Climb Into the Cold
Any of us that have hiked the peaks in the Northeast know one thing. The higher you climb the colder it gets. A few weeks ago I was snow shoeing in the Green Mountains of Vermont. When we left the valley floor to drive up to the trail head it was a balmy 20 degrees F. As we put on our gear I noticed that the temperature was only about 15 degrees. We’d only traveled a few miles and climbed just a few thousand feet in elevation, but that was enough to feel what is called the environmental lapse rate of temperature.
The decrease in temperature with elevation is called the environmental lapse rate of temperature or normal lapse rate of temperature. The normal lapse rate of temperature is the average lapse rate of temperature and is generally considered to be about 1.8o F per 500 feet in elevation. But the environmental lapse rate of temperature is the actual vertical change in temperature on any given day in any given place and can be greater or less than the average.
Recently, researchers from Yale and Dartmouth wanted to know what the lapse rate might be in the northern Appalachians. They erected weather stations at low, mid and high elevations on three mountains – Whiteface Mountain in the Adirondacks, Mt. Mansfield in the northern Green Mountains and Mt. Moosilauke in the White Mountains. The stations logged data from 2000 to 2002 through rain, snow, heat and cold.
As expected, mean annual temperatures had a direct relationship with elevation. On average, temperature decreased at a rate of 1.5 F per 500 feet in elevation. There was a clear diurnal lapse rate pattern, with lapse rates during the day generally being much stronger than those at night.
The location of the alpine treeline is thought to be driven by growing season air temperature. Mean July air temperature at treeline was 55 F on both Whiteface Mountain and Mt. Moosilauke, and 57 F on Mt. Mansfield. Treeline in western North America correlates with a 50 F July isotherm.
Treeline is apparently at a lower elevation than would be expected from air temperature alone in the Northeast. This is especially true for Mt. Mansfield, where it is located at an elevation over 650 feet lower than on the other mountains they studied. Other factors such as wind and blowing snow and ice scour could also be affecting treeline. These trees live a long time and once established, they are resilient to environmental change. Treeline represents a long-term average marked by decades or centuries of climatic history. Because of this lag time, when there is an upward trend in temperatures, treeline will be found at an elevation lower than expected.