Wednesday, August 10, 2011

Lessons in Lapse Rates

Meteorologists define lapse rate as the rate of decrease in temperature with height.  There is perhaps no place or time in the western United States to get a first-hand feel for lapse rate than Great Basin National Park, which has to be the best kept secret in the National Park System.  

Great Basin National Park contains the Snake Range, which rises abruptly out of the deserts of Utah and Nevada to as high at 13,063 feet at the summit of Wheeler Peak.

Wheeler Peak (center) and the Snake Range of Great Basin National Park
One can drive up a spectacular road from the tiny town of Baker, NV (elev. 5300 ft/1615 m), up the alluvial bench of the Snake Valley, and then into the Snake Range to an elevation of about 10,000 feet at the Wheeler Peak campground.

This puts you near the base of the spectacular, glacially carved alpine ridge of Wheeler Peak.

A 4 mile hike then puts you on the top of Wheeler Peak, nearly 8000 feet above the surrounding desert.

Looking from near the summit of Wheeler Peak,
northeast down Lehman Canyon toward the Snake Valley
Looking from near the summit of Wheeler Peak, northwest toward the
Spring Valley
Thus, our trip up Wheeler Peak brought us from the searing heat of the desert to the cool alpine of the highest peak in Nevada.  As can be seen in the Elko sounding, temperatures at valley level were near 32C (Elko's 1608 m elevation is very similar to Baker's 1615 m), 15 C at the Wheeler Peak campground (~3000 m/700 mb), and 3C at the top (~3982m/600 mb).  That's a remarkably large mean lapse rate of ~12C/km.

Typically in the atmosphere the lapse rate does not exceed the dry adiabatic lapse rate of ~9.8C/km.  When the lapse rate is this large, density is constant with height.  Any larger and you have lower density air beneath higher density air, and buoyancy driven turbulence (e.g., thermals) tends to mix the atmosphere to a lapse rate that is dry adiabatic.

A larger mean lapse rate exists in the Elko sounding because the layer near the Earth's surface is superadiabatic (i.e., greater than dry adiabatic).  Such layers are often found in the afternoon when intense surface heating warms the atmosphere faster than the thermals can mix it back to dry adiabatic.  They are very common over the Great Basin.

The temperatures we experienced on Wheeler Peak were, in general, somewhat warmer than found in the Elko sounding.  For example, we observed 21C at the Wheeler Peak campground at about 2:00 PM MST.  This higher temperature reflects surface heating, which typically leads to somewhat higher temperatures over mountains during the afternoon than in the free atmosphere.

The lapse rate isn't always large.  Sometimes the lapse rate is quite small or even negative (i.e., temperature increases with height), as is the case when an inversion is present.  Your mileage may vary, but during the summer in the Great Basin, you can usually count on an afternoon lapse rate ~9.8C/km.

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