This is a map showing cloud frequency over 2017, as measured by the MODIS satellite sensor. A cloud frequency value of 20% means that the MODIS pixel was marked as cloudy (by the MODIS cloud masking algorithm, specifically the MOD09GA state_1km variable) for 20% of the days in 2017.
The colour shows the range of cloudiness from very cloudy (red) to very clear (green). Click anywhere on the map to get a cloudiness percentage for that location. (If nothing happens when you click then wait a few seconds, the statistics are calculated on the fly and sometimes take a while to be processed).