Historical Changes in the Heat Index

Historical Changes in the Heat Index

This tool is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2023.

Tool Background

The Heat Index is a combined measure of temperature and humidity that reflects how hot it feels to individuals. High Heat Index values can be dangerous to human health and serve as an important metric for setting policies or regulations that reduce exposure and protect public health. Table 1 shows four classifications of the Heat Index that represent levels at which human health can be impacted.

Table 1. Heat Index Classifications

Classificiation Heat Index Effect on the Body
Caution 80°F–90°F Fatigue possible with prolonged exposure and/or physical activity
Extreme Caution 90°F–103°F Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity
Danger 103°F–124°F Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged esposure and/or physical activity
Extreme Danger 125°F or higher Heat stroke highly likely

Source: National Weather Service

Key Findings

  • The Mid-Atlantic region has seen two to thee times more days with heat index values above the Caution classification, when human health may be impacted with prolonged exposure, compared to 1981–1990.
  • The region has also seen three to five times more days with heat index values above the Extreme Caution classification, at which heat stroke, heat cramps and heat exhaustion are possible.
  • Much of Virginia has regularly experienced more than 100 days per year above the Extreme Caution classification, at which heat stroke, heat cramps and heat exhaustion are possible.

Figure 1. Days with High Heat Index Values

How to Use the Tool

Selecting Temperature Thresholds and Time Periods Use the filters to the right of the maps to adjust the Heat Index threshold and the 10-year time period.

Technical Notes

Heat Index values were calculated based on station data for air temperature and relative humidity using the Rothfusz equation employed by the National Weather Service.1

Footnotes

  1. https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml Return to text ⤴

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