The explanation lies in air pressure.
You may already know about the relationship between temperature and pressure: When you pressurize air (or any gas), it gets hotter, and when you release the pressure on air it gets colder. So a bicycle pump gets hot when you pump up a tire, and a spray paint can or a C02 cartridge gets cold as you release the pressurized gas. A refrigerator puts both of these processes together, pressurizing gas on the outside of the refrigerator to release heat and decompressing it inside the refrigerator to absorb heat (see How Refrigerators Work for details).
John Foxx/Getty Images
Lower pressure at higher altitudes causes the temperature to be colder on top of a mountain than at sea level.
You may also know that air pressure decreases as altitude increases. This table shows the pressure (in pounds per square inch) at different altitudes:
Air Pressure Sea level 14.7 PSI 10,000 feet
10.2 PSI 20,000 feet
6.4 PSI 30,000 feet
4.3 PSI 40,000 feet
2.7 PSI 50,000 feet
As air rises, the pressure decreases. It is this lower pressure at higher altitudes that causes the temperature to be colder on top of a mountain than at sea level.
Lower elevations are subjected to greater pressure because more air is stacked on top; higher elevations have less and thinner air above.
But arctic air masses are different. Locally, the Bridger Bowl ski area rises 2,700 feet from the parking area to the ridge on top of the Bridger Range. Normally temperatures drop 10 to 12 degrees going from the base to the top. Not so today.
Earlier this afternoon the ridge temperature was 11 degrees higher than the base. The base temperature was -13 F while the ridge topped out at at -2 F.
Convection has taken over. As anyone can attest by waving a hand over flame or a boiling pot, hot air rises. Cold air falls because it is heavier and less active than hot air. These reversals are typically localized and most extreme in canyons, stream bottoms and off of steep ridges, areas that are low and protected from wind. High wind speeds have a mixing effect and counteract the tendency of different temperature layers to maintain separation. An important contributing factor is the presence of an Arctic high pressure system, where winds from the outside whirl in to compound air pressure (and the weight of air) in the center.
Such is the situation we have at Bridger Bowl today. The winds are light. The topography is severe. And a strong Arctic high pressure system has settle in on this side of the Divide. Skiers at the top can bask in the relative comfort of near zero temperatures, such as it is.
Below, is a web cam view of life on the ridge, Saturday, December 7, 2013 with temperatures hovering around 0 degrees. I hope the fellow standing on top was dressed warm, very warm.