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.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
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 measured air temperature.
High temperatures can place a lot of stress on one’s body, especially for young children, older people, those with certain health conditions or on certain medications and those who don’t have ready access to air-conditioning. “No one is immune from the effects of heat, but there are some who are more susceptible,” McMahon says. “It’s a really personalized hazard.”
When the human body overheats, it can become dehydrated, which causes the blood to thicken and the heart to work harder to pump. Heat exposure can lead to heat exhaustion, which can feature nausea, dizziness and muscle cramps. Exposure can also lead to heat stroke and even death.
And though we’re all familiar with how the temperature feels according to whatever scale we use (Fahrenheit in the U.S.; Celsius nearly everywhere else), this figure isn’t always the only factor in summer heat. There’s another measure that plays a big role in how we experience heat, especially when it’s extreme.
Humidity
Most of us are broadly familiar with humidity—the amount of water vapor in the air—and how it can make a hot day much less pleasant and sweatier. But though we know a humid day feels worse, not everyone makes the connection “that an 85-degree-F [29-degree-C] day that is very humid is worse on your body than an 85-degree day that is dry,” says Casey Ivanovich, a Ph.D. student who studies extreme heat at Columbia University.
Humid heat can be more of a health risk because the more moisture there is in the air, the less the body can naturally cool itself off through the evaporation of sweat. So humidity is a key component to consider when thinking about your experience with the day’s weather.
Relative humidity
Forecasts typically communicate this experience through relative humidity, which expresses the amount of water vapor in the air relative to what the air could hold at a given temperature. That means relative humidity is, as its name suggests, relative to the temperature, which can make it difficult to gauge how muggy conditions will feel. An 80-degree-F (27-degree-C) day with 50 percent humidity feels a lot more humid than a 30-degree-F (minus-one-degree-C) day with 100 percent humidity, for example.
Relative humidity can also change during the day: the humidity can be, say, 90 percent in the morning and 50 percent in the afternoon, when the temperature is higher, and yet conditions won’t feel any less humid later in the day.
Dew point
For this reason, a lot of meteorologists and weather enthusiasts prefer to use another measure, the dew point. The dew point is the temperature the air would have to be cooled to for the water vapor in the atmosphere to start condensing out, meaning the air would reach 100 percent relative humidity. The nice thing about the dew point is that it doesn’t change with the temperature, though it can change during the day if a moisture-laden air mass moves in or out of the area.
But you don’t have to remember that definition to use the dew point to gauge how humid it is outside: using a rough guideline, a dew point below 55 degrees F (13 degrees C) is dry and pleasant; between 55 and 65 degrees F (18 degrees C), it’s starting to get sticky; and above 65 degrees F, it can start to feel like you’re swimming through the air. Once you get into the 70s F (low to mid-20s C), it’s oppressive.