During the corona spring of 2020, we reported very blue skies. This week, again, clear blue skies were observed. How do these days compare to those in 2020? We performed the same calculation as back then to determine an objective degree of blueness. And what turns out? The blueness of the sky during the current week is comparable to that during the lockdown.

Cleaner Air During the Corona Lockdown

The interesting thing is that the situation regarding air traffic and industry is now very different than it was then: during the corona lockdown, the emission of air pollution was much lower than normal, which generally led to cleaner air. Yet, we found in 2020 that the air can also be very blue in situations with more or less normal emissions. The high solar radiation and blue skies in the spring of 2020 had much more to do with the meteorological situation than with the exceptional circumstances due to the then lockdown (see this study).

Supply of Clean Air on March 18

This week, a high-pressure area determined our weather with the absence of low clouds and a very dry upper air, allowing airplane trails to dissipate quickly. The origin of the air in the Netherlands on Tuesday, March 18, was above eastern Germany and Scandinavia, and moreover, it descended along the way (image 1). The result was clean air with exceptionally few suspended particulate matter (aerosols). Aerosols scatter the different colors of sunlight relatively evenly compared to air molecules, making the air whiter. Fewer aerosols therefore means a bluer color. The air was also very dry, which further contributed to the blueness on March 18 (image 2).