Q Why do orange growers spray their crops with water on freezing nights?
A The recent cold snap across the country garnered a lot of news attention. As seems to happen every winter, this coverage was capped in early January with a picture of Florida oranges encased in icicles.
When arctic air reaches Florida, farmers may spray their crops with water to keep them from cooling to the temperature of the cold air. This seems counterintuitive, but the energy associated with the phase change of water makes it exactly the right thing to do.
Water exists in our atmosphere in three phases; solid (ice), liquid (water), and gas (water vapor). When water changes phase from one form to another, energy is involved. When the change is from a low energy state (like ice) to a higher energy state (like liquid or gas), energy is required to accomplish the change.
A common example is the evaporation of a puddle of water. Energy from the sun on a sunny morning after a rain is used to change the water from its original liquid state to the invisible gaseous state. When the change is from a high energy state (liquid) to a lower energy state (ice), energy is released to the environment.
So when the citrus farmer sprays liquid water on her crop in anticipation of an overnight freeze, she is taking advantage of the fact that when that liquid water freezes, the process will release energy (in the form of heat) to the fruit, thus preserving it against the ravages of the cold.