Has this ever happened?
How often?
I never recall seeing this on my local news weathercast.
I think you can get extremely low humidities when the temperature is very low. Maybe someone who lives through -40F Michigan winters can give numbers.
relative or absolute? 
Well, I guess in those cases, it is the heat.
Well, I guess in those cases, it is the heat.
I saw 5% predicted once for the desert outside of Phoenix. I don’t think you can get any lower than that, for practical reasons that water is all around us.
bbeaty may be right. It looks like the humidity at Amundsen-Scott Station in Antarctica is currently at N/A.
Of course, thanks to sublimation I’m probably wrong… :rolleyes:
I suspect that absolute 0% humidity is impossible. The driest place on earth is the Atacama desert (in terms of precipitation), but apparently the driest air is found in Antartica, where the average humidity is less than 1% year-round.
vanilla, the water vapor given off by your potato chips will bring the level above zero.
very funny.
For you metric kids out there, -40F is the same is -40c, that is where the two scales intersect. Moisture in the air becomes crystalized at that point, so any measure of humidity no longer becomes applicable. Moisture in exposed skin is also quickly evaporated, making it feel as though there is negative humidity. I forget the name of the effect, but the crystals in the air play some tricks with light sources too, halos and rainbows and double suns to an unfocused eye.
Fun games at this temp include listening to nostrils slam shut, tossing boiling water into the air and watching it freeze before it hits the ground (doesn’t work with non-boiling water, the air content in the boiling water allows it to crystalize faster)
So I guess my answer is that if measured, it technically would be 0% humidity, but I suspect that N/A is probably the better answer.