Latitude vs Cloudiness

The plot shows the cities cloudiness in relation to the latitude. I don't know of any expectations in relation to cloud coverage.

From the plot I can conclude that the cloud coverage is concentrated between 40 and 60 degrees latitude. This is most of Europe and the top of the USA and the bottom of Canada. This could be because that is where it is currently fall. There is a peculiar concentration of cloud percentage at the 20%, 40%, 75%, 90% and 100% lines going across all latitudes.