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Bitterly cold weather descended on Montana in early February, leading some people to question how that can happen during global warming. Actually, not only can it occur during global warming, but it is in fact expected and even fostered by global warming.

First, we should emphasize that weather is a short-term condition (days or weeks); climate is a long-term condition (decades or longer). Global warming is just that — a long-term rise in average temperature worldwide; some areas become warmer, others become colder, but the global average is warmer. Also, global warming is just one aspect of climate change. Other aspects include changing large-scale patterns of atmospheric and oceanic circulation, more unstable weather such as storms in some areas, more drought in others, more floods in still others, rising sea level and many other aspects.

Here is a simplified explanation of where the polar blast came from.

It all depends on overall temperatures in the Arctic and how they interact with warmer temperatures to the south to form the jet stream. Note that air rises in low-pressure weather cells, pulling in air from outside the cell and circulating winds counterclockwise.

1. When the Arctic was very cold (the Arctic Ocean and land permafrost stayed frozen winter and summer — pre-global warming) the difference between Arctic air and warmer air to the south was great and the jet stream was strong and fast, circling the Northern Hemisphere — like a fast-spinning child’s top.

2. When the Arctic warmed — now much of the Arctic remains unfrozen, both ice on the ocean and former permafrost on land; it is warmer year-round — the difference between Arctic air and warm air to the south is less, so the jet stream is slower, and begins to meander and wobble like the slowing spin of a child’s top.

Where the jet stream wobble reaches farther south, cold air reaches much farther south, as in the Midwest in early January 2014 and late this January, we are affected by a weather phenomenon that meteorologists call a “polar vortex.” These cold cells slowly migrate eastward with the jet stream; one moved over Montana in early February.

Not only are such cold times normal as the jet stream bulges southward, but they are expected. We had better get used to them; more will come in the future.

Don Hyndman is a professor emeritus of geosciences at the University of Montana.

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