Understanding Wet-Bulb Temperature
Wet-bulb temperature is the lowest temperature air can reach when water evaporates into it at constant pressure. Traditionally measured with a thermometer wrapped in water-soaked cloth and ventilated to simulate evaporative cooling, it reveals how effectively the human body can shed heat through perspiration.
Unlike the dry-bulb temperature you see on standard thermometers, wet-bulb factors in atmospheric moisture. At 100% relative humidity, wet-bulb equals dry-bulb since no further evaporation occurs. As humidity drops, the gap widens—dry air allows more evaporative cooling, lowering the wet-bulb value significantly. This difference, called wet-bulb depression, is critical for assessing heat risk.
The relationship between these two measurements underpins physiological responses to heat. Construction engineers, military personnel, athletes, and industrial hygienists rely on wet-bulb data to prevent heat-related illness and plan safe working conditions.
Wet-Bulb Temperature Formula
The Stull formula is widely used for calculating wet-bulb temperature. It provides accurate results across relative humidities from 5% to 99% and temperatures between −20°C and 50°C. The formula performs reliably even in extreme conditions, with maximum error bounds of −1°C to +0.65°C.
Tₘ = T × arctan[0.151977 × √(RH% + 8.313659)]
+ arctan(T + RH%) − arctan(RH% − 1.676331)
+ 0.00391838 × (RH%)^(3/2) × arctan(0.023101 × RH%) − 4.686035
T— Dry-bulb temperature in degrees CelsiusRH%— Relative humidity expressed as a percentage (0–100)Tₘ— Wet-bulb temperature in degrees Celsius
Wet-Bulb Globe Temperature and Heat Stress Assessment
Wet-Bulb Globe Temperature (WBGT) extends the wet-bulb concept by incorporating solar radiation and globe thermometer readings. This composite index better represents the thermal stress experienced outdoors compared to wet-bulb alone.
Outdoor WBGT (with direct sunlight):
WBGT = 0.7 × Tₘ + 0.2 × Tg + 0.1 × T
Indoor WBGT (no solar radiation):
WBGT = 0.7 × Tₘ + 0.3 × T
where Tg is globe thermometer temperature. WBGT guides activity restrictions: at 26°C, vigorous exercise becomes hazardous; above 28°C, most outdoor physical work must cease. Military units, sports teams, and occupational health programs use WBGT thresholds to schedule breaks, mandate hydration, and prevent heat casualties.
Practical Applications Across Industries
Construction and building design: Materials respond differently to humidity and temperature combinations. Architects and engineers consult wet-bulb data when specifying HVAC systems, insulation, and ventilation strategies for different climates.
Snow production: Snowmaking requires both cold temperatures and low humidity. Operators monitor wet-bulb depression to time optimal production windows, since higher humidity reduces evaporative cooling and raises the effective air temperature.
Meteorology and forecasting: Weather specialists use wet-bulb temperature to predict rain, freezing rain, and snow formation. It helps explain why certain storms produce specific precipitation types.
Occupational safety: Heat stress prevention programmes in agriculture, mining, construction, and manufacturing rely on WBGT to establish work–rest cycles and hydration schedules that protect workers from heat exhaustion and heat stroke.
Key Considerations When Using Wet-Bulb Data
Understanding wet-bulb limitations and proper application prevents misinterpretation of heat risk.
- Account for wind speed and acclimatisation — WBGT does not include wind speed effects, which can significantly enhance evaporative cooling and lower perceived heat stress. Individual acclimatisation levels also vary; unaccustomed individuals face greater risk at any given WBGT value than trained personnel.
- Distinguish wet-bulb from heat index — Heat Index measures apparent temperature in the shade using different weighting factors. WBGT incorporates direct solar radiation and is more accurate for outdoor occupational settings. They produce different numerical values and risk assessments—use the correct metric for your context.
- Monitor actual conditions, not forecasts alone — Localised pockets of humidity and solar exposure can diverge significantly from broad weather predictions. Athletes and outdoor workers should measure or monitor real-time conditions on-site, especially in variable terrain or near large water bodies that alter local humidity.
- Recognise the 35°C threshold — A wet-bulb temperature of approximately 35°C represents a physiological limit: sustained exposure becomes life-threatening for most humans regardless of fitness, hydration, or clothing, since core body heat cannot dissipate. Approach such conditions with extreme caution.