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Ask Eric: Where and How Does Severe Cold Develop?

Now, what I want to know is where and how does this kind of intense cold develop before it starts to move south? It has to come from somewhere!!  A strong low on the part of the earth tilted away from the sun? - John Day, Foxboro, MA

 

It's January. It's cold. People who love it rejoice like it's the 4th of July (on Hoth) and everyone else grumbles. January is typically the coldest month of the year in New England, and so far it's been living up to its reputation. But what are the origins of all this frigid air? Well John, it's cooking in the Arctic kitchen.

As we head into autumn, the sun's most direct rays are moving away from us in the northern hemisphere and focusing farther south. By the time we hit the winter solstice, those direct rays are overhead on the Tropic of Capricorn in the southern hemisphere. They get to enjoy summer, but meanwhile we start freezing our butts off in New England. Farther north, it's not only getting colder but they daylight goes away. Inside the arctic circle, the sun doesn't even rise for part of the winter (in some places for more than 4 months). You've got the perfect breeding ground for intense cold.

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In those far northern latitudes, the cooling process is already well underway by September. By October there's generally a whole lot of snow. November comes around and not only has sea ice expanded and snow covered the landscape, but darkness dominates most hours of the day. There's so little energy from the sun helping to warm the ground (and therefore the air) that the temperatures can really start to bottom out to their coldest possible point for that spot on the planet. The air is stable and dense, with deep cold pooling near the surface. But it doesn't just sit there - every once in a while the freezer door opens and it pours southward toward the equator.

The jet stream is the main driver for all of this. Think of it as a fence that separates the freezer (arctic) from the grill (tropics). The jet stream itself exists because of this thermal gradient. The difference between warm and cold generates thermal wind, and so the jet stream flows generally west to east between the two. The polar jet isn't static though, it undulates constantly. Storms can rapidly develop and add energy to it, buckling its flow. Numerous factors can help change its course around the globe, and where it eventually flows dictates whether or not we get a blast of arctic air!

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We get particularly intense cold in the eastern U.S. because of our geography. The coldest air in the world breeds over land masses, not water. We call this continental polar air. Even though Seattle is a city farther north than we are, it's a lot more temperate because it sees more marine air masses than continental air masses. The Rockies act as a massive wall that helps focus a lot of the cold which blasts down from Siberia, the arctic, or Canada into the central/eastern U.S.. So long as the trajectory stays mainly over cold surfaces, the air will tend to hold on to its cold properties for a long distance. This is especially true in mid-winter, when much of the land the air travels over is covered with snow and ice. This helps to keep it from modifying too much as it settles south. Eventually though, it will warm and run out of gas...so that even the coldest air masses typically can't make it into the 30s into South Florida  (only on rare occasion).

 

 

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