NWS Heat Index: A Problematic Heat Stress Standard

The US National Weather Service (NWS) is responsible for weather forecasts including estimates of heat stress. Since about 1979, the NWS has utilized a “heat index” to notify the American public of heat-related environmental risk. Why this heat index was chosen by the NWS is not known to the author. It is suspected that it was chosen at least in part because it could be calculated using weather data (dry bulb temperature and relative humidity) that was universally available from the Automated Surface Observation System (ASOS) readings that had been in place at airports nationwide for many years.

This heat index, based on the work of R.G. Steadman, is supposed to provide an “apparent” temperature. That is, the NWS heat index is supposed to reflect how a person would “feel” in a given set of conditions. It seems to the author of Heat Stress Adviser that the NWS Heat Index is ill suited for application to estimates of heat stress in outdoor sports, at least in part due to its underlying assumptions. Among these questionable assumptions are the following:

1.  It is assumed that the person is in shady conditions.

2.  The clothing of the person is assumed to be long trousers and a short-sleeved shirt.

3.  Wind speed is assumed to be 5.8 mph.

4.  It is assumed that sweating is uniform and sweat is not dripping from the body.

5.  The activity of the person is assumed to be walking at a speed of 3.1 mph.

6.  It is assumed that the user of the NWS Heat Index will understand this: “Full sunshine can increase Heat Index Values by up to 15°F.” Whether the user of the NWS Heat Index will be willing to add a certain number of degrees to the announced Heat Index, and if so how many degrees to add, is unknown. No guidelines are given for this crucial aspect of the application of the NWS Heat Index. A 15°F change in Heat Index can cause substantial change in the guidelines provided by the NWS.

So, at least for outdoor sports, it seems safe to say that hardly anyone (or, more likely, no one) has used the NWS Heat Index correctly in the 25 years since its introduction. To the best of our knowledge, there has not been a single scientific paper published on heat stress in sports that utilized the NWS Heat Index as its heat stress standard.

References:

Heat Wave: A Major Summer Killer. Anonymous. U.S. Department of Commerce, National Oceanic and Atmospheric Administration. NOAA/PA 85001 (pamphlet)

Rothfusz LP: The heat index “equation” (or, more than you ever wanted to know about heat index). Technical Attachment.  Scientific Services Division, NWS Southern Region Headquarters, Fort Worth, TX. SR 90-23, 7/1/90.

Steadman RG: The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science. Journal of Applied Meteorology 18:861-873, 1979.

Steadman RG: The assessment of sultriness. Part II: Effects of wind, extra radiation and barometric pressure on apparent temperature. Journal of Applied Meteorology 18:874-885, 1979.

Here are the NWS Heat Stress Guidelines:

 HI = 80-90°F     Fatigue possible with prolonged exposure and physical activity.

HI = 90-105°F    Sunstroke, heat cramps and heat exhaustion possible.

HI = 105-130°F    Sunstroke, heat cramps and heat exhaustion likely, and heat stroke possible.