There was heated scientific debate after the 9/11 attacks on New York, when all airline flights were grounded for three days, about the effect the absence of aircraft contrails had on temperatures. The main conclusion was that daytime temperatures rose and the nights were cooler, the argument was by how much. Some put the afternoon temperatures as much as 1.5C higher and the nights similarly cooler, others said it was a fraction of a degree.
As it happened, the weather was exceptionally clear at the time so the contrails crossing the crowded skies over the US would normally have been highly visible. Since then there have been attempts to replicate the unlooked-for experiment by measuring temperatures in different parts of the US with similar landscapes and weather conditions, the only difference being the volume of flights passing above. These seemed to confirm that the diurnal temperatures, the difference between daytime highs and nighttime lows, increased significantly with the absence of aircraft.
Now with the coronavirus pandemic causing most flights to be cancelled it is a chance to test this theory over several different landmasses at once. This matters because in a heat wave it could be dangerous for some people if temperatures rose significantly, or for farmers if it produced late spring frosts.
