[mjhess]

Question: Why would you design a car to have a light come on at 39 degrees F ?? Is it a warning, is damage going to occur, or is it telling you to go put on your long johns?? Seems strange to me! :?

 

[bd041556]

That is the temperature at which you might encounter ice on the road. Why not 32 degrees? Because radiant heat losses can freeze water on clear nights even when the temperature is above 32 degrees. This light coming on at 39 degrees is not unique to Prius. I've had "frost warning" lights on several Audis and BMWs, and they came on at about 39 degrees also.

 

[nmpriusman]

FYI, the light comes on at 37 degrees...

 

[veggivet]

That's when my MINI Cooper's alert comes on, as well.