this is more proper weather api with clouds, but they only have last 5 days
Weather in December 2017 in Longyearbyen, Svalbard, Norway
they have the custom weather data exposed for free, we could scrape it
Cloud Fraction (1 month - Aqua/MODIS) | NASA
cloud mask, they use satellites to figure out where clouds are
Birulya Monthly Climate Averages
month by month cloud per location, you can query the website by gps but if not close to a city returns null.