Heat Index vs. WBGT vs. HeatRisk: Which Heat Forecast Should You Use?

Short Answer

If you are trying to decide whether to run, work outside, send kids to practice, or move a trip itinerary, do not rely on one “feels like” number.

Use Heat Index for a quick sense of how hot humid air feels in the shade. Use WBGT, or Wet Bulb Globe Temperature, when people will be active in sun, heat, or gear. Use HeatRisk when you want the health-risk picture for your area over the next several days.

A simple rule works well: HeatRisk for the week, Heat Index for the day’s general feel, WBGT for outdoor exertion.

Which Heat Forecast Should You Check?

Situation	Start with	Why
Quick weather-app check before errands	Heat Index	It combines temperature and humidity into a shade-based “how hot it feels” estimate.
Running, soccer practice, marching band, camp, landscaping, roofing, or road work	WBGT	It includes sun, radiant heat, wind, humidity, and temperature, which better reflect heat stress during activity.
Travel, school events, family care, and multi-day planning	HeatRisk	It shows local health risk on a 7-day color scale.
Hot pavement, roofs, construction sites, protective gear, or blocked wind	WBGT or site-specific heat assessment	Heat Index can miss important worksite and surface-heat conditions.
Planning for people sensitive to heat, people with chronic illness or limited mobility, workers, athletes, or anyone with prolonged exposure	HeatRisk	The color scale is built around health risk, not just comfort.

Heat Index: Useful, But Built For Shade

Heat Index combines air temperature and relative humidity to estimate how hot conditions feel to the body. Humid air matters because sweat cools the body mainly by evaporating. When the air is already holding a lot of moisture, sweat evaporates less easily.

The National Weather Service gives a striking example: an air temperature of 96 degrees F with 65% relative humidity can produce a Heat Index of 121 degrees F.

The catch is the assumption behind the number. NWS describes Heat Index values as based on shady conditions with light wind. In direct sun, NWS says the apparent temperature can be up to 15 degrees F higher. OSHA also notes that Heat Index does not account for wind, sunlight, radiant heat, or workload.

That makes Heat Index a good starting point for “How oppressive will it feel outside today?” It is weaker for “Can this team practice safely in full sun?” or “Is this jobsite heat exposure acceptable?”

WBGT: The Better Fit For Workouts, Practices, And Outdoor Jobs

WBGT stands for Wet Bulb Globe Temperature. OSHA describes it as a measure that reflects four major environmental heat factors: temperature, humidity, radiant heat, and wind.

That matters because the body is not just absorbing heat from the air. A person can also gain heat from direct sunlight, hot pavement, roofs, machinery, reflected light, and protective clothing. At the same time, running, lifting, marching, and other physical work create metabolic heat inside the body.

Two 90-degree days can be very different. A slow walk under trees is not the same heat load as football practice on an artificial-turf field, or roofing work in direct sun. WBGT is designed to get closer to that real exposure.

NWS describes WBGT as especially useful for outdoor workers, athletes, and marching bands. OSHA says workplace environmental heat should be evaluated with WBGT when possible, and treats Heat Index as a screening tool rather than a replacement for a more complete assessment.

HeatRisk: The Local Health-Risk View

HeatRisk is not just another temperature number. CDC describes it as a health-based heat forecast developed by NOAA’s National Weather Service and CDC. It provides a 7-day outlook and uses a 0 to 4 color scale.

The scale runs from green 0 to magenta 4. Higher levels mean a greater chance that heat will harm health.

HeatRisk level	Meaning in CDC guidance	How to use it in planning
Green 0	Little to no heat-related health risk	Normal warm-weather planning may be enough.
Yellow 1	Minor risk, mainly for people sensitive to heat	Add cooling, hydration, and check-ins for heat-sensitive people.
Orange 2	Moderate risk for heat-sensitive people and some healthy people with long exposure	Move activity to cooler hours, plan shade and rest, and reduce long exposure.
Red 3	Major risk for most people	Consider rescheduling outdoor activity and make cooling breaks realistic, not optional.
Magenta 4	Extreme risk for everyone	Plan around air-conditioned indoor space when possible.

HeatRisk also adjusts for local context. CDC says it reflects local heat-health relationships, location, time of year, and humid air. NWS also describes the tool as accounting for whether temperatures are unusual for the time and place, how long the heat lasts, and whether overnight temperatures allow recovery.

That is why the same afternoon high can mean different risk in different regions or at different points in the season.

Why The Numbers Can Disagree

Heat risk is not one physical measurement. It is a mix of weather, environment, activity, and human vulnerability.

Heat Index is strongest for the humidity question: how hard will it be for sweat to cool the body under shaded, light-wind conditions?

WBGT is stronger for the exposure question: how much heat is a person taking on from sun, surfaces, air, humidity, and limited cooling while active?

HeatRisk is stronger for the planning question: how unusual and health-relevant is this heat for this location and time of year?

The numbers can point in different directions because they are answering different questions. A mild-looking Heat Index may still be a poor guide on a sunny jobsite. A high HeatRisk level may reflect local vulnerability, early-season heat, warm nights, or several hot days in a row, not only the afternoon high.

A Practical Check Sequence

1. Check HeatRisk first for the next several days. If your area is orange or higher, review outdoor schedules, cooling options, and who in your group may be more heat-sensitive.

2. Check Heat Index for the day’s general heat feel. Treat it as a shade-based estimate, not a full sun or high-exertion safety threshold.

3. Check WBGT for sports, camps, outdoor work, marching practice, or long events. Use an NWS WBGT forecast or an on-site WBGT reading when available.

4. Add activity intensity. Standing, walking, carrying loads, fast work, and repeated sprints produce different amounts of body heat.

5. Add site conditions. Hot pavement, roofs, reflective surfaces, blocked wind, direct sun, and protective gear can make the real heat burden higher than the app number suggests.

For official tools, start with the NWS Heat Forecast Tools, then use CDC’s HeatRisk guidance when health risk is the main planning issue.

Common Misunderstandings

“Feels like” does not mean “safe.” OSHA notes that a worker died of heat stroke on a day when the maximum Heat Index was 86 degrees F, and that less severe heat-related illness can occur at lower Heat Index values. Workload and site conditions matter.

WBGT is not only for specialists. On-site measurement is common in occupational safety, but NWS also provides WBGT forecasts and describes the measure as useful for outdoor workers and athletes. Schools, camps, teams, and outdoor event planners have a reason to look beyond Heat Index.

HeatRisk is not a plain temperature forecast with colors. It is a health-risk tool that considers local conditions and timing. NWS also describes HeatRisk as an experimental product, so it should be used with local official warnings and forecasts rather than treated as the only source.

Limits And Exceptions

No forecast tool can know every detail of a person’s body, schedule, surface, clothing, sleep, hydration, or local shade. Heat-risk decisions get more cautious for people sensitive to heat, people with chronic illness or limited mobility, workers, athletes, and anyone who will be exposed for a long time.

For symptoms of heat illness, use medical and emergency guidance rather than forecast interpretation. The practical role of these tools is planning: changing time, location, intensity, breaks, shade, cooling access, and supervision before heat becomes a problem.

What To Do Next

Before a hot day with outdoor plans, open the local HeatRisk forecast, check the day’s Heat Index, and look for WBGT if the activity involves exertion or work. Then ask one concrete question: “What can we change: time, shade, rest, intensity, gear, or indoor cooling access?”