Flow and Thermal Errors in Current National Weather Service Wind Chill Models

Abstract

The two National Weather Service (NWS) wind chill models in operation since 2001 have inherent errors. The first model attempted and failed to make a facial surface temperature correction on the existing Siple and Passel model. The second model, intended as an improvement on the first, erred by mistakenly defining the wind chill temperature in terms of an internal body temperature rather than the facial surface temperature. To account for a boundary-layer reduction in the free-stream velocity at head level, both models incorrectly apply a constant high-percentage reduction to this velocity as measured at the NWS 10 m level. As a result of these errors, both models predict wind chill temperatures much warmer than the actual values. These warmer temperatures can instill a public complacency whereby facial freezing is viewed as a remote possibility when in reality it may be imminent.