How to define the sky (condition of currently weather) as windyty did?

[nampq2]

I would like to ask:
How to define the sky as these icons were show on windyty.
E.g. I will use data forecast of GFS.
So, what is the variable need for that and define how?

I appreciate for your help!

[www999wap]

Cloudiness and schematic maps related to navigation. Thank you！