This has to do with the latent heat of phase change between ice and water. The most common method of active frost protection is the use of overhead sprinklers while the frost risk is present. When water is applied to crops under zero or sub zero temperature conditions, it freezes. On freezing the water releases heat, which offsets the heat lost by the crop to its cooler surroundings. To achieve successful frost control a film of continuously freezing water must be maintained on the surface of the formed ice. When the ice begins to melt, usually after dawn when the air temperature begins to rise, heat must be applied to the ice or this heat will be drawn from the crop. This heat is supplied by water applied to the crop via the irrigation system. Generally an application rate of 3.5 mm/hr is adequate for frosts down to 4.0C.
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