The jet stream that circles Earth's north pole travels west to east. But when the jet stream interacts with a Rossby wave, as shown here, the winds can wander far north and south, bringing frigid air to normally mild southern states. NASA/GSFC hide caption

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They're down there somewhere: The Great Lakes as seen from space on Wednesday. Lake Ontario, which has less ice than the others, is at the lower right. A bit of open water can be seen in Lake Superior, at the upper left. hide caption

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