When the dew point is below freezing, does a sling psychrometer still function properly? I would think the wet bulb would ice up. That would both release latent heat and reduce evaporation, thus distorting the wet bulb measurement, no?
I prefer GIFs to javascript-driven sites like Tidbits because javascript comes with so much baggage.
Coolwx has GIF of the GFS at
http://coolwx.com/ptype/movie.gif