Understand your weather
WBGT (Wet Bulb Globe Temperature)
A view of outdoor environmental heat stress.
WBGT stands for Wet Bulb Globe Temperature. It is used to assess environmental heat stress by bringing together the effects of air temperature, humidity, air movement and radiant heat. It is expressed in degrees Celsius, but it is a different quantity from the temperature on an ordinary thermometer.
Corydal estimates outdoor WBGT using personal weather station observations. We call this Estimated WBGT because it models the sensor temperatures used in WBGT rather than measuring them directly.
Why we say Estimated WBGT
Measured WBGT
Uses a globe thermometer and a naturally ventilated, wetted sensor, together with air temperature for outdoor sun exposure.
Corydal’s Estimated WBGT
Models those sensor temperatures from weather-station measurements. Corydal does not have a direct globe-thermometer or natural wet-bulb measurement.
A calculated ordinary wet-bulb temperature is not interchangeable with a sun-exposed natural wet bulb. Wind and radiation affect the latter. Our model accounts for both.
How it differs from air temperature and Heat Index
Air temperature describes the surrounding air. Heat Index combines temperature and humidity under shaded, light-wind assumptions. Outdoor WBGT also accounts for wind and solar exposure. Two places with the same air temperature can have different WBGT values.
Corydal models outdoor conditions with direct-sun exposure when sunlight is present. It does not estimate the conditions inside a building or beneath a particular shade structure. Indoor and shade assessments require separate exposure assumptions.
How is WBGT calculated?
The calculation uses stored air temperature, relative humidity, wind speed, solar radiation and absolute station pressure, plus station latitude, longitude, wind-sensor height and the observation’s timestamp. Timezone controls the displayed times; solar position uses the UTC instant.
Corydal implements the globe and natural wet-bulb heat-transfer calculations from Liljegren et al. (2008). The outdoor result combines 70% modeled natural wet-bulb temperature, 20% modeled globe temperature and 10% air temperature. Reference-code tests check numerical agreement; they do not establish the accuracy of a personal station.
The model uses sun position to estimate direct and diffuse radiation. It retains the reference radiation cap and fixed sensor/surface properties. Station pressure affects air density, vapor diffusion and evaporation; sea-level-adjusted pressure is not used as a substitute.
When the wind sensor is not at 2 m, Corydal estimates the 2 m wind using FAO-56 equation 47. That assumes a neutral wind profile over short grass. This is an explicit approximation, not a measurement of local airflow or the original model’s terrain-dependent wind correction. V1 supports sensor heights from 1 to 10 m.
For calm conditions, the heat-transfer model uses its reference minimum wind of 0.13 m/s. At night, a measured zero solar input still allows longwave heat exchange. Positive solar readings inconsistent with the modeled horizon are unavailable. Calculation details show the latest inputs and adjustments.
WBGT from a Personal Weather Station
Corydal can estimate WBGT from compatible personal weather station observations when the required weather station data and station settings are available. If you use a connected Ambient Weather station, Corydal can use your station observations to estimate WBGT over time. These remain modeled estimates, not direct WBGT measurements.
Reading the latest value and 24-hour history
The latest estimate uses the latest observation, even if an earlier observation was more complete. Old observations are marked delayed. The recent maximum is the highest estimate available in the last 24 hours; missing samples may hide a higher actual value.
Gaps indicate missing or unavailable estimates, not zero heat stress. The chart also breaks across gaps longer than ten minutes. Coverage counts five-minute intervals with a valid estimate. All values are recalculated from saved weather data and current station metadata.
What this estimate cannot tell you
Accuracy depends on station siting, wind-sensor height and sensor quality. Buildings, roofs, trees and nearby hot surfaces can change exposure substantially. The model assumes ground temperature equals air temperature and uses fixed surface properties. Sensor averaging times and response delays may differ from the stored timestamp.
Missing required inputs return unavailable. V1 accepts air temperature 0–50°C, relative humidity 0.1–100%, wind 0–50 m/s, solar radiation 0–1500 W/m² and station pressure above 800 through 1100 hPa. Solar dates must be in 1950–2049. Nonconvergent calculations and a modeled wet bulb below freezing are unavailable. These software limits are not a guarantee of accuracy within the ranges.
WBGT alone does not define a universally safe activity level. Interpretation depends on workload, acclimatization, clothing or PPE, the population and the applicable organization or standard. Corydal V1 provides no universal risk categories, work/rest recommendations or WBGT alerts.
Sources and attribution
- Liljegren et al. (2008) — Modeling the wet bulb globe temperature using standard meteorological measurements
- James C. Liljegren — WBGT v1.1 reference source (mirror)
- FAO-56 — Wind profile relationship, equation 47
- OSHA — Heat hazard recognition
- National Weather Service — WBGT and Heat Index
This product includes software produced by UChicago Argonne, LLC under Contract No. DE-AC02-06CH11357 with the Department of Energy. Adapted to TypeScript by Corydal, with the wind conversion and availability checks described above. Reference software license and disclaimer.
For other ways to interpret your station observations, learn about Growing Degree Days and Chill Hours.