windchill is irrelevent to non people or animals. fluid freeze goes by actual temps.
Inside car temp 75F ---- outside temp 0F (example).
1)Parked car : blowing hot air on inside of windshield elevates windshield temp enough to melt anything on the outside of the glass. This is, IIRC, named conduction.
2) Same car, same inside and outside temps, same blowing hot air on windshield, but car moving at 60mph. Do you think stuff will melt on the outside as quickly/easily as when the car was parked ? Nope... and this, IIRC, is convection. Throw a fast moving cold fluid (in this case air) on a surface and it will cool down significantly faster than with zero air movement.
So this isn't "wind chill factor" as we all know and love, but it's physics at work. When the cool washer fluid hits a warm (warm enough) windshield, no problems. Get that car moving through cold air and the game changes.
I remember when I first hit a hale storm in 90F weather driving on the highway in a Ford Econoline. The metal roof made cracking sounds (not the same sound as the ice balls hitting) and also the outside of the windshield fogged up instantly. There was so much cold fluid hitting the van at once the temp differential was very high, the contracting metal made noises and dew point on the outside of the glass. Fascinating...
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