Wet-Bulb Temperature Calculator

About the Wet-Bulb Temperature Calculator

The **Wet-Bulb Temperature Calculator** determines wet-bulb temperature from dry-bulb temperature and relative humidity. Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling, which is why it matters for heat safety, HVAC sizing, and cooling-tower design.

The page also reports dew point, wet-bulb depression, heat index, and a simplified WBGT-style risk level so you can see how the same weather conditions look from several related angles. For most meteorological use, the Stull approximation gives a practical estimate without requiring a full psychrometric chart.

That combination makes it easier to tell when humidity is merely uncomfortable and when evaporative cooling starts to lose its effectiveness.

When This Page Helps

Wet-bulb temperature is the number that connects weather conditions to cooling limits. It helps you see when heat is still manageable, when sweating becomes much less effective, and when an evaporative process will no longer perform the way you expect.

How to Use the Inputs

1. Enter the dry-bulb (actual air) temperature.
2. Enter the relative humidity as a percentage.
3. Optionally adjust atmospheric pressure for altitude effects.
4. Read the wet-bulb temperature and heat stress risk level.
5. Compare dew point, wet-bulb, and dry-bulb on the visual scale.
6. Use the RH table to see how wet-bulb varies with humidity.
7. Check WBGT for outdoor work safety compliance.

Example Calculation

Tips & Best Practices

• At 100% RH, wet-bulb = dry-bulb = dew point — no evaporative cooling possible.
• Cooling towers are rated at design wet-bulb; hotter/more humid locations need larger towers.
• Desert swamp coolers work great (low Tw) but are useless in humid climates (Tw ≈ Td).
• Wet-bulb depression (Td − Tw) indicates evaporative cooling potential — larger is better.
• OSHA uses WBGT for workplace heat standards; the military uses it for training schedules.
• A sling psychrometer measures wet-bulb directly by swinging a wet-wick thermometer in air.

Wet-Bulb Temperature and Human Health

The wet-bulb temperature is increasingly recognized as the key metric for heat mortality risk. Traditional heat indices combine temperature and humidity but can be misleading. A dry 50°C day (Tw ≈ 25°C) is survivable with shade and water, while a humid 38°C day (Tw ≈ 34°C) at the same heat index is far more dangerous because evaporative cooling fails.

Early-2010s work by Sherwood and Huber established Tw = 35°C as the theoretical human survivability limit. More recent controlled-lab studies found that in practice, young healthy adults begin struggling at Tw = 31°C under moderate activity. Elderly and chronically ill individuals face danger at even lower thresholds.

Applications in Engineering

**HVAC Design:** Building cooling systems are sized based on outdoor wet-bulb design conditions. ASHRAE provides 0.4%, 1%, and 2% design wet-bulb temperatures for locations worldwide. In Houston: 0.4% Tw = 26.7°C. In Phoenix: 0.4% Tw = 21.1°C — explaining why evaporative cooling works in Phoenix but not Houston.

**Data Center Cooling:** Large data centers emit massive amounts of heat. Free cooling (using outdoor air or evaporative methods) is possible when the outdoor wet-bulb temperature is below the supply air temperature — typically below 18-22°C. Locations with low Tw (like Oregon or Iceland) save millions in cooling costs compared to humid locations.

Climate Change Implications

Global warming increases wet-bulb temperature through two mechanisms: higher air temperature and increased evaporation (more moisture in the atmosphere). The Persian Gulf, South Asian subcontinent, and equatorial regions are most vulnerable. By mid-century, wet-bulb temperatures exceeding 32°C may occur for weeks at a time in heavily populated areas, threatening billions of people who lack air conditioning.

Frequently Asked Questions

• What exactly is wet-bulb temperature?

  It is the temperature a thermometer reads when wrapped in a wet cloth with air flowing past it. Evaporation cools the thermometer: the drier the air, the more cooling, and the lower the wet-bulb temperature. At 100% RH, wet-bulb equals dry-bulb (no evaporation possible).

• Why is 35°C wet-bulb considered the human survivability limit?

  Humans cool by sweating: sweat evaporates, removing heat. When Tw = 35°C (body core temperature), evaporation stops because the air can hold no more moisture at body temperature. Without cooling, core temperature rises uncontrollably, leading to heat stroke and death within hours.

• What is WBGT and how is it used?

  Wet-Bulb Globe Temperature combines wet-bulb (70%), black-globe radiation (20%), and dry-bulb (10%). OSHA and military use WBGT to set work-rest cycles: <25.6°C = unrestricted, 25.6-27.8°C = limited heavy work, >32.2°C = minimal activity. It gives a simplified outdoor estimate.

• How does wet-bulb temperature relate to cooling towers?

  Cooling towers cool water to within 3-8°C of the wet-bulb temperature (this difference is called "approach"). A location with Tw = 25°C can cool tower water to ~28-33°C. Lower wet-bulb = better cooling performance. Cooling tower design is based on the 1% design wet-bulb temperature.

• Is wet-bulb temperature increasing due to climate change?

  Yes. Several locations have already briefly approached or exceeded Tw > 35°C, and climate projections show that heavily populated regions in South Asia, the Persian Gulf, and the US Gulf Coast could see far more frequent Tw = 32-35°C heat-wave conditions over time.

• What is the difference between wet-bulb and dew point?

  Both involve moisture, but dew point is the temperature at which condensation begins (cooling air at constant moisture). Wet-bulb is the temperature of evaporative cooling (adding moisture to air). Relationship: dew point ≤ wet-bulb ≤ dry-bulb, always.