Over the past couple of weeks, the word "clipper" has appeared in the forecasts in Chicagoland constantly.
While you may not know what a clipper is, you no doubt have experienced the effects of a string of them. One day, it gets cloudy, then there's some snow, and then next day it gets clear and much colder. And this has happened numerous times, like with the snow events on January 14, 16, 21, and 24, with other occurrences as well.
Basically, a clipper is a type of low pressure system that moves relatively quickly. They usually occur in winter, and bring light snow and then much colder air. They are also known as "Alberta Clippers", because they travel southeast from the Canadian province of Alberta.
As with any low pressure system, there is a convergence of warmer air with colder air. The warmer air is located southeast of the low pressure center, and the cold air wraps around the low's other three quadrants.
These low pressure clippers haven't put down too much snow as the cold temperatures in the area have limited the amount of moisture available for accumulating snow.
They've been coming one after another because the upper-level air pattern has remained stagnant, high up in the troposphere (the layer of the atmosphere starting at the surface and going 60 miles up). As I referred to in my post about the extreme cold, this pattern is manifested best in the upper levels as a belt of strong winds moving from west to east in the Northern Hemisphere. This belt of winds, known as the jet stream, is buckling in such a way so that warmer air is surging north toward Alaska, forming a ridge. In compensation to that movement, the jet stream is dipping south across North America, forming a trough, keeping conditions cold in our area throughout much of this month. There hasn't been anything on a broad scale to move along this pattern of ridging near Alaska, troughing over the US. Therefore, these clippers have generally been riding the air flow that is diving south into the Coterminous United States.
(As an aside, the WGN midday news broadcast today, Tom Skilling remarked that the ridging up into Alaska may be the result of unusually warm ocean waters in the Pacific, which is pushing warmer air further north. It's an interesting explanation for what's going on.)
So that's a word on clippers.
By the way, if you're interested and you have the time, check out this video on the White House's website, from one of their "We the Geeks" Hangouts. It features a panel of people in the field of meteorology discussing the polar vortex and the associated extreme cold outbreak that occurred earlier this month, as well as other things related to climate change. It's about 45 minutes long, and they all engage in some really good discussion, presenting great insights. Pay attention especially to what Stephanie Abrams says about weather being an effort to restore balance, and doing good for the environment just for the sake of doing good.
Here's the main page for "We the Geeks": http://www.whitehouse.gov/we-the-geeks
The video should be on that page, after you scroll down a little. If not, try this YouTube link: http://www.youtube.com/watch?v=AWZA85GUsi0
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