Heat Index, WBGT, or HeatRisk: which heat number should you use?

Choose the question first

The three tools measure different parts of heat risk

There is no single number that perfectly describes heat exposure for every person, place, and activity. Before acting on a map or app value, identify the question you are trying to answer.

Heat Index

Use Heat Index for the combined effect of air temperature and humidity

The NWS Heat Index estimates how hot conditions feel when relative humidity is combined with the actual air temperature. It is the familiar "feels like" value used in many warm-season forecasts. It is most useful for a person in shade with light wind—not for every outdoor setting.

• Read the hourly value because temperature and humidity change through the day.
• NWS says full sunshine can increase heat exposure represented by the Heat Index by as much as 15°F.
• Very hot, dry, windy conditions can also be dangerous even when humidity does not produce the day's highest Heat Index.
• Do not compare a shaded Heat Index number directly with a sun-exposed WBGT number; the scales and intended uses differ.
• Use the number to reduce exposure, not to determine whether a specific person is safe. Clothing, health, exertion, acclimatization, and access to cooling matter.

A Heat Index forecast tells you about expected apparent temperature. It does not tell you whether an NWS Heat Advisory or Extreme Heat Warning is active. Alert criteria and expected impacts can vary by local climate and forecast office.

Wet Bulb Globe Temperature

Use WBGT for outdoor work, sports, and exercise in direct sun

WBGT estimates heat stress by combining temperature, humidity, wind, solar radiation, sun angle, and cloud cover. NWS describes it as especially useful for active, acclimatized people such as outdoor workers and athletes. Forecast WBGT can help move, shorten, slow, or add rest to strenuous activity before exposure begins.

• Use a WBGT forecast for the exact time and location of the activity.
• Follow the established WBGT policy for the school, league, workplace, military unit, or event; do not invent a universal cutoff.
• Account for protective equipment, uniforms, heavy clothing, workload, recovery time, shade, water, and cooling access.
• New workers and athletes, or anyone returning after time away, may not be acclimatized even when others are.
• WBGT guidance varies geographically, so follow local officials and the organization responsible for the activity.

NWS currently labels forecast WBGT an experimental tool. It can improve planning, but it is not supported as an around-the-clock warning service and it does not replace an official alert or an organization's heat-safety rules.

NWS HeatRisk

Use HeatRisk to understand local impact potential over the next seven days

HeatRisk assigns a color and number to the forecast level of heat concern for a specific location. It uses NWS gridded forecasts through seven days and considers more than the afternoon high: how unusual the temperature is locally, the time of year, how long the heat lasts, whether nights provide relief, and whether the forecast reaches levels associated with increased heat-health impacts.

NWS says HeatRisk is supplementary to official heat watches, warnings, and advisories. It is also experimental and not supported 24/7. Treat it as planning guidance, then check the current forecast, active alerts, and instructions for your exact location.

Read the product name

An outlook, forecast, watch, advisory, and warning require different decisions

A Heat Index, WBGT, or HeatRisk value describes part of expected heat stress. The official product label tells you how specific and urgent the information is. Always read its issue time, valid period, affected area, and instructions.

CPC heat outlook

A probability-based signal for a broad region and future period. Use it to review plans and monitor later updates; it is not a local forecast or alert.

Local NWS forecast

Expected hourly and daily conditions for a specific place. It may include temperature, humidity, Heat Index, and timing, but a forecast is not automatically a watch or warning.

Extreme Heat Watch

Conditions are favorable for an extreme-heat event, but occurrence, timing, or duration remains uncertain. Prepare to act if later forecasts confirm the threat.

Heat Advisory

Dangerous heat is expected or occurring, but conditions are not expected to reach warning criteria. Reduce exposure and follow the advisory instructions.

Extreme Heat Warning

Extremely dangerous heat is expected or occurring. Take action, use effective cooling, and follow the warning and local emergency instructions.

Turn the number into a plan

Match protection to exposure, activity, and the people involved

1. Check the local hourly forecast and active alerts before outdoor work, sports, travel loading, yard work, or an event.
2. Use Heat Index for shaded humid exposure, WBGT for hard activity in sun, and HeatRisk for the broader daily health-impact picture.
3. Move strenuous activity earlier or later, reduce intensity, add rest, and provide water and shade when heat stress rises.
4. Confirm that the cooling plan still works during a power outage and know a safe air-conditioned place you can reach.
5. Check on older adults, young children, pregnant people, people with disabilities or chronic conditions, outdoor workers, and anyone without reliable cooling.
6. Never leave a child, dependent adult, or pet in a parked vehicle.

Personal medical conditions and medicines can change heat risk. Use guidance from a healthcare professional for individual health decisions. For immediate danger or signs of heat stroke, call 911.

Recognize heat illness

Heat exhaustion needs prompt cooling; heat stroke is an emergency

Heavy sweating, weakness, cool or clammy skin, dizziness, nausea, headache, muscle cramps, or fainting can signal heat exhaustion. Move the person to a cooler place, loosen clothing, apply cool wet cloths, and offer sips of water if the person is alert and able to drink. Seek medical help if symptoms worsen, vomiting occurs, or symptoms last longer than one hour.

Confusion, slurred speech, loss of consciousness, very high body temperature, or hot red skin can signal heat stroke. NWS says to call 911 immediately, move the person to a cooler place, and cool with wet cloths or a bath. Do not give fluids to a person with suspected heat stroke.