The Post is tracking the potential for dangerous heat using the heat index, which accounts for the combined impact of temperature and humidity — the higher the humidity, the more difficult it is for the body to cool itself off through sweating.
Extreme heat kills more people in the United States than any other weather hazard, and the risk of longer and more frequent heat waves is only expected to increase as climate change worsens. Heat disorders such as heat stroke, heat cramps and heat exhaustion are possible with any extended exposure to a heat index at or above 90 degrees.
Heat illness can set in quickly — in as little as 10 to 15 minutes — when your body overheats and can’t properly cool itself off. This can lead to muscle cramps or spasms, heavy sweating, weakness or tiredness, abnormal pulse rate, dizziness, nausea, vomiting, headache, confusion, fainting, loss of consciousness or death.
See how high the heat index is expected to reach where you live in the next seven days using the lookup tool below.
Multiple days of extreme heat, including warm nights that don’t allow our bodies to cool down, are especially dangerous. A Washington Post analysis of data provided by the nonprofit First Street Foundation estimated that the average number of Americans experiencing at least three consecutive days of temperatures 100 degrees or higher each year will climb from 46 percent today to 63 percent over the next 30 years.
Hottest cities today, by heat index
Urban centers, which have fewer trees, less grass, and heat-absorbing pavement, can be up to 20 degrees hotter than nearby neighborhoods, according to the National Oceanic and Atmospheric Administration.
The Weather Service issues heat watches, warnings and advisories when extreme heat — generally a heat index of 100 degrees or higher — is expected or imminent. Any watch, warning or advisory in effect for your location can be seen by entering your location into the lookup box at weather.gov.
How many people may be exposed to heat
Infants and children up to four years old, adults 65 years and older, and people who are overweight, ill, or on certain medications are at the highest risk for heat-related illness, according to the Centers for Disease Control and Prevention. Outdoor workers and athletes are also at greater risk.
The Weather Service recommends wearing light, loosefitting clothing, drinking water often before you get thirsty, reducing or rescheduling strenuous activity, and staying in air-conditioned places during extreme heat.
Dan Stillman and Niko Kommenda contributed to this report.