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Dear Tom,

What are the triggers that cause the arctic air to spill into the Midwest and all the way down to Texas?

Bill Ooms, Alsip

Dear Bill,

There’s an old saying that there is nothing between the North Pole and the United States east of the Rockies except a barbed-wire fence. This allows frigid arctic air masses to plunge south into the United States, often penetrating all the way to the Gulf of Mexico and in extreme cases even farther south, deep into the tropics. This tends to happen when a strong northwesterly jet stream, often dubbed the “Siberian” or “Polar” Express sets up, driving the cold air south. The only blocking mechanism to prevent these arctic intrusion would be a high mountain range, and since none exist, the cold air has unimpeded access. In most winters, arctic outbreaks routinely penetrate into the Deep South, and a few will sweep through Florida and Texas. However, some cold surges have reached Central America and the northern portion of South America. Topography plays a limiting role, with Central American mountain ranges halting the southern push of cold air. Some of the natural barriers include the Sierra Madre range and the Costa Rican mountains south of Lake Nicaragua. A few vigorous cold outbreaks have made it to about 7- or 8-degrees north latitude where they were finally halted by the mountains of Columbia. At those low latitudes, the cold air is very shallow and greatly modified, and only lowers the temperature a few degrees.