News•December 31, 2015

Two Charts Show December’s Crazy Warmth

If you live in the eastern half of the U.S., you probably don’t need anyone to tell you that it has been ridiculously warm this month.

Just how warm is illustrated by two eye-popping charts that show there have been more than 20 times as many daily heat records as cold ones this month and that nearly a third of the Lower 48 has seen temperatures an astonishing 10°F (5.5°C) or more above normal.

> 10,000 daily U.S. warm records this month. That's 21 times more than daily cold records. https://t.co/WgsQQhcWAr pic.twitter.com/6eEbmUoO81

— Jonathan Erdman (@wxjerdman) December 30, 2015

The month is likely to be the hottest December on record for most, if not all, states to the east of the Mississippi, capping off a year that is virtually a lock to be the world’s hottest on record.

As the first chart, from the Weather Channel, shows, there were about 10,000 combined daily record highs and record warm lows compared to just around 500 record cold highs and lows this month — a ratio of about 21-to-1.

The higher ratio of record highs to lows is a hallmark of global warming, which skews the odds in favor of the former. Without warming, the ratio would be about 1-to-1, at least on the scale of years or decades. But over the past two decades, those figures have tilted 2-to-1 in favor of record heat.

That so many overnight low temperatures also registered as record warm in December shows another key aspect of warming: colder times of day (just like colder places and colder times of year) warm faster than warmer ones.

Of course, warming isn’t the only reason for this incredible December heat. Jake Crouch, a climatologist with the National Centers for Environmental Information, chalks it up a combination of the effects of El Niño and climate patterns over the North Atlantic.

“It appears they have been reinforcing each other in terms of the Eastern U.S. weather the past couple of weeks,” Crouch said in an email.

1/3 of the country is 10 degrees or more above average for December—darn impressive! Map by @islivingston #climate pic.twitter.com/MtkWv8nKNd

— Eric Blake (@EricBlake12) December 30, 2015

That interplay has led to the incredible temperature departures that can be seen in the second chart, which shows about a third of the country experiencing temperatures at least 10°F above normal for the month.

Departures that high virtually guarantee that numerous states and cities will see their hottest December on record, and by a long shot.

Come January, the weather is expected to take a more wintery turn for the East as climate patterns in the North Atlantic flip. One scientist says that flip will release energy into the polar atmosphere and could weaken the infamous polar vortex, which would allow cold air to spill out over North America.

As Crouch put it, “Goodbye spring-like temperatures, hello winter.”