Saturday, December 7, 2013

Arctic Air Is Different

Under normal conditions the impact of elevation is to reduce temperature as elevation increases. Daytime temps in the summer, spring and fall usually decrease about 3 to 4 degrees for every thousand foot increase in elevation. In keeping with its mile depth, it is commonly 20 degrees warmer on the floor than at the rim of the Grand Canyon. It can snow like the dickens at the top while it rains at the bottom. Temperature differences are primarily why mountains are snow-capped when valleys are clear and mountain snows remain long after the seasonal snow melt finishes on valley floors.  

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). 
picture of mountain
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
1.6 PSI

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.


  1. I observed a microcosm of what you describe a few years ago.

    I was working along the coast in a petroleum coke stockpiling/shipping facility. The facility was surrounded by levees, including a hurricane protection levee, which were from 10' to 18' above grade.

    The ambient temperature for most of the area at dawn was around 35 degrees. Due to the nature of the terrain, and the weather conditions, the temperature in the "bowl" was at 28 degrees, which allowed ponding water to freeze.

    When I told others about my experience, they looked at me like I lost my mind. They couldn't fathom the dynamics of the event.

    1. Used to play golf on a course down along the Potomac on an ancient flood plain heavily forested on three sides, would be frost delays when the air temp was 37, 38 degrees a couple miles away, same phenomenon.