The physics of protecting fruit plants from freezing

[Maxo]

Fruit growers sometimes protect their crops by spraying them with water when overnight temperatures are expected to drop below freezing. Some fruit crops, like the strawberries in the figure, can withstand temperatures down to freezing (0° C), but not below freezing. When water is sprayed on the plants, it can freeze and release heat, some of which goes into warming the plant.

So when the water freezes it released heat which warms the fruits. But what about when the ice melts? Doesn't that mean it then takes heat from the fruit, thereby making the fruits colder?

 

[Borek]

Saying that freezing water releases the heat that warms up the fruit is incorrect. What is happening is that if the temperature drops down, heat is transferred from the fruit to the surroundings. If the fruit is covered with water, heat transferred to the surroundings will first come from the water and from the latent heat of freezing water. Hopefully that will take long enough to let the fruit survive through the night.

 

[sophiecentaur]

The fluids in the fruit (sugar solutions etc.) will have a lower freezing point than pure water. The water will freeze 'first' and, if there is enough of it, the latent heat released as it freezes will delay any further drop in temperature and the fruit fluids will remain liquid until the surrounding air warms up again in the morning. So it's not so much a matter of 'warming' the fruit - it's more a delay in cooling that's achieved.
The action of the water is only to delay the freezing if the ambient air is much below zero C.

And, yes, there will be a delay in warming up again - but that will not matter if there is 1kW per msq falling on it from the morning sun.