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I'm thinking of compiling a map showing how often it was sunny at a given location on a given day of the year, one map for every day of the year. The exact definition of "sunny" will depend on what cloud cover data I can obtain.

Are any such data sets freely available? I've found MODIS image archives, but they are fairly limited, especially in that they are raw images, not interpreted as cloud cover. I've also found a company that seems to sell exactly what I want, but I don't expect this to come cheap.

Can I get historic cloud cover data for free?

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  • Are you looking for global data? What time coverage and temporal interval are you looking for? And if you want sunny days, why not go for sunshine hour data directly?
    – gerrit
    Commented Mar 20, 2015 at 15:44
  • @gerrit I expected that I wouldn't have much choice, but the perfect data would be global, 50km grid or finer, hourly intervals or finer. Sunshine data would simplify this slightly, but seeing as it's simply cloud cover * illumination, I thought I'd go after the one I expected to be more readily available. (and yes, satellite era since given what I'm after, before satellites such data simply didn't exist)
    – user6024
    Commented Mar 20, 2015 at 15:59

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For the satellite era, the best you can get is likely reanalysis data:

A meteorological reanalysis is a meteorological data assimilation project which aims to assimilate historical observational data spanning an extended period, using a single consistent assimilation (or "analysis") scheme throughout.

The two most widely used reanalysis are the ones from ECMWF and NCEP. Of those, the former is European and semi-free, whereas the latter is fully free. After making an account, you can download data through this page at UCAR. Hourly data includes fields like Cloud Amount/Frequency, and Cloud Liquid Water/Ice.

Normal users of this data are atmospheric and climate scientists, so you might need to be quite careful to see you are interpreting the data correctly.

For before the satellite data, there exists no global record of cloud cover.

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Another option is the Climate Research Unit (CRU) at the University of East Anglia.

High-resolution gridded datasets (and derived products)

Cloud cover (cld) is provided as a percent. Retrieving data requires registration.

An example dataset:

A gridded time-series dataset. This version, released 4 July 2014, covers the period 1901-2013

Coverage: All land areas (excluding Antarctica) at 0.5° resolution

Variables: pre, tmp, tmx, tmn, dtr, vap, cld, wet, frs, pet

(You can also access this data from Google Earth)

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