[John Rogers]

I used the term "chill factor" to cover several types of cool down. As the previous post noted you can not get the temp below ambient but that is in error, especially if there is any moisture on the surface and you have air movement which you will, then what is called latent heat of vaporization comes into play which will act similar to what we use as a refrigeration system now days. With water that equates to about 970 BTU/LB of water. In the days before me (and I am old but not that old) ice was used and the heat absorbed by melting the ice kept food cold. I do remember my grandmother having an "ice box" until about 1952 or so!

So where does the moisture come from? Condensation mainly especially when the car is hot and then cools down at night! Sooooo, be safe and add some anti freeze if allowed.