Archive for February 7th, 2019

Sometimes a Cold Snap is Just a Cold Snap

February 7th, 2019

On Wednesday, January 30, 2019, The College Wooster closed due to cold. This cold snap was felt across much of the central and eastern USA. The message Wooster staff and faculty received included this statement:
“The National Weather Service is forecasting daytime temperatures tomorrow between -3 and -7 degrees Fahrenheit, and wind chills of -25 to -30. In these weather conditions, exposed skin can begin to show signs of frostbite in as little as 10 to 15 minutes outside.”

However, in the past week, I’ve heard diverging narratives from people across the eastern USA about their experience. Some lament that this was horrible weather, the worst ever — how can climate change be real? Others lament that it used to get much colder — we have it easy today because of global warming. So what’s the truth? 1) Was Wooster’s cold spell out of the ordinary? 2) Is winter not as harsh as (or harsher than) it used to be? 3) Is climate change to blame?

1) Last week’s cold was not out of the ordinary.

There’s two ways to think about how cold it was.  One is that the daily high was only 9.6°F, which is frigid — it never topped 10°F. Another is that the daily low was -6.0°F, and that occurred while students would have been walking to classes in the morning. I can’t speak much to the wind chill because the OARDC station is too far from campus to give an accurate assessment.  Wind varies a lot more temperature from place to place, so it’s hard to know exactly how bad the wind chill was for any random person walking with exposed skin outside.  For temperature, though, the weather forecast was spot on.

Figure 1: The distribution of the annual low temperature in Wooster (the lowest daily low) from 1900 to 2018, with 2019 marked.

That temperature, however, was not exceptional.  Funny enough, January 22, 2019 actually had a lower low of -7.2°F — it just wasn’t as windy. In Wooster, the average annual low temperature since 1900 is -7.6°F. The average lowest daily high is 11.0°F. Our 2019 is currently right in line with those numbers (although the winter is not yet over).

2) The coldest days might be getting less severe.

This is actually a tricky one to answer. If you look at the coldest temperature recorded each year at the OARDC, nearly every year before 1930 had at least one day in which the temperature fell below 0°F — but not so from 1930 to 1960.  The coldest cold was above 0°F about in about 25% of the years in that second period.  From 1960 to 1990, the reliable sub-zero temperatures returned.  Since 1990, the annual coldest day has been less severe again on average.  In other words, if you only look back to 1960, yes, the worst days have been getting less severe.  But if you look back to 1900, the last 120 years suggest that Wooster still gets plenty of cold.  So if you were born in the 1950s, no, the new generation doesn’t have it easier, but they may be more sensible about preventing frostbite. 

Figure 2: Time series of the the lowest daily low temperature (the coldest temperature each year) in Wooster from 1900 to 2019 (so far). Edit: With 99% confidence interval on the trend line.

3) There’s not a clear climate change signal here.

The problem with evoking climate change is that weather extremes are by definition rare, so it’s hard to pinpoint the immediate cause of local-scale weather extremes to long-term, global-scale warming.  There is some evidence out there that the polar jet stream (a.k.a. the “polar vortex”) is becoming more erratic as the world warms, leading to more days like January 30 when the Arctic Ocean is warmer than Minnesota, but that is not settled science. Plus, there’s no clear trend with this particular measure. In other words, it’s premature to blame climate change for every weather event you don’t like.