Why will spraying a fruit tree with water before a frost help protect the fruit from freezing?
The additional water slows the rate at which the cold weather can freeze the fruit. In order to freeze the fruit on the tree, the cold weather will have to cool/remove heat from the additional water as well. Water has a very high heat capacity. It takes a lot of energy to change the temperature of water by 1 degree. So without the water, a lot less heat would have to be removed to lower the temperature of the fruit alone by 1 degree — making it easier to freeze. So spraying a fruit tree with water before a frost can help protect the fruit from freezing. (well, provided the cold/frost is a temporary over-night condition rather than persistent one. if it is persistent (below freezing temperatures all day and night) then no amount of spraying will protect the fruit from freezing.
