The summer sun is rising in the sky as you’re getting ready for the day. You check the forecast to see how hot it will be: The high is 94 degrees Fahrenheit (34 degrees Celsius). The heat index is 102 degrees F (39 degrees C). The relative humidity is going to be 50 percent in the afternoon. There’s a heat advisory in your area, and the “heat risk” is orange. Clearly, it’s going to be a warm, muggy day. But what exactly does this jumble of numbers and terms actually mean for what you’ll experience outside and what precautions you should take?
“It can be confusing, having multiple parameters or indicators of heat and heat stress,” says Kimberly G. McMahon, public weather services program manager at the National Weather Service (NWS) and co-lead at the National Integrated Heat Health Information System.
Scientific American is here to help unpack the various measures and tools used to communicate heat and its associated risk to help you prepare for the day.
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Temperature
When you’re talking about a heat event, the baseline measurement is air temperature.
The NWS has a series of stations that measure the temperature of the air a few feet off the ground. Of course, temperature can vary widely over even a small area, so the reported figure for, say, New York City may not be the same across the entire metropolitan area. The verdant expanse of Central Park is generally a few degrees Fahrenheit cooler in the summer than a neighborhood with plenty of paved surfaces and little green space.
And our experience of temperature varies depending on whether we’re in the shade or the sun: standing in direct sunlight can feel 10 to 15 degrees F (six to eight degrees C) hotter to the body than the…
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