So where would I post this?

[tandkvd]

I don't know how accurate this therory is but here is my thought.

Water freezes at 32 deg. F. But to have frost to develop on the grapes the actual temperature of the grapes would have to drop below the dew point of the air, which in a low humidity situation would be well below 32 deg. F. Thus the grapes would be getting to a lower temperature before the ice forms on them.

But with the water allready on them by use of sprinklers the protection from the ice layre starts at a higher temperature.

IMHO of course.