Chinook had Alaskans dreaming of green Christmas

December 20th, 2006

By Ned Rozell
Fairbanks, Alaska - On December 8, 2006, in the middle of Alaska, the air temperature on the roof of the Geophysical Institute at the University of Alaska Fairbanks 37 degrees Fahrenheit. The normal high temperature for this date is 5 degrees. A chinook wind is blowing over Alaska.

The "chinook gap" as seen from Fairbanks on December 8, 2006. This band of clear sky over the Alaska Range is the sign of a chinook wind blowing from the Gulf of Alaska northward. Chinooks mean higher temperatures for Alaska's Interior, but no snow because the air is dry.

Chinook winds happen when rivers of warm air flow over mountain ranges, in this case the Alaska Range. Also known as "snow-eaters," Chinooks happen several times each winter in Alaska, but rarely do they have enough punch to melt large amounts of snow. A notable exception was the December of 1934, which featured the only green Christmas in the 100-year weather record of Fairbanks, and all-time high temperatures in Anchorage and Nome.

At the beginning of December 1934, Fairbanks had five inches of snow on the ground (in 2006, another lean year, there's also only five inches). On December 3, 1934, the airflow known as a jet stream shifted to flow from south to north, bringing warm, moist air from the North Pacific over Alaska. Weathermen sometimes call this "the pineapple express" for its ability to transport what feels like tropical air northward.

"In 1934, the jet stream probably scooped into really low latitude, moderate air," said Eric Stevens of the National Weather Service in Fairbanks.

Stevens tracked down the weather records for 1934 that tell the following tale of a chinook that robbed Fairbanks and much of Alaska of its meager snow. On December 2, 1934, the high temperature in Fairbanks was 9 degrees and the low, minus 12 degrees. Highs for the next seven days were 44, 52, 58, 51, 52, 55, and 36 degrees Fahrenheit. Even worse for the snow, the daily low temperatures on three of those days was higher than 32 degrees.

"That's what killed off the snowpack," Stevens said. "The warm temperatures were just sustained for so many hours. It takes a long time to ripen a snowpack before it melts."

The snow went from five inches to "a trace," and Fairbanks children could no longer construct snowmen.

"That's the only brown Christmas in 100 years of data for Fairbanks," Stevens said.

The 1934 chinook stands up as the strongest on the Alaska record, Stevens said. In reviewing weather data for 100 years, he saw that in November through February, the Fairbanks temperature had risen above 50 on just six days. Five of those days were during the great chinook of 1934. Anchorage had seven straight days with high temperatures above freezing during the chinook, with a peak of 53 degrees Fahrenheit on December 7. Nome also had seven straight days with the high temperatures at, or warmer than, freezing.

Warm air from the Pacific is one reason why Alaska chinooks peg thermometers, but chinooks gain much of their warmth by a process called compressional heating, when dry air descends the leeward side of the Alaska Range. This process is often visible from Fairbanks as a cloudbank that extends northward caps a clear gap of dry air above the Alaska Range. When a person looks southward from Fairbanks on this December day, he or she can see the "chinook gap."

Chinooks happen worldwide, wherever big airflows meet mountain ranges. In some places, they have strange effects on people.

"These warm, dry winds have sometimes adversely affected human behavior," the editors of the textbook, Meteorology Today wrote. "During periods of chinook winds some people feel irritable and depressed and others become ill. The exact reason for this phenomenon is not clearly understood."

One way to get people irritable and depressed is to take away their snow before Christmas. Stevens said that chinooks like the one in 1934 are so rare he doesn't expect it to happen this year.

"Chinooks usually fade out before they melt all the snow."