Summarizing, the following points are emphasized as being important considerations in the proper heating of orange groves in Florida to protect the fruit against low temperatures:
1.) With a fast drop of the air temperature, the fruit temperature tends to parallel it closely. With slowly falling or stationary air temperature, the fruit temperature invariably drops lower than the air temperature, except occasionally when freezing of the fruit has already begun. On dry nights, exposed oranges tend to maintain a temperature as much as 3° or more lower than a stationary or slowly falling air temperature. On nights when dew and frost form on the fruit, this temperature difference is usually reduced to from 1° to 2°. The factors believed responsible for this radically different characteristic of Florida oranges, as compared with California oranges, are the thinner rind and the greater amount of moisture in the rind of the Florida orange. Both these properties cause a faster rate of cooling by allowing more rapid conduction of heat from within the orange to the surface than does the thicker, drier skin of the California orange.
2.) The proper use of fruit thermometers provides the best temperature index in scientific grove heating. On nights of dry cold, unshielded fruit thermometer records are representative of true grove conditions. From experimental data, however, it appears that to obtain reliable records on nights when the dew point temperature is high and frost forms, the puncture in the fruit made by the thermometer must be sealed with petroleum jelly, or some other suitable substance, to prevent the frost particles from coming in contact with the fruit juice at this point; otherwise, premature crystallization in the fruit will occur.