Wet-bulb temperature
Wet-bulb temperature measures how much evaporation can cool air, making it a key way to understand humidity, heat stress, and limits on human cooling.
What wet-bulb temperature is
Wet-bulb temperature is a measure of cooling by evaporation. Imagine a thermometer bulb wrapped in a wet cloth and exposed to moving air. As water evaporates from the cloth, it removes heat and lowers the reading. The final reading is the wet-bulb temperature, which depends on both air temperature and moisture in the air.
Dry bulb, wet bulb, and dew point
Dry-bulb temperature is the ordinary air temperature measured by a dry thermometer. Dew point is the temperature at which air becomes saturated enough for water vapor to condense. Wet-bulb temperature sits between those ideas: it shows how far evaporation can cool the air under current humidity conditions.
Why humidity changes the number
Dry air allows water to evaporate quickly, so a wet bulb can cool far below the dry-bulb temperature. Humid air is already holding more water vapor, so evaporation slows and the wet-bulb reading stays closer to the air temperature. At 100 percent relative humidity, the wet-bulb and dry-bulb temperatures are the same.
Connection to human cooling
People cool themselves partly by sweating. Sweat must evaporate to remove heat from the body. When the wet-bulb temperature is high, the surrounding air has little capacity to accept more water vapor, so sweating becomes less effective. This is why humid heat can be dangerous even when the air temperature is lower than in a dry desert.
The 35 C threshold
A sustained wet-bulb temperature near 35 C is often discussed as an approximate physiological limit for healthy people under idealized assumptions, because the body can no longer shed enough heat by evaporation. Real-world danger can begin at lower values, especially with sun, exertion, dehydration, illness, age, heavy clothing, or poor ventilation.
Wet-bulb versus WBGT
Wet-bulb temperature is not the same as wet-bulb globe temperature, or WBGT. WBGT is a heat-stress index that also accounts for sunlight, radiant heat, wind, and air temperature. Weather services, workplaces, militaries, and sports organizations often use WBGT for operational safety decisions because direct sun and wind matter for the human body.
Climate and extreme heat
Climate scientists track wet-bulb temperature because warming can increase both heat and atmospheric moisture. Coastal regions, river valleys, irrigated areas, and places near very warm seas can face especially dangerous humid-heat combinations. Short bursts of extreme wet-bulb conditions are already a concern in some regions.
Why it matters
Wet-bulb temperature links weather physics to public health. It helps explain why the same air temperature can feel tolerable in one place and dangerous in another, why nighttime humidity matters during heat waves, and why heat planning must consider moisture, shade, wind, workload, housing, and access to cooling.