Approximating Wet-Bulb Temperature Using the One-Third Rule

The accuracy of the “One-Third Rule” approximation for wet-bulb temperature is investigated using 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 approximation 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 determining precipitation type in winter storms. The arithmetic mean however, is more accurate for warmer temperatures. This approximation can be useful in determining precipitation type, preventing crop damage from sub-freezing temperatures, and also for the creation of snow at ski resorts.