Wet bulb temperature (T_wb) is the lowest temperature that air can reach through evaporative cooling alone—imagine a thermometer with a wet cloth wrapped around its bulb, placed in moving air. As water evaporates from the cloth, latent heat is absorbed from the air, cooling it. At 100% relative humidity, no further evaporation occurs, so the wet bulb temperature equals the actual air temperature (dry bulb temperature). In dry climates (low RH), evaporative cooling is powerful, and T_wb can be much lower than air temperature; in humid climates (high RH), evaporation is suppressed, and T_wb approaches air temperature. The wet bulb is superior to simple temperature for assessing heat stress because it directly quantifies the body's ability to lose heat through perspiration. Humans cool themselves via evaporative sweating when body temperature rises; but if ambient humidity is high, sweat cannot evaporate efficiently, body heat accumulates, and core temperature rises pathologically. Critically, a wet bulb temperature exceeding ~35°C (~95°F) is considered to be beyond human physiological tolerance—even healthy individuals cannot maintain thermal equilibrium through sweating alone (the human body generates ~100–200 W heat metabolically at rest, higher during exercise; at each gram of sweat evaporated, ~2.4 kJ of heat is removed; but evaporation fails when RH approaches 100%). The equation used here is Stull's empirical formula (2011), a highly accurate approximation valid across typical meteorological ranges, derived from extensive psychrometric data. For HVAC design, wet bulb temperature determines cooling tower performance (cooling towers exploit evaporation, so their effectiveness diminishes at higher wet bulb); for outdoor work safety, OSHA and military guidelines use wet bulb thresholds to restrict exertion (no exercise above 28–30°C T_wb; mandatory breaks above 26°C). Heat index (also output here) is a perceived temperature measure combining dry bulb and humidity for casual interpretation, but wet bulb is the thermodynamically correct parameter for heat stress prediction. Dew point (also shown) indicates moisture content independent of temperature; it's where air becomes saturated if cooled at constant pressure.

Practical implications span occupational safety, climate adaptation, and equipment design. In industrial settings (construction, military training, athletics), wet bulb thresholds are enforced: simultaneous high temperature and high humidity create life-threatening conditions (e.g., 35°C T_db + 85% RH → T_wb ~32.5°C, dangerous zone requiring hydration, rest, medical monitoring). Climate change is raising wet bulb temperatures globally—regions with historically manageable combinations (e.g., 40°C dry bulb but dry climate, T_wb ~25°C) are experiencing increased humidity, pushing T_wb into dangerous ranges, potentially rendering some locations uninhabitable during peak summer. Data centers and power plants use wet bulb for sizing cooling systems: a facility designed for 25°C T_wb cooling tower approach cannot operate its design load at 30°C T_wb (reduced capacity forces load shedding or shutdown). Passive evaporative coolers (swamp coolers) are highly effective in arid climates but useless in humid ones; their outlet temperature approaches wet bulb. Conversely, vapor-compression air conditioning doesn't depend on wet bulb—it can cool to any setpoint regardless of humidity, but requires significant energy and refrigerant. For future resilience, urban planners now assess "wet bulb heat waves"—periods where T_wb exceeds 28–31°C for sustained durations—as a critical climate hazard, alongside traditional heat waves (dry bulb only). Understanding wet bulb is essential for heat illness prevention (athletic trainers, military medics), infrastructure planning (HVAC engineers, grid operators), occupational health (OSHA, workplace hygienists), and climate impact assessment (environmental scientists, urban planners).