White Christmas or Black Christmas?

I was just reading an article in the current New Scientist about our Winter weather, and it was very illuminating, and not at all complicated or difficult to understand.

jetstreamsThe key concept to grasp is the polar jet stream, which, in the Northern Hemisphere, is a high-speed ribbon of air rotating around the globe at about 60°N (about the latitude of Bergen, or Anchorage). It flows West-to-East at about 10km above ground level and has the effect of coralling in the cold, polar air and preventing it from flowing South.

This means that the air temperatures either side of the polar jet stream can differ by a large amount, much warmer on the Southern side. So far, so good.

But the polar jet stream is wriggly. It refuses to rotate around the pole in a neat circle. So, at times, a meander will loop South and let the polar air reach us; while at others, it might slink North and leave room for warmer tropical air to cover us.

The particularly cold spell of a year ago (I recorded -10.2°C) was accompanied by a big loop in the polar jet stream which stayed in place for a number of weeks. (American weather broadcasts often show the jet streams and their effect on the forecast, but European ones generally don’t, for some reason.)

What I find interesting is that the wriggliness of the polar jet stream depends on its speed — the faster it’s going, the straighter its path. And the speed depends on the temperature difference between the air masses on either side of it. Global warming is affecting the poles more than the rest of the world, meaning that the temperature difference is declining, so the polar jet stream is becoming slower.

jetstream loops

Slower, and more wriggly. The effect of that is that Winter weather is less predictable than it used to be, given that there is more chance of a loop bringing us either unusually cold or unusually warm air. The unpredictability can only increase as well, since the failure of the international community to deal with carbon dioxide emissions makes increased polar temperatures a certainty.

So, in the near future, Winter weather will have more random extremes of warm and cold. Get used to it.


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