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Heat Index

Heat Stress Risks and Exercise Guidelines

By Coach Gary Westlund

 More of this Feature
• Risk Factors of Heat Illness
• Heat Index
• How to Feel Cooler
• Signs of Heat Illness
• Fahrenheit and Celsius

Heat Index

The heat index combines air temperature and relative humidity to determine an apparent temperature, or how hot it actually feels. Think of it as you do a wind-chill index.

High heat-index days can be health and life threatening even to the non-exerciser. Imagine how much riskier internal heat-producing aerobic endurance activities are, when one starts out in a heat-hazardous environment.

Heat Index Chart

Temp          Dew Point(F)
(F)     50.0  55.0  60.0  65.0  70.0  75.0  80.0  85.0 
  65:    62.7  63.8  65.0  66.6                         
  70:    67.8  68.7  69.8  71.1  72.6                   
  75:    73.1  73.9  74.8  75.9  79.2  80.7             
  80:    79.8  80.6  81.6  82.8  84.4  86.9  90.9       
  85:    83.5  84.7  86.1  88.0  90.5  94.0  99.0 106.6 
  90:    87.9  89.4  91.2  93.6  96.9 101.2 107.2 115.6 
  95:    92.9  94.5  96.7  99.6 103.4 108.4 115.2 124.3 
 100:    98.1  99.9 102.4 105.6 109.8 115.3 122.7 132.3 
 105:   103.4 105.4 108.1 111.6 116.1 122.0 129.7 139.7 
 110:   108.7 110.9 113.8 117.5 122.3 128.4 136.3 146.5

Any value less than 80 is considered comfortable.
Any value greater than 90 is considered extreme.
Any value greater than 100 is considered hazardous.
Any value greater than 110 is considered dangerous.


Humidity is the amount of moisture in the air. Humidity is of particular concern to aerobic exercisers whose primary cooling mechanism is perspiration evaporating. It's the evaporation of that perspiration that causes some cooling effect, not the process of perspiring itself. In other words, in water-vapor-saturated air (high humidity), there is no evaporation of perspiration, and therefore, our principle cooling mechanism is not functioning for us.

Dew point

Dew point is the temperature at which a vapor begins to condense. Dew points are sometimes reported and/or used rather than relative humidity. Beware of dew points above 70.

Heat Factors to Consider

Temperature: Actual air temperature. You'll want to learn both reporting systems, Fahrenheit and Celsius.

Radiant heating: The increase in heating due to direct sunlight. Heat is transferred from the surface of one object to the surface of another with no actual physical contact.

Convective heating: The increase in heating due to wind above a certain temperature. Wind chills below freezing, and wind heats above about 72 degrees. Think of how convection ovens speed up the cooking process. Don't be a "turkey cooking" faster, and wondering why the wind on a hot day isn't cooling you. It "cooks" you.

Conductive heating: The increase in heat from contact. This would be the added heat transferred from hot pavement or macadam through your shoes to your hot feet.

Heat reducing, or heat-loss processes are the reverse of the above. What determines whether you're going to be heated up or cooled down is the temperature gradient, or the temperature differences between you and your environmental heating/cooling mechanisms. Add to these the processes of evaporation mentioned above, which is our main heat-loss mechanism when the temperature rises.

Next page:> How to Feel Cooler When You Walk or Race in the Heat
Signs and Types of Heat Illness
Converting Fahrenheit and Celsius Temperatures
First Page: Risk Factors of Heat Illness with Exercise

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Steadman, R. G., 1979: The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. J. Appl. Meteor., 18, 861–873. doi: 10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2.

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