But it's a "Dry Heat"
I know people in Arizona hate to hear it when we say, “113º? Yeah, but it’s a dry heat.” It doesn’t make 113º any cooler but, when we consult the National Weather Service Heat Index Chart, it can be 88º with a humidity of 90% and the body perceives it as the same temperature. Heat stroke might not have been on the radar at 88º but it should be depending on the humidity.
Remember the methods of heat transfer…conduction, convection, radiation, and, in the heat, evaporation? One of the most important ways the body stays cool is through evaporation. However, at humidities over 75%, evaporative cooling is no longer effective and our perceived temperature is very different.
By looking at the ambient temperature across the top of the chart and the relative humidity on the left side, we can determine the perceived temperature. Other factors such as wind speed and acclimatization will have an impact too, so the chart is just a starting point but it should be considered as we plan for strenuous outdoor activities.
Source: National Weather Service
103º?? We’re not complaining…we’re in Mexico
Other factors that should be considered are that these temperatures are measured in the shade. If you are in the sun, you might have to add 15º. Another consideration in the wilderness is when you are moving through different terrain, you will cross different microclimates. You might be traveling into different conditions depending on sun exposure, vegetation, and wind conditions. Maintain your situational awareness.
As we talk about heat illness in a clinical setting, hyperthermia is determined by measuring the core temperature. However, in the field without a thermometer, we should be considerate of the body's perceived temperature (a combination of ambient air temperature and humidity) and look for clinical signs like Altered Levels of Responsiveness that would indicate someone is deteriorating past the typically heat affected or heat exhausted patient.
More information can be found at the National Weather Service website article What is the heat index?. While the chart is a handy tool, a calculator can be found at Heat Index Calculation. Forecasts are available at WPC Heat Index Forecasts (Days 3-7).
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References:
WPC Heat Index Forecasts (Days 3-7)
Author:
Peter Robbert is the Clinical Coordinator for BMG. He is a retired Fire Captain/Paramedic and has been instructing medical and rescue skills since 2007. He has a masters degree in Outdoor Adventure and Expedition Leadership from Southern Oregon University specializing in trip planning and risk management.