Pictures from around western Colorado and eastern Utah
Heat Index
What is the Heat Index?

Hot Sun with Sunglasses Considering the tragic death toll of heat-related incidents, the National Weather Service has stepped up its efforts to alert more effectively the general public and appropriate authorities to the hazards of heat waves -- those prolonged excessive heat/humidity episodes.

Based on the latest research findings, the NWS has devised the "Heat Index" (HI), (sometimes referred to as the "apparent temperature"). The HI, given in degrees Fahrenheit, is an accurate measure of how hot it really feels when the relative humidity (RH) is added to the actual air temperature.

To find the Heat Index, look at the Heat Index Chart or Calculator below. As an example, if the air temperature is 95° F, and the relative humidity is 55%, the HI -- or how hot it really feels -- is 110° F. This is at the intersection of the 95° row and the 55% column.

Important: Since HI values were devised for shady, light wind conditions, exposure to full sunshine can increase HI values by up to 15° F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Note on the HI chart the shaded zone above 105° F. This corresponds to a level of HI that may cause increasingly severe heat disorders with continued exposure and/or physical activity.

Heat Index Chart
Heat Index Calculator
(by Tim Brice and Todd Hall)
Enter a temperature that you would like and choose your units: What the temperature feels like to your body:
Fahrenheit Celsius ° F
Enter your relative humidity:  
% ° C
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Heat Index / Heat Disorders
Heat Index Possible heat disorders for people in higher risk groups
130°F or higher Heatstroke/sunstroke highly likely with continued exposure.
105° - 130°F Sunstroke, heat cramps or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity.
90° - 105°F Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
80° - 90°F Fatigue possible with prolonged exposure and/or physical activity.
Heat Index Calculation

The computation used for the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23). The regression equation of Rothfusz is:
HI = -42.379 + 2.04901523 * T + 10.14333127 * RH -.22475541 * T * RH - .00683783 * T * T - .05481717 * RH * RH + .00122874 * T * T * RH + .00085282 * T * RH * RH - .00000199 * T * T * RH * RH

where T is temperature in degrees F and RH is relative humidity in percent. HI is the heat index expressed as an apparent temperature in degrees F. If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI:
ADJUSTMENT = [(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17}

where ABS and SQRT are the absolute value and square root functions, respectively. On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI:
ADJUSTMENT = [(RH-85)/10] * [(87-T)/5]

The equation for HI above with the appropriate adjustment is used to compute a maximum heat index using the Hydrologic Prediction Center forecast maximum temperature and the 00 UTC MOS dew point temperature at each forecast point location for each forecast projection day. Similarly, a minimum heat index is computed. The forecast average heat index for the projection day is the average of these two values, the maximum heat index and the minimum heat index.


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