GFS/WRF freezing line

Dan Robinson

Quick question about the freezing line (blue dotted line) on surface pressure/wind model output. Is that the freezing line at the surface or at 850mb?
I've always assumed it was the 850mb freezing line but I've never been positive about that. From UCAR:

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Ahh, that makes sense - thanks for the link. It always confused me because the line rarely matches up with the freezing temps at either the surface or 850mb.