Live Higher, Stay Warmer (At Least in Fairbanks)

For most of the year, Fairbanks, Alaska, defies meteorological logic. Unlike what happens in most places, Fairbanks temperatures often increase with altitude in the first few thousand feet above the ground. The blame goes to one of the most powerful temperature inversions on the planet, a phenomenon recently measured by Rick Thoman, lead forecaster for the National Weather Service Forecast Office in Fairbanks.

Thoman is a hill-dweller, living west of town in Lincoln Creek subdivision, about 1,600 feet above sea level. For the past two winters, he's compared temperatures at his home with those of a Weather Service observer who lives in the Goldstream Valley, about 590 feet above sea level and about 20 feet above Goldstream Creek. Thoman gathered his statistics in interior Alaska's season of strong temperature inversions, from October until about mid-March.

These inversions, in which warmer air forms a lid that sits above cold air, happen in part because Fairbanks sprawls in the bowl of the Tanana Valley, which is hemmed in by mountains and hills on all sides but the southwest. The other inversion ingredients are clear skies that allow heat to escape, low solar radiation in the winter that doesn't allow the earth to be warmed much, and calm winds. The combination of factors causes inversions rivaled only by those at the South Pole.

In the winter of 1997 to 1998, Thoman found that his home on the hill averaged about 16 degrees warmer than the Goldstream Valley. On 94 percent of the days, the minimum temperatures at his home were higher than those in the valley. More than 25 percent of the days, the daily low on the hill was warmer than the daily high temperature in the valley. On some days, the hills were more than 25 degrees warmer than the lowlands.

Thoman, who heats exclusively with wood, thought all that warmth must be saving him logs. To see how many, he calculated his number of heating-degree days, which are tallied by taking the average temperature of the day and subtracting 65 (though he used 50 degrees because he wouldn't build a fire when his home was 65). He found that it took him 18 to 20 percent less energy to heat his home than if he lived in the lowlands. He also said that people don't need to live as high as he does to benefit from the temperature inversion. He estimated that those just a few hundred feet off the valley floor probably save about 10-to-15 percent.

Temperature inversions are strongest in the winter, but sunshine and breezes combine to knock them out in early spring. From June to August, the temperature inversions are a memory, but when the sun wanes in September, the inversions once again begin setting up and making high places warmer.

Thoman says the phenomenon makes forecasting a challenge for Weather Service people newly stationed in Fairbanks, who will typically miss their first few forecasts by 20 degrees or so. Unlike most other places, where air masses thousands of feet above the ground will warm the air at ground level, Fairbanks' inversions form a cap that doesn't allow air to mix. "People are really amazed at how disconnected the lowest few thousand feet of the atmosphere is with that above it," he said.