Using UK Met data from here, I noticed that Tmax for Decembers are cooling at -.45C / decade for 30 years. Thats a lot!
And then I wondered why? A theme I have returned to over and over is that hours of bright sunshine has increased because of cleaner air. Could more sunshine in December lead to cooler Decembers since clear skies can lead to colder temperatures in winter?
Guess what. Sunshine is up an average of 5.5 hours per year in December for the last 30 years. And compared to 1988 sunshine is up 80% in December. Instead of 28 hours of sunshine in December, the average is closer to 48.
Click on the graphs to enlarge.
sunshine hours 