A Simple Rule For Calculating Wet-Bulb Temperature Near Freezing, With Application to Irrigation for Frost Prevention

The accuracy of the “One-Third Rule” approximation for wet-bulb temperature is investigated. It is an empirical simplification that uses the dry-bulb temperature and dew point. This approximation is a weighted average, as opposed to an arithmetic mean of air temperature and dew point, which is another, better-known simplification for wet-bulb temperature. The results of the One-Third Rule indicate that this approximation is more accurate than the simple arithmetic mean for temperatures near freezing (0°C), which is a critical threshold for many applications.

The One-Third Rule can be useful in determining precipitation type, preventing crop damage from sub-freezing temperatures, and also for the creation of snow at ski resorts. Its ease of use makes it ideal for communication to and use by the public.

In this presentation, we focus on calculation of wet-bulb temperature for scheduling irrigation for frost prevention in fruit crops, such as blueberries. Current practice provides templates for determining when to start irrigating that depend on dry-bulb temperature, regardless of humidity. Using wet-bulb temperature instead of dry-bulb temperature allows refinement of irrigation scheduling in cases of unusually dry air. This could reduce crop loss due to earlier irrigation, or increasing savings in water and energy when irrigation will be ineffective in preventing frost damage.