A shot of truly polar air moved into the Lower 48 this week, sending temperatures plummeting. Of course, what is considered extreme cold today is not as cold as winters in previous decades.

The latest cold shot brought some of the most teeth-chattering temperatures to the Dakotas and Minnesota. But as seen in the chart below comparing observed temperatures to the records for Dec. 14, many were more than 10°F shy of the records.

December temperatures and records

While the western Dakotas are having one of their five coldest Decembers on record, some other parts of the country are tilting the other way. Many locations in South Florida are having one of their five warmest Decembers on record. Most places in the U.S. have had near-normal December temperatures so far, according to the Southeast Regional Climate Center.

The eastern U.S. is much colder than it was last year when, for example, temperatures in Boston and Washington, D.C., were more than 10°F above normal. Even though this year is chillier, the temperatures so far this month in both cities only represent of a normal level of December cold.

December temperatures

As the planet continues to warm from the increasing amount of greenhouse gases in the atmosphere, cold outbreaks will still happen in the winters, but they won’t be as powerful or long-lasting. In many states, winter is the fastest warming season, further muting the cold weather.

Seasonal warming

This trend isn’t surprising. It takes less energy to raise the temperature of something cold, rather than something that is already warm. This is one of the main reasons that the Arctic has warmed about twice as fast as the rest of the world. As a result, the air coming from the Arctic and into the U.S. is not as cold as it used to be during the winter, making record lows more difficult to come by in the Lower 48.

Temperature outlook

The worst of this cold will retreat by the middle of next week, and much of the country east of the Plains will be warmer than normal for the end of the month. This will likely cement 2016 as the second warmest year on record in the U.S. And of course globally, 2016 is basically a lock to be the warmest year on record.