Cloudiness and Latitude

It seems that the cloud coverage tends to increase within the region from 20 t0 65 degrees latitude. By viewing the graph, it doesn't seem there exists a strong correlation between cloudiness and latitude. By taking a closer look, it seems that at certain latitudes the cloudiness would form certain clusters. For example, some cities in the region from 20 t0 65 degrees latitude tend to have very high cloudiness. However, on the same lattitude, there is a certain amount of cities that have almost no cloudiness.

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