It is clear from the the cloudiness plot that there is low cloudiness range centered around latitudes -30 and 30. There are also high cloudiness ranges centered around lats 0 and 60. These high cloudiness ranges are correlated with the low pressure band formed at the boundaries between Polar cells and Mid-latitude cells, which occur at lat 60 and -60.

There aren't many cities at latitudes less than -60, as it is mostly ocean, but if we were to look at cloudiness data for that area we would expect to see similar high cloudiness conditions. The existence of these cells and their boundaries makes a linear regression a poor choice for finding meaning in the cloudiness data.