Guide
Heat index vs. WBGT
Updated 2026-07-10
Quick answer
Heat index uses only air temperature and humidity, and assumes shade. WBGT (wet-bulb globe temperature) adds solar radiation and wind, so it reads higher in direct sun. A phone can only produce a WBGT estimate from available weather data — a governing body that requires WBGT compliance wants a physical black-globe meter reading, not an estimate.
Heat index is a shade metric
The NWS heat index is calculated from air temperature and relative humidity only. It was designed to describe how hot shaded, light-wind conditions feel — it does not account for direct sun exposure or how much air is moving across your skin, both of which change how quickly your body can offload heat.
WBGT adds solar load and wind
Wet-bulb globe temperature (WBGT) is built from three separate readings: a natural wet-bulb thermometer (captures humidity and evaporative cooling), a black globe thermometer (captures radiant heat from the sun), and a dry-bulb thermometer, combined with wind. That is why WBGT is the metric OSHA, NIOSH, and many high-school and college athletic associations prefer for outdoor work and sport — it reflects direct-sun conditions that a shade-only number misses.
Why a phone can only estimate it
A true WBGT reading needs a physical black-globe thermometer exposed to actual sun and wind at the exact location — software cannot manufacture that measurement. What an app can do is compute a simplified WBGT estimate from temperature and humidity. Swelter’s Pro WBGT figure is exactly that: a shade estimate from temperature and humidity, always labeled as one — never presented as a measured reading, and no substitute for an on-site meter.
When you need a real meter
If your workplace, league, or state athletic association has a policy that requires a WBGT reading for a go/no-go decision, that policy is asking for an on-site black-globe measurement, not a modeled estimate from a phone. Use Swelter’s WBGT estimate as a general planning signal, not as a substitute for an on-site WBGT meter where one is required.
Sources
Not affiliated with, endorsed by, or connected to OSHA, NIOSH, CDC, or the National Weather Service. Threshold values and guidance are based on their published public information. Swelter is an informational tool, not a medical device. It does not diagnose or prevent heat illness.