Although active protection methods have been used extensively for the protection of horticultural crops from freezing, little effort has been devoted to understanding the effects of protection practices on the energy balance. Understanding the effects of practices on energy transfer is crucial for determining the best management and to address the economic viability of a given practice. Clearly, the air temperature falls during radiation freeze nights because there is a bigger loss of energy to net radiation than is gained from sensible and soil heat flux density. The rate of temperature decline depends on the relative contributions and losses of energy. Management factors like soil water content and the presence of ground cover affect the energy balance and hence temperature fall rates. In addition, weather factors like wind speed, cloud cover or fog, and humidity will influence the net radiation loss and heat transfer from the air and soil. Also, condensation or deposition (dew or frost) will release latent heat to slow declining temperatures as well.
Active protection methods (e.g., sprinklers, surface irrigation, wind machines, etc.) provide a source of heat in addition to that coming from sensible and soil heat fluxes. However, there are also losses of energy (e.g., evaporation or sublimation) when active methods are employed for freeze protection. In this paper, a nighttime energy balance model for freeze protection of an orchard will be presented. This model accounts for net radiation, soil and sensible heat flux, etc. and it allows for the introduction of other sources of energy (e.g., from freezing water) used to add heat for freeze protection. The model is useful for evaluating the management of protection methods, cost benefits, and for teaching energy balance concepts.