@idefix37 Thanks for your reply. That was indeed my question, i.e. does the grey colour mean there are no clouds ? Psychologically one tends to associate grey with obscured sky and blue with clear sky, hence the colour scheme could be improved to become more intuitive.

Hi
Firstly a big thank you for Windy. I use it for flying and find the forecasts incredibly precise and reliable.
When flying VFR the cloud base is of particular interest especially when it's getting marginal. And here's the question : The colours are explained at the bottom except the grey areas which return an X (no value) in terms of cloud base altitude. Does it mean the forecast is not possible to determine or rather that the ceiling is indefinite ?

Windy is a great tool for aviation. An really useful (and safety related) addition would be to see the freezing level. Maybe by importing information from the significant weather charts ?