Spraying fruit trees with water before a frost helps protect the fruit from freezing because as the water freezes, it loses heat to the cold surrounding air and at the same time it is loses heat to the leaves, fruits, stems and other parts of the plant; so in reality, it is warming up the parts of the plant. Because water is a great insulator, once the ice is frozen on the tree it also acts as insulator to prevent more cold from getting on to the tree and its fruits.
