Heat Index & Heat Safety Explained

Heat doesn't knock down buildings or show up on radar, so it rarely feels like a weather emergency. Yet over the long term heat kills more Americans than any other weather hazard, more than hurricanes, tornadoes, floods, or lightning. Most heat deaths are preventable, and they usually happen to people who were indoors without cooling, working or exercising outdoors, or left in a closed vehicle. This guide explains how to read the numbers and recognize when heat has become dangerous.

Why humidity matters: the heat index

Your body cools itself mainly by sweating: when sweat evaporates from your skin, it carries heat away. Humid air already holds a lot of water vapor, so sweat evaporates more slowly and cooling becomes less effective. The heat index combines air temperature and relative humidity into a single "feels like" number that reflects this.

Values are calculated with the National Weather Service heat index formula and rounded. Two caveats matter more than the exact figures:

• The heat index assumes shade and a light breeze. Working or exercising in direct sunshine can add up to about 15 °F to the effective heat stress.
• It doesn't account for exertion, clothing, or acclimatization. Someone in protective gear doing heavy labor can get into trouble at a heat index well below 100.

For outdoor work and sports, many organizations now use the wet bulb globe temperature (WBGT) instead, which accounts for sun, wind, and humidity together. If your employer, school, or athletic program publishes WBGT-based guidelines, follow those.

Heat exhaustion vs. heat stroke

Heat illness tends to progress, and recognizing the stages early is the most important skill in this guide.

Confusion or a change in behavior is the key warning sign that heat exhaustion is becoming heat stroke. People with heat stroke often don't realize anything is wrong, so watch out for each other.

Who is most at risk

• Older adults, especially those who live alone, have chronic illness, or take medications that affect fluid balance or temperature regulation (some blood pressure, heart, and psychiatric medications).
• Infants and young children, who heat up faster and depend on others to keep them cool.
• Outdoor workers and athletes, especially early in the season before their bodies have adjusted. Acclimatization takes about one to two weeks of gradually increasing exposure.
• People without air conditioning, especially in upper floors of apartment buildings, which can stay dangerously hot overnight.
• Pets, particularly short-nosed dog breeds and any animal left outside without shade and water. Hot pavement can burn paws: if it's too hot for the back of your hand for seven seconds, it's too hot for a dog's feet.

Hot cars

A closed car parked in the sun heats up remarkably fast: the temperature inside can rise about 20 °F in the first ten minutes, and keep climbing from there. Even on a 70 °F day the interior can reach well over 100 °F. Cracking the windows makes little difference. Dozens of children die in hot cars in the United States each year, most of them unintentionally forgotten by a caregiver whose routine changed. Simple habits help: put something you'll need, like your phone or bag, in the back seat, and always check the back before locking the car.

Nights matter as much as afternoons

The most dangerous heat waves are often the ones where it doesn't cool off at night. When overnight lows stay in the upper 70s or 80s, bodies that went without relief all day don't get the chance to recover, and indoor temperatures in buildings without air conditioning keep rising day after day. Cities make this worse: pavement and buildings absorb heat during the day and release it at night, so urban neighborhoods can stay much warmer than surrounding suburbs and countryside. This is called the urban heat island. If a forecast shows several days in a row with warm nights, treat it as more serious than a single hot afternoon.

Heat alerts

Heat alert criteria vary by region, because people in Phoenix and people in Seattle are acclimatized to very different conditions. In general:

• Heat Advisory — heat is expected to be hazardous, especially to sensitive groups and anyone working outside.
• Extreme Heat Watch — dangerous heat is possible in the next few days.
• Extreme Heat Warning — dangerous, potentially life-threatening heat is expected. (These were called Excessive Heat Watches and Warnings before 2025; the meaning didn't change.)

The National Weather Service also publishes HeatRisk, a 0 to 4 forecast scale that factors in how unusual the heat is for the time of year and location and whether nights will cool off, so it can flag early-season heat that people aren't yet adjusted to.

On the RadarBug map, heat alerts are drawn as outlines only, so they don't cover other layers over large areas. The outline color shows severity: purple for extreme, light red for severe, orange for moderate, and yellow for minor.

Staying safe

• Find cooling. Air conditioning is the strongest protection against heat illness. Electric fans help in moderate heat but can't prevent heat illness once indoor temperatures are in the high 90s. Many cities open cooling centers during heat warnings; libraries, malls, and community centers are options too.
• Drink before you're thirsty. Water is best. Limit alcohol and very sugary drinks. If you're sweating heavily for hours, a drink with electrolytes helps.
• Shift your schedule. Do strenuous activity in the early morning, when it's coolest, and take frequent breaks in the shade.
• Dress for it: loose, lightweight, light-colored clothing and a wide-brimmed hat.
• Check on people at least twice a day during a heat wave, especially older neighbors and relatives who live alone.