We’ve always known that the seasonal cycle lags the length of the day. But by how much? To find out I fit a sinusoidal curve to the monthly average daytime highs for Providence, RI assigning the average temperature to the 15th of each month. The fit was rather tight with a 0.34°C standard deviation. The annual average = 15.66°C ranging from 3.26°C (Jan. 22) to 28.06°C (July 24). While solar equinox is 11:06 PM March 19 this year, we must wait another full 5 weeks until ‘thermal equinox’ on April 24. Is there a word for this day?
Since I grew up in Stockholm I had to check the numbers there and found that the thermal lag was 2 days less, i.e., 33 days. Given Stockholm’s maritime climate I had expected a greater lag, not a lesser one (probably not significantly different). The fit wasn’t quite as tight, 0.55°C. The annual average daytime high = 9.83°C ranging from -0.67°C to 20.32°C. The climate tables didn’t show the years used. Given warming in recent years the average temperatures are probably on the low side. This is no doubt the case for Providence as well.
Addendum: At lunch today my colleagues suggested Stockholm may not be as maritime as I thought and suggested I consider a station on Ireland. They were right, ‘thermal equinox’ arrives even later there at May 6, almost 7 weeks after vernal equinox!
I also checked climate data for Bismarck, ND thinking that is about as continental as one can get, but amazingly. ‘thermal equinox’ is only 4 days earlier than here on April 20. Of course, the thermal swing is huge, daytime high is -4.2°C January 19 and 30.0°C July 19.
https://en.wikipedia.org/wiki/Seasonal_lag