December 2021 was the 2nd hottest on record for Philadelphia, as part of a climate change trend


As to wipe the slate for 2022, temperatures have plunged and lawns snowed in the Philadelphia area and on the Jersey Shore Monday, leaving December as a quickly fading memory.

But the last day of 2021, Dec.31, recorded an average daily temperature of 51 degrees in Philadelphia – 16 degrees above normal.

In fact, the entire month averaged 45.3 degrees, the second hottest December on record for the city dating back to 1874, according to an Inquirer analysis of records kept by the National Oceanic and Atmospheric Administration. Six of the 10 hottest years have occurred in the 21st century; 2015 was the hottest year on record.

As the Inquirer recently noted, temperatures are rebounding, often varying wildly from winter to winter. Of all the seasons, winter experiences the most dramatic temperature swings, and although the climate is constantly changing, over the past few decades temperatures have moved in one direction.

READ MORE: Winters in Philadelphia are not what you think

Indeed, the coldest December months all fell in the 19th or 20th century.

In December 2021, the average temperature in Philadelphia was 6.7 degrees above normal (it was also dry with just 1.64 inches of precipitation, or 41% of normal).

Dave Dombek, senior meteorologist at Accuweather, said the weather in the region was dominated by a winter version of a Bermuda High – a subtropical area of ​​high pressure in the Atlantic Ocean that migrated east.

“It has certainly been an exceptionally hot month,” said Dombek. “We had a few cold days, but if you look at the low temperatures, a lot of those days didn’t even get below freezing at night. It was quite large. It just wasn’t a stretch. One day that stands out, it is the 68 degrees which occurred on the 11th ”.

The heat is part of a long-term trend linked to climate change, according to Climate Central, a nonprofit made up of scientists and journalists. The group found that December in Philadelphia is getting milder, causing the average temperature to rise 4.4 ° F since 1970.

December is also the month with the second fastest rate of warming in Philadelphia.

To calculate this average increase, Climate Central used linear regression, a standard statistical method for modeling change over time.

Although Philly’s records date back to the 19th century, Climate Central chose 1970 for its analysis to be consistent with calculations for locations across the United States, some of which did not keep reliable records before that.

“I’m still a little cautious about adjusting trends across the record” going back to 1874, said Andrew Pershing, director of climate science at Climate Central. “While we normally think that adding more data gives you a more accurate picture, in the case of climate change this is not always true. Basically, global warming is accelerating – it was weak at the start of the record high and has grown much stronger in recent decades. “

For example, Pershing said that the entire 147-year record available for Philly shows a fairly standard pattern of ups and downs, with an increase of 0.02 degrees per year. However, a clear trend emerges around 1970 with a change of 0.09 degrees per year so far, resulting in a change of 4.4 over that 51 year period.

NASA and NOAA are expected to report on Thursday how hot 2021 has been overall across all seasons. While the agencies aren’t supposed to be announcing a banner year of heat, they should say it has been much hotter than normal.

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