Why does my rear window defogger remove rain? I've noticed this before and still don't have a picture, but...
Today it was lightly raining in the morning. As always, I turned on the rear defogger. I do this because I have noticed it clears the water from the rear window.
It takes maybe a minute to start, and then you can see the water being repelled from the areas where the resistor stripes are, and eventually, the entire area covered by the stripes is clear. The area above and below remains covered in water.
If you turn it off again, the water fills back in and the rear window is covered again, more rapidly, perhaps a minute.
I can't imagine why this might happen. Any ideas?
UPDATE: yes, it's clearing water on the outside of the window, not frost on the inside. It does that quite well too, but that one I completely understand.
 A: Without more quantitative data regarding the ambient temperature and the temperature of the external window surface it would be difficult to know for sure if this can explain your observation, but my hypothesis is that the heating is decreasing the surface tension of water in the area of the heater - enough to cause the water to form a film as opposed to a bead that's happening at the lower temperature. I have seen this also on the rear window of my Mitsubishi!
Decreasing surface tension generally decreases contact angle, the projected angle at the liquid solid interface. You can learn more detail by googling surface tension and/or contact angle, but the following link provides a good explanation as well as data of contact angle vs temperature of water on silicon (not glass - but close)
Thesis titled: Temperature-Dependence of the
Contact Angle of Water
on Graphite, Silicon, and Gold
A: I don't have anything to back this up beyond the assumptions I've always made regarding this phenomenon (I often run the rear window heater during rain).
I've always just assumed it's due to the evaporation in that area.  It's the same reason it gets rid of condensation; just the energy barrier would be a bit higher due to the increased mass of rain compared to condensation.  Given that the glass seems to stay clear; I just always assumed it was still warm enough to cause the rain to evaporate in those areas.
As soon as you turn off the rear defrost/defogger, the rain starts to cool the window panel back down (through evaporation and convection), so the effect is short lived once the power is taken away.
