Just how cold was it earlier this month? It was so cold in North Dakota, that liquid thrown into the air from a glass crystalized instantly; it was so cold that the all time record low temperature was reached in Minnesota (-60 degrees F), it was so cold that at zoos, penguins and polar bears were demanding to be let inside, well ok, that's a bit of an exaggeration. The point is, it was bitterly cold.

Outbreaks of Arctic air generally occur several times during the winter months. The most recent outbreak was the coldest in the last seven years, and in some locations it has never been colder. The noontime temperature at the Air Force base in Minot, North Dakota was -33 degrees, and the windchill was -93! In order for such extreme temperatures to occur, a number of factors, both in the atmosphere and at the surface, must come into play.

Of course, there must be a source of cold air. During the late fall and winter as the nights become longer and longer, the Arctic air becomes more and more chilled. After several snowfalls, the already cold air is chilled even further. Usually, this cold air is confined to polar areas (Alaska, northern Canada, Siberia, Greenland, and the Arctic Ocean). However, after a period of time, the refrigerated air increases in density, and the pressure near the surface begins to build. Sometimes a massive high pressure system extends from Siberia to Canada. The cold air is kept in place by the polar jet stream, which typically moves in a serpentine fashion from west to east at the top of of the troposphere (the troposhere is the layer of air between the Earth's surface and the stratosphere). Eventually, the jet stream will meander to the south, allowing the high pressure system to plunge south as well, like water breaking through a dam.

This is what happened two weeks ago. Generally, the higher the air pressure, the colder the air. The coldest temperatures occurred at places with at least about 15 cm (6 in.) of snow on the ground, under clear skies during the early morning hours (near dawn). Temperatures can fall quite dramatically at night due to radiational cooling, when the air is clear and dry. Not only does the presence of snow reflect shortwave solar energy away from the surface during the day, but snow emits longwave radiation (heat) at a more efficient rate than other surfaces, thus at night the temperature will drop more if snow covers the ground than if the ground is free of snow. The combination of the strong high pressure system, clear skies, and snow-covered ground was responsible for the headlines.

During the Arctic outbreak last week, some all-time low temperature records were set. What contributed to these extreme temperatures?

2/16/96