WBGT: Handheld Measurements vs Numerical Weather Model Forecasts

Learn about Wet Bulb Globe Temperature (WBGT) and why it provides the most reliable real-time measurement for accurately assessing the risk of heat stress.

The Wet Bulb Globe Temperature (WBGT) is a measurement of heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle, and cloud cover (which affects solar radiation). The WBGT can be calculated by using this equation:

WBGT = 0.7TW + 0.2Tg + 0.1Td

• TW is the natural wet-bulb temperature (humidity)
• Tg is the black globe thermometer temperature (temperature affected by solar radiation/cloud cover)
• Td is the dry-bulb temperature (actual air temperature)

The best way to get an accurate WBGT is by using a reliable handheld meteorological instrument, such as the Kestrel 5400 Heat Stress Tracker, at the location of concern. However, with the advancement of hyper-local forecasting, the WBGT has now started to be forecasted using numerical weather forecast models. Those who use the WBGT for critical decision-making now face the question of which measurement they should use. The traditional handheld measurement or the newer forecast estimation? While both values of the WBGT can help in the decision-making process, only one can really provide the true measurement of what is happening at a given location at a specific time. The handheld WBGT meter takes into account all the meteorological measurements needed to calculate the WBGT as shown in the formula above. The Kestrel 5400 has the accurate sensors required to directly measure the natural wet-bulb, black globe temperature, and dry-bulb temperature at a specific location.

Onsite Measurement vs. WBGT Forecasting

To understand why the handheld device is a better choice for WBGT measurement over forecasted WBGT, you must first understand how forecast models work. Numerical weather models are based on a grid system where little squares make up the surface of the Earth and the atmosphere. For example, most weather models have grid squares that are between 50km x 50km to 1km x 1km. In these grid spaces, all meteorological measurements are calculated over the Earth. The issue for meteorological models is that the smaller the measurement that is trying to be resolved, such as WBGT at a specific location, the smaller the grid spacing needs to be and the longer it takes to run the model.

Running a super high-resolution weather model to forecast meteorological measurements is not practical due to the computational cost of running such a super high-resolution model. So most weather models are done at a coarser resolution to save computer resources and time. That means one 50km x 50km square may include several cities in a grid while providing one WBGT value based on the forecast model. However, experience tells us that on one side of a city, it could be cloudy and humid; on the other side of the city, it could be partly cloudy and dry. There lies the problem with numerical weather forecasts of any meteorological value. The grid spacing of meteorological models cannot see or resolve the local effects of weather at such a small spatial scale. Not to mention that most weather models forecast meteorological measurements at specific time intervals, either hourly or 3-hourly steps. This means if an event of measurement occurs between these time steps, the weather model doesn't see it. This is where the strength of handheld devices, like the Kestrel 5400, is demonstrated. The Kestrel 5400 can measure the local effects of the current weather conditions and can provide the most accurate, real-time WBGT reading needed for making the most informed decisions about safety.

"Onsite WBGT measurements are recommended before physical activity to best approximate the true heat-exposure effects on athletes' health, safety, and performance."

Key Research Discoveries
Comparing Observed WBGT vs Modeled WBGT

• Onsite WBGT measurement remains the best option for determining environmental conditions and the need for heat-safety physical-activity modifications or cancellations.
• The National Weather Service WBGT underestimated the local athletic-surface heat stress, especially when the surface was black or red or made of synthetic material.
• Using National Weather Service wet-bulb globe temperature (WBGT) estimates resulted in heat-safety category misclassification across all athletic surfaces.

Changing Conditions Along the Marathon Route

Let's look at marathons to explore how microclimate conditions might change even within the same general area. For example, the Boston Marathon covers ~42km and has an elevation change of almost 130 meters. Across that distance and elevation, the atmospheric conditions can change quickly. That is why the Boston Marathon has handheld weather stations across the course to measure the WBGT for heat stress for their racers. As stated earlier, forecasted meteorological measurements do not have the spatial resolution to forecast across a tiny region.

In conclusion, handheld devices that measure the actual conditions onsite to calculate WBGT, consistently outperform forecasted WBGT measurements. Forecasted WBGT measurements do not have the spatial or temporal resolution to adjust for small changes in meteorological conditions at this time or across distance. These small changes can make a large impact on heat stress conditions. It should be noted that forecasted WBGT information are better than having no measurement at all if a handheld device is not available for heat stress monitoring.

To learn more about the Kestrel Heat Stress Monitors, data loggers, and WBGT Trackers and how they help safety managers and health professionals keep people safe, please visit the following:
Heat Stress Monitoring System for worker safety and job site monitoring
Kestrel 5400 Heat Stress Tracker
Kestrel Heat Stress line for Heat Safety Management
All Kestrel heat stress meters and loggers