We arrived at the base lift at Jackson Hole one morning and were greeted by a sign reading, "Temperature, -20. At the top of the lift, 0. At the top of the next lift (the top of the mountain) +15."
Not every day is like this, but they happen several times each winter in Jackson. This is the weather phenomenon called a temperature inversion, known around the Intermountian West simply as "The Inversion".
Normally, wind stirs the atmosphere, making temperatures drop as altitude increases. When the winter wind stops, weather turns upside down and colder, denser air sinks to the valley floors while the warmer air rises to the top of the mountians. I saw it happen this morning.
Our house is at the bottom of a mountain with Deer Valley Ski Resort on top, 4,000 feet higher than our house. There is a recording thermometer at the top of Deer Valley on the Internet. At the top of the mountain at 8am yesterday, it was 4°. Went up to 15° around 2pm, then down to 5 at 9pm. After 9pm, the temperature rose every hour until 8am this morning when it was 19° on top and 1°at our house. The Inversion was back.
Over the years, we’ve made about 10 ski trips to Jackson, and caught inversions two to three times. They were all the same—frigid in the valley (town), but pleasant on the mountain, though cold. The Inversion is created by large high pressure systems which produce sparkling, cloud free skies and brilliant sunshine in the mountians. Some valleys, like Salt Lake City, turn into cold, smog-choked hell holes. Jackson doesn’t yet have enough polluting cars to get too bad on pollution, but it will be well frozen in the valley, where all lodging is.
We have also run into The Inversion in Bozeman, MT, (Big Sky) and a god awful Inversion week in Salt Lake City where we had to drive with headlights on to see the road at noon. But in all, go up the mountians to ski, and it gets warm and sunny.
By far, The Inversion is at its worst in Salt Lake City, where too many cars create a foul smelling—you can actually taste it—choking smog which the Inversion traps in the valley. The Inversion may sometimes be seen at work by watching the smoke from a chimney. The smoke plume will rise for a while, then it stops going up and spreads horizontally. As the air gets warmer with increasing attitude, the smoke which started rising at the surface because it is hotter than the outside air, soon reaches air that is hotter than the smoke, so it can’t go up any more.
Salt Lake City’s Inversion creates a dense blanket of smog, looking a lot like a cloud that hangs over the valley. At the ski resorts above the smog line, there won’t be a cloud in the sky. One day, we were in a really bad smog cloud that reached halfway up the first lift at Solitude Ski Resort. We got on the chair in a smelly fog, and about halfway up the mountain, came out of the fog into brilliant sunlight, with blue skies above us and a solid cloud blanket below.
The good news is that western ski resorts and valleys rarely get an Alberta Clipper, the big cold air systems that sweep down across the Great Plains from the Canadian Arctic and put most of the nation west of Denver in the deep freeze. Once, we were skiing at Bridger Bowl just outside Bozeman when a Clipper struck. We could look east from the top of the mountain and see the start of the plains, where the temperature was 35° colder than we were.
We have been skiing the west for over 30 years, and living there for 10, but only once has an Alberta Clipper been so strong that it managed to cross the mountain wall where the Rockies meet the plains into the intermountian zone. That was memorable, however. The high temperature at the top of the mountain where we were skiing was -17°. Wearing all the layers we could muster, and wrapped in the finest northern goose down, we could stay outside for about 2 hours at a time between warmups in the lodge.
The lesson here is not to worry about temperatures when planning a ski trip to the west. The incredibly cold temperatures shown in the weather reports are almost always valley inversion temperatures, and they won’t bother you skiing, up on the mountain where it will be warmer and gloriously sunny. The low humidity of the west also makes cold weather feel less cold than the same temperature feels in more humid places