fog forecast

huxm

Hi Windy team,
I have a question regarding the global “fog” layer when using the ICON model.
As far as I understand, the ICON global GRIB output does not include fog or visibility fields, and visibility seems to be available only in limited-area configurations (e.g., ICON-EU / ICON-D2). However, Windy displays a global fog layer when ICON is selected.
Could you please clarify how the fog layer is generated outside Europe?
I’m trying to better understand whether the global fog layer represents a native model output or a post-processed/diagnostic product.
Thanks in advance for your help!

Suty

@huxm Hello, we use ICON data that we process on our end.

tauqirashraf

@huxm Hi there,

Great question — this is something that often causes confusion.

You’re right that the global ICON output does not directly provide a native “fog” or visibility field like ICON-EU or ICON-D2 do. In those limited-area models, visibility is explicitly calculated and available as a proper model output.

For the global ICON layer on Windy, the fog visualization is actually derived indirectly. It is not a direct model variable, but rather a post-processed diagnostic product calculated from available atmospheric parameters such as humidity, temperature, and boundary layer conditions. This allows an estimation of areas where fog or low visibility is likely, even in regions where explicit visibility output is not provided.

So in short:

ICON-EU / ICON-D2 → direct visibility/fog output
Global ICON → derived/diagnostic fog estimation used for visualization on Windy

Hope this helps clarify it 👍

huxm

@tauqirashraf I understand. Thank you very much for your explanation.