NOAA's National Weather Service Heat Index Program

Considering this tragic death toll, 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. As an example, if the air temperature is 95F (found on the left side of the table), and the relative humidity is 55% (found at the top of the table), the HI -- or how hot it really feels -- is 110F. 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 15F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Note on the HI chart the shaded zone above 105F. 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 / Heat Disorders
Heat Index
Possible heat disorders for people in higher risk groups
130F or higher Heatstroke/sunstroke highly likely with continued exposure.
105 - 130F Sunstroke, heat cramps or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity.
90 - 105F Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
80 - 90F Fatigue possible with prolonged exposure and/or physical activity.

The "Heat Index/Heat Disorders" table relates ranges of HI with specific disorders, particularly for people in the higher risk groups.

Summary of NWS's Alert Procedures

The NWS will initiate alert procedures (advisories or warnings) when the Heat Index (HI) is expected to have a significant impact on public safety. The expected severity of the heat determines whether advisories or warnings are issued. A common guideline for the issuance of excessive heat alerts is when the maximum daytime HI is expected to equal or exceed 105F and a nighttime minimum HI of 80F or above for two or more consecutive days. Some regions and municipalities are more sensitive to excessive heat than others. As a result, alert thresholds may vary substantially from these guidelines. Excessive heat alert thresholds are being tailored at major metropolitan centers based on research results that link unusual amounts of heat-related deaths to city-specific meteorological conditions.

The alert procedures are:

  • Include HI values in zone and city forecasts.
  • Issue Special Weather Statements and/or Public Information Statements presenting a detailed discussion of (1) the extent of the hazard including HI values, (2) who is most at risk, (3) safety rules for reducing the risk.
  • Assist state and local health officials in preparing Civil Emergency Messages in severe heat waves. Meteorological information from Special Weather Statements will be included as well as more detailed medical information, advice, and names and telephone numbers of health officials.
  • Release to the media and over NOAA's own Weather Radio all of the above information.
 
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Designed by Jerren Saunders
Last Updated: June 12, 2002