Note that the value for b depends on the temperature units. It is important to remember that this method will only work during radiation freezes with calm, clear nights when the temperature drops because of long wave radiation losses from the surface. However, radiation freezes do not occur during cloudy, windy conditions. Generally, subtropical crops are most sensitive to freeze damage just before and during the harvest period when the temperature is lowest. The sensitivity of deciduous tree and vine crops to freezing temperature increases from first bloom to the small nut or fruit stages.
Sensitivity is also higher when warm weather has preceded the frost night.
Damage to a crop depends on how low the temperature goes and how long it is at the minimum temperature. Most published values for freeze damage to crops give the T10, T50, and T90 for 30 minutes at a given minimum air temperature. At any given time during the night, the corresponding temperature is the predicted hourly mean temperature for the preceding hour. Therefore, the critical time for starting any active freeze protection method is one hour before the critical damage temperature is expected. Figures 3 and 4 show the predicted versus observed temperatures over several days.