In this presentation, we demonstrate that there is, in fact, a simple approximate formula--the "One-Third Rule"--that relates the three variables of wet-bulb, dry-bulb, and dewpoint temperatures that is quite accurate for wet-bulb temperatures in the general vicinity of the melting point of water. This is fortunate, because several important applications of the wet-bulb temperature are made for conditions near zero degrees Celsius, including precipitation type forecasts in synoptic meteorology and protection from frost damage in agriculture. We first present the empirical roots of the One-Third Rule, then justify it for near-freezing temperatures, illustrate its accuracy using real-world data and examples, and close with suggestions for how to teach this in the atmospheric thermodynamics classroom.
We propose that this work partly fills a small but frustrating gap in atmospheric thermodynamics pedagogy, and hope that it becomes incorporated in future textbooks on the subject.
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