons, we know it will soon rain in town. When a smooth-capped mass of clouds forms on the western horizon, we know that Chinook winds will blow soon.

In 1961, I helped a university Natural Areas Committee to explain the scientific values and need for preservation in its natural state of the Flatirons-Mesa trail area. Many folks have forgotten what these values were. It is time for us to remember.

The abruptness of the outer foothills creates locally moist microclimates on the east-facing mountainsides partly because of the deep shade afforded by the cliffs and the steep ravines that face northeast. In addition, however, the great height and steep faces of the Flatirons have the frequent effect of stimulating the formation of a cloud veil just in this area, when warm moist air from the southeast meets with colder air from the west.

To the west of the Flatirons-Mesa Trail area, this cloud veil is usually absent. The localized effect of the cloud veil is to raise the relative humidity on the slopes of the Flatirons. This in combination with the more shaded, steep slopes provides localized relatively moist microclimatic conditions.

These more moist conditions provide unique habitats on which many plants not ordinarily suited to our dry climate can survive and reproduce. The effects of this moist situation are reflected even on the back side of Green Mountain and Eldorado Mountain.

The steep gradient of this area has caused a telescoping of the flora and fauna belonging to a great range of habitats, altitudes and temperature tolerances into a relatively small area where they live side-by-side. Because of the steep north and south slopes, this actually results in some of the higher-altitude life forms occurring lower on the north-facing slopes than the low-altitude forms on the south facing slopes. This situation provides unique settings of extraordinary aesthetic and scientific values.

One curious and unique fact about the flora of this strip is that a number of plants typical of the Deciduous Forest Region of Eastern North America occur in very small and scattered stands. On the back of the Flatirons is the only stand of paper birch in Colorado (and the southernmost one in the United States). This is true also of plants such as the hazelnut, wild sarsaparilla, carrion flower, bird-foot violet, wood lily, rattlesnake fern, dwarf creeping raspberry, yellow lady-slipper and several extremely rare orchids.

Other plants in the Flatirons-Mesa Trail area seem to be “living fossils,” relics of the time long ago when the redwood forests were present over all this region. Some relic species are the Oregon-grape and several of the yellow-flowered parsleys common in the lower foothills and largely restricted to this area.

It is the many spots of unusually moist microclimatic conditions that certainly account for the presence of these outpost or relic species of plants in the Flatirons-Mesa Trail area. During the Pleistocene ice ages, when the local climate here was probably cooler and moister, there was apparently a westward movement of the eastern flora along the moist drainage systems as well as an eastward movement of Rocky Mountain species out onto the plains, resulting in the meeting and mixing of these elements along the base of the Front Range.

This wild mixture of unusual geographic vegetation types has all but disappeared in Colorado except for a few moist sites along the Front Range, of which the one represented by the Flatirons-Mesa Trail group is probably the best still remaining.

It would be a real tragedy if these last remnants were destroyed through our ignorance. Filling in the details of their climatic origins and potential for survival in an urban setting would enhance appreciation by the Boulder community of these unique gifts of nature to our open spaces

William A. Weber is a professor emeritus at the University of Colorado at Boulder.