Watch out for the heat index. If the feels-like temperature is 103 degrees or higher, the National Weather Service says people outside for long periods or doing physical activity in the heat are , As shown in the table below, heat indices meeting or exceeding 103 ° F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat. Use this weather calculator if you prefer to enter numbers manually instead of reading a chart., The "heat index" is how hot it feels, combining temperature and humidity. Dangerous heat is the No. 1 weather-related killer in the United States. Find a heat index chart plus sun safety tips., Checking the temperature reading during a heat wave won’t tell the whole sweltering story. It’s going to feel a lot hotter. Blame the heat index, also called “apparent temperature.” It’s a, According to NOAA, excessive heat was the leading cause of weather-related death in the United States from 1992 to 2021. In 2024, excessive heat took 199 lives in the U.S., more than floods (89 , .