This post by The Chiefio correlates with my gut feel for the data.
Summers aren’t getting hotter except due to UHI.
Winters are less cold. According to Chiefio it is because high altitude thermometers are disappearing. I just believe in waste heat produced by billions of humans.
Here are the highlights:
Basically I [The Chiefio] found that for some Continents (regions) the Winter Data is vastly more volatile than the Summer data. Most (all?) of the “warming” comes out of a reduction of the volatility to the downside of those cool season data. Hot seasons are just not very volatile. Yet they DO have an up tilt at the very end with the advent of electronic thermometers and lots more asphalt around airports.
My conclusion (up here at the top 😉 is that this leans strongly toward “hot black asphalt in the sun” causing the summer “lift” and “loss of high cold volatile places” causing the cool season “lift”. Remove those stations at altitude that tend to have the strongest cold excursions, you get a flatter winter curve with the cold anomalies pruned. Have an airport add 10,000 foot more runway and 20 acres of paved parking for Jet Travel, you get hotter summer readings.
It is my opinion that accounts for most of whatever warming is in this the unadjusted data. (That then gets enhanced with “all one way warmer” adjustments in the adjusted version).
What would happen if post W.W.II airports added more concrete and asphalt and more of the thermometers were placed at airports and inside the “Concrete Jungle” of cities? More solar heated pavement carryover into night warmth, less vegetation transpiration making cool and damp.