# Understanding the Heat Index During Extreme Heat
The heat index tells you how hot the air actually feels on your skin, not just what the thermometer reads. When humidity rises, your body's natural cooling system—sweating—becomes less effective because moisture cannot evaporate from your skin as easily. The result: the air feels hotter than it actually is.
This measurement has scientific origins. Researchers developed the heat index concept in a 1979 journal paper titled "The Assessment of Sultriness," which studied how humans perceive temperature when combined with moisture in the air. The metric has since become standard in weather forecasting and public health alerts across the United States.
A heat index of 90 degrees Fahrenheit might feel like 100 degrees or higher, depending on humidity levels. The National Weather Service uses heat index values to issue warnings and advisories. When the heat index climbs above 103 degrees, the risk of heat exhaustion and heat stroke increases dramatically for vulnerable populations, including older adults, young children, and people with chronic illnesses.
Understanding this distinction matters for your safety. The actual air temperature is less important than the heat index when planning outdoor activities or assessing health risks. A day with 85 degrees and high humidity can pose greater danger than a day with 95 degrees and dry air.
During heat waves, checking the heat index before heading outside helps you decide whether to exercise, run errands, or stay indoors. Symptoms of heat-related illness—dizziness, nausea, headache, rapid heartbeat—can develop quickly when the heat index is dangerously high. Wearing lightweight clothing, staying hydrated, and taking frequent breaks in air-conditioned spaces are essential protective measures.
Weather services now prominently display heat index readings alongside temperature forecasts. This shift reflects growing recognition that perceived heat, not actual temperature, better predicts health