Clouds in the Eyes of the Beholder.

Observed Cloudiness (%) = 0.5*Painted Cloudiness (%) +26% (r^2 = 0.90)

Artists do have a bias. They put in an average of 26% too much clouds in the sky compared to meteorological observations. So when oil-on-canvas indicates 70% cloud cover read 44% cloud cover and you will be close to what the weather station observer would record. The Y-axis on the right hand side of the illustration above has the bias removed.

The observed-in-the-modern (post 1910) increase in cloudiness in North American and Europe is around 17%. A 17% increase in cloudiness is illustrated by the small red indicator in the illustration above. The change is consistent with the artists impressions and expressions. If these artists are indeed faithful weather artists then Earth's experience is one of real meaningful climate changes and we have been having them for hundreds of years.