The operative word for today's weather is breezy. The winds are gusting up to 25 mph and are predicted to get stronger overnight. It should remain windy through tomorrow. Shocking as it may seem, tomorrow may be downright brisk. Morning wind chills will be in the lower teens. The cool weather will be brief. We should have very pleasant weather for all the fine athletes running around with shopping carts on Saturday.
Could the warmer weather we'll have this weekend be the January thaw? Or was the thaw last Friday and Saturday's extreme warmth? Then again, twenty-one of the first twenty-four days of the year have been warmer than normal. Could the entire month be in a thaw? As it turns out the January Thaw is an elusive beast. In the comments a couple of weeks ago a Gothamist reader called the thaw "false spring", we kind of like that name, and said it occurs on a semi-regular basis. Inspired by the comments, we did a little research and what we found surprised us.
First, a definition. According to the Glossary of Meteorology sitting on Gothamist's desk, the January thaw is "…a period of mild weather popularly supposed to recur each year, in later January; most pronounced in the Northeast…" The warm air just doesn't appear, there has to be a physical mechanism to bring it here. The source of the mild weather has to be warm air from the south that is either pulled up by a storm to our west, a high pressure system that has passed over us, or ridging of the jet stream above us. If the thaw recurs at about the same time each year then one of those mechanisms has to be in place at the same time each year. Here's where the January thaw story begins to fall apart.
A handful of studies have examined the timing of various meteorological conditions, like sea level pressure and upper-level air flow, to see if those conditions repeat on a consistent basis year after year. No consistently repeating weather patterns have ever been found. Without a physical mechanism to bring warm air here, there's little reason to believe that the January thaw is real. Unless…. Unless, we're still ignorant of how the atmosphere works.
It could be that the thaw is happening and we just don't know why. In that case, we can temporarily ignore the physics and perform some statistical hypothesis testing. Central Park has more than a century's worth of daily temperature data, surely that is enough to have the January thaw stand out. Right? No. There is no statistically significant warm spells in January. The analysis gets hairy, and you do see a slight warm weather bump where there should be a warm weather bump (see graph above), but it isn't a meaningful bump. As Charles Martin, Chief of the Weather Bureau wrote in 1919 (pdf file) "Each striking feature on a long record is, therefore, no evidence of the persistent irregularities, but is simply the residual scar or imprint of some unusual event, or a few which have been fortuitously combined at about the time in question."
This begs the question: If there's no known meteorological reason for a January thaw, and there is no statistical evidence for a January thaw, why then is there a January thaw? The simplest answer is because we want there to be a thaw. Temperatures in January fluctuate a lot. January is often cold. A warm spell after a cold period in January is memorable, but our memories are faulty. We string together last year's early-January warm spell with this year's late-month warming with next year's mid-month warm-up and thirty years from now we'll be telling our grandkids "Why, when I was young it got really warm for just a few days each January…"
January thaw graph from Godfrey, Wilks and Schultz