When One Agency Publishes Two Heat Numbers, the Difference Is in the Inputs
Two numbers from the same agency A National Weather Service forecast page can carry two temperature-like numbers for the same hour and the same location, and they will not agree. One is the heat index, the number that appears beside the air temperature in most forecast products. The other is Wet Bulb Globe Temperature, or WBGT, which several NWS field offices publish as a separate gridded product. When the two are compared on a hot afternoon, the natural assumption is that one of them is a correction of the other, or that a newer instrument has replaced an older one. Neither is the case. The gap between them is not a disagreement about the weather. It is a difference in what each number was defined to take in. The heat index accepts two variables. WBGT accepts five. Everything else that separates them follows from that. The heat index published by the NWS is not a reading from any instrument. It is the output of a multiple regression fitted to a table, and the equation is public....