What data is used for the Clouds overlay?

The Clouds overlay on the Map and Profile View uses data from the GFS Cloud Coverage product which provides a global 24-hour forecast of cloud coverage and altitudes. Rather than forecasting the actual positions of clouds, this layer depicts the likely extent of cloud coverage over a given area at the specified time and altitude, with each shade of gray roughly corresponding to standard cloud coverage intensities: few, scattered, broken, and overcast.