Weather

Wind chill, heat index and dew point are each defined by a published equation, and each is only valid over a range. Wind chill, the National Weather Service's 2001 formula, applies at or below 50 °F with a wind above 3 mph; outside that it gives numbers that mean nothing, so the page reports the air temperature instead. Heat index follows the Weather Service's own procedure: a simple formula first, and the Rothfusz regression with its two corrections only when conditions are hot enough to need it. Dew point uses the Magnus approximation with Alduchov and Eskridge's coefficients, and runs in reverse to give relative humidity from a dew point. Enter temperatures in °F or °C and they are converted exactly, including the −40 where the two scales meet.