Last week's historic cold snap in the central United States is now well and truly thawing out, with some cities warming up by nearly 50ºC over the last seven days.

A vagrant pool of Arctic air was driven across the United States early last week by a big bend in the northern hemisphere's polar jet stream.

This abnormal weather pattern, which was instigated by a phenomenon called sudden stratospheric warming, saw more than 70 percent of the United States blanketed in snow at the start of last week.

But over the last few days, much warmer air has returned to the United States and temperatures have been rapidly on the rise.

Images: Cold air over the United States on February 14th (top) was replaced by much warmer air on February 23rd (bottom).

In Texas, the mercury dipped to a limb-numbing -18.9ºC at Dallas Airport on Tuesday February 16. Just one week later, it reached 27.2ºC at the same site. This is a weekly temperature change of 46.1ºC in the space of one week.

Further north, Oklahoma City went from -25.6ºC last Tuesday to 22.2ºC this Tuesday, a temperature rise of 47.8ºC.

According to the Washington Post, some places have seen even more extreme temperature swings this month. Valentine, Nebraska registered temperatures of 21.1ºC on February 3rd and -36.1ºC on February 16th. That's a temperature range of 57.2ºC within a fortnight.

Image: The impressive temperature range in Texas over the last week.

Temperature extremes like this are possible in the United States because of its unique position in the northern Hemisphere.

Canada's large landmass to its north allows frigid Arctic air to reach the United States without warming up over any oceans. Meanwhile, much warmer air masses can push over the country from the south.

Its mountainous terrain can also cause sharp wind-driven temperature fluctuations in very short periods of time.

The largest daily temperature change on record in the United States (and possibly the world) was 57.2ºC at Loma, Montana in January 1972. This occurred as Loma went from -47.8ºC at 9am on January 14th, 1972 to 9.4ºC at 8am on January 15th, driven by a downslope chinook wind event.

You can read more about extreme temperature changes in the United States here.