@gukel
Hi,
In Windy the Extreme weather layer is expressed as % because it directly comes from the Extreme Forecast Index (EFI) from ECMWF. See what that means:
https://charts.ecmwf.int/products/efi2web_2tmin?area=Europe&base_time=202312010000&day=1&quantile=1
(Note that Windy shows the EFI from the chart on the left, but not the Shift of Tail, nor the chart on the right).
Here is the detail of the colour scale in Windy
D4931AA9-C5F4-4EFA-8C7B-A18FA47EAD9B.jpeg
So this colour scale is not based on Kelvin.