Health library

July 13, 2026—How hot is it out there? That's an important question when it comes to your comfort and your safety. And there's more than one way to answer it.

In fact, the National Weather Service (NWS) uses several tools to forecast heat hazards and issue heat watches, warnings and advisories to the public. Knowing about these tools can also help you prepare for hot weather. Here are some of them.

Heat index

Humidity can make hot weather feel even hotter. That's because humidity messes with the body's ability to cool itself through sweating. This is where the heat index comes in.

Some people call the heat index the "feels like" or "apparent" temperature. Whatever you call it, the heat index calculates how hot it really feels outside based on the relative humidity and the actual air temperature. For example, a temperature of 94 degrees Fahrenheit and a relative humidity of 60% will feel like 110 degrees!

The higher the heat index, the hotter it's going to feel and the higher the risk of heat-related illness is, the NWS reports.

When to use it: The heat index is good for planning most of your day-to-day activities. But keep in mind: The heat index assumes you're in the shade with a light wind. This means that:

• If you're in direct sunlight, it can feel up to 15 degrees hotter than the heat index indicates.
• Strong, hot winds can make the heat riskier than the heat index reports.

Wet bulb globe temperature (WBGT)

Do you work or exercise outside in the sun? If so, you may want to check the WBGT instead of (or in addition to) the heat index.

This tool uses temperature (measured in full sunlight), humidity, wind, sun angle and cloud cover to estimate the potential for heat-related stress on the human body. It is based on combined temperatures from three thermometers: a black globe for the sun's rays and angle; a wet bulb for humidity and a dry bulb for the air temperature.

When to use it: Checking the WBGT may be a better way to predict how the heat may affect your health, according to the American Council on Exercise. It may be especially useful for people engaged in outdoor work, sports or exercise. In fact, many school athletics programs use the WBGT for deciding the intensity of their practices.

HeatRisk

Here's another way to tell if it's going to be hot enough to potentially hurt your health.

The NWS HeatRisk tool forecasts the risk of heat-related health effects for the contiguous United States over the next seven days. HeatRisk uses a color code and a number to show how risky the heat could be for your area. It gives information for heat-sensitive groups, such as older adults, young children, outdoor workers and people sensitive to poor air quality.

For example, if the HeatRisk value is 2 (orange), there will be a moderate risk of heat-related impacts. Those impacts are most likely to be experienced by people in sensitive groups, those who are exposed to the sun for long periods and those without air conditioning.

Unlike other heat forecasting tools, the NWS HeatRisk tool identifies unusual heat and accounts for what your body is used to. For example, an early season heat wave or a period of very warm overnight lows and daytime highs could pose more risks because people aren't yet acclimated to the heat.

The HeatRisk tool also recommends steps those at risk can take to stay safe when a heat-related impact is forecast.

When to use it: Consider the NWS HeatRisk another way to monitor and prepare for hot weather. It might be especially useful when planning activities to avoid the heat early in the season.

Where to find these heat values

Phone apps and local weather reports often show heat index, temperatures and other values. You can also find charts and calculators on the NWS website.

Knowing when it's dangerously hot is an important part of preventing heat illness. Learn more ways to stay safe in extreme heat.

Sources

• American Council on Exercise. "Rethinking the Heat Index: An Evidence-Based Approach." https://www.acefitness.org/resources/pros/expert-articles/8914/rethinking-the-heat-index-an-evidence-based-approach.
• National Oceanic and Atmospheric Administration. "HeatRisk – Overview." https://www.wpc.ncep.noaa.gov/heatrisk/pdf/HeatRisk-v2.6-Overview.pdf.
• National Weather Service. "Heat Forecast Tools." https://www.weather.gov/safety/heat-tools.
• National Weather Service. "Heat Index." https://www.weather.gov/arx/heat_index.
• National Weather Service. "HeatRisk—Understanding HeatRisk." https://www.wpc.ncep.noaa.gov/heatrisk/pdf/HeatRisk-v2.5-Understanding-HeatRisk.pdf.
• National Weather Service. "HeatRisk—What's in HeatRisk?" https://www.wpc.ncep.noaa.gov/heatrisk/pdf/HeatRisk-v2.6-Whats-in-HeatRisk.pdf.
• National Weather Service. "Wet Bulb Globe Temperature vs. Heat Index." https://www.weather.gov/ict/WBGT.
• National Weather Service. "What Is the Heat Index?" https://www.weather.gov/ama/heatindex.
• National Weather Service. "Why Is It Called Wet Bulb Globe Temperature?" https://www.weather.gov/images/safety/WBGT-05.jpg.