Saturday, December 20, 2008

Tuckerman and Huntington Ravines on Mt. Washington

Image courtesy www.washburngallery.org

Curiously, the western side of Mt. Washington is devoid of glacial cirques. In fact, it has noticeably less developed topography, with nearly all the runoff from the mountain’s alpine zone draining off into Ammonoosuc Ravine, a massive V-shaped valley that plunges several thousand feet to the foot of the mountain. Its shape stands in dramatic contrast to the cirques on the mountain’s opposite side.

The discrepancy is caused by the prevailing western wind in the middle latitudes, which dramatically affects snow accumulation in the region. The White Mountains are very wet. New England’s climate is generally humid, and like all mountains, the Whites block clouds as well as wind, leading to a regional orographic precipitation effect in which water vapor cools as it moves up mountainsides, eventually forming droplets of rain when its temperature reaches the dew point. The mean annual precipitation observed on Mt. Washington (averaged between 1971 and 2000) is just shy of 102 inches per year (www.mountwashington.org/weather/normals.php). The average temperature atop the peak is 27.2 degrees Fahrenheit, so precipitation on the summit falls as snow rather than rain for more than half the year. In fact, the record snowfall for one season is an astonishing 566.4 inches, in the winter of 1968-69 (snowfall’s liquid equivalent is used to calculate annual means and for comparisons with the summer months).

With such prodigious snowfall, one might expect to find large accumulations of snow on Mt. Washington’s heights, and indeed, where sheltered, deep drifts do form. In general, however, the west wind is so strong that it blows the snow off the ridges into its eastern ravines, where an enormous amount of powder builds up. Snow is so deep in Tuckerman Ravine that it often does not completely melt until August, two months after the last of the snow has disappeared from the rest of the mountain. By April, however, it is packed with skiers, for whom “Tucks” has the best backcountry skiing in the region. The glaciers that once filled the cirques grew as a result of the wind’s scouring the ridges, as well. While the eastern ravines were originally shaped like Vs due to erosion from runoff, glaciers formed in them during ice ages because windblown snow accumulated to such a depth that the pressure at their base was strong enough to turn snow into firn and firn into ice. This process continued feeding the glacier, which dug out and enlarged the ravine into a cirque until temperatures rose high enough that ablation exceeded accumulation. The last episode of glaciation ended about 13,000 years ago, towards the end of the Pleistocene (Slack & Bell, 17, 1995).

Ammonoosuc Ravine, on the western side of Mt. Washington, maintained its V-shape because it was never subject to glacial erosion. Relatively little snow built up in its confines, so liquid runoff continued to be the main agent of erosion. Winds on the mountain are usually highest there, however, so the region is fully exposed and notorious among hikers as a setting for death due to hypothermia.

Ecological Effects

The quantity and variety of wind’s effects on the flora and fauna of the Presidential Range is enormous. No book, let alone a paper, could enumerate all the strategies plants employ to gain shelter from the wind and retain heat. But in describing how wind has helped create an alpine zone on Mt. Washington, a few of these techniques come to light.

Treeline is influenced by many factors, not all of which are climatic. Wind, temperature, and precipitation are usually primary causes, however, particularly in alpine environments. On Mt. Washington, wind can be so strong that it strips a tree from the ground, tearing up its root system, or shears off its needles so that the tree can no longer photosynthesize. In the krummholz (the word comes from German, meaning twisted wood), the mountainside belt just below treeline, spruce and fir trees are gnarled and stunted by the wind. Winter winds often carry ice fragments, which scour the branches, particularly on the windward side of the tree. If the wind is steady, the conifers lose all the needles on one side, giving them a flaglike appearance, or, if the lead branches survive while most other branches die, the tree will eventually resemble a broomstick

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