Wet Bulb Temperature
Wet bulb temperature is the lowest temperature achievable through evaporative cooling. It represents the theoretical limit of cooling and is critical for assessing human heat stress, designing HVAC systems, and evaluating evaporative cooler effectiveness.
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Wet bulb is always ≤ dry bulb; high humidity makes them converge. At 35°C wet bulb, evaporative cooling fails—critical for survival. Swamp coolers work best when dry-wet bulb difference is large. WBGT is the standard for athletic and occupational heat safety.
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Why: Wet bulb above 35°C (95°F) means the human body cannot cool through sweating—a life-threatening threshold. HVAC engineers use it for cooling tower design and swamp cooler sizing.
How: The Stull formula calculates wet bulb from dry bulb and RH. WBGT combines wet bulb, globe, and dry bulb temperatures for outdoor heat stress assessment.
Run the calculator when you are ready.
☀️ Hot Dry Summer Day
Typical desert or arid summer conditions with high temperature and low humidity
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🌴 Humid Tropical Conditions
High temperature with very high humidity - tropical coastal conditions
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💨 Swamp Cooler Efficiency
Evaluating evaporative cooler effectiveness in hot, dry climate
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🏥 Heat Stroke Risk Assessment
Dangerous conditions requiring heat safety evaluation
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🏭 Industrial Cooling Tower
Industrial cooling system design parameters
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Input Parameters
Defaults to standard sea-level pressure (1013.25 hPa) if not specified
For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Wet bulb is measured with a thermometer wrapped in a water-soaked cloth.
— WMO
Above 35°C wet bulb, humans cannot regulate core temperature.
— NOAA
Cooling towers approach wet bulb as their theoretical minimum.
— ASHRAE
Tropical humid conditions often have wet bulb near dry bulb.
— Engineering Toolbox
What is Wet Bulb Temperature?
Wet Bulb Temperature is the lowest temperature that can be achieved by evaporative cooling of water. It is measured using a thermometer wrapped in a water-soaked cloth (wet bulb) and ventilated. This temperature represents the theoretical limit of evaporative cooling and is crucial for understanding human heat stress, HVAC systems, and industrial cooling processes.
Unlike dry bulb temperature (the air temperature we typically measure), wet bulb temperature accounts for the cooling effect of evaporation. When humidity is high, evaporation is limited, so wet bulb temperature approaches dry bulb temperature. When humidity is low, significant evaporative cooling can occur, making the wet bulb temperature much lower than the dry bulb temperature.
Key Characteristics:
- Always less than or equal to dry bulb temperature
- Represents the lowest temperature achievable through evaporative cooling
- Critical for assessing human heat stress and safety
- Essential for HVAC and cooling system design
- Used in meteorology for weather analysis
How Evaporative Cooling Works
The Evaporation Process
Evaporative cooling occurs when liquid water changes to water vapor, absorbing heat energy from the surrounding environment. This process is the basis for how our bodies cool through sweating, how swamp coolers work, and how cooling towers operate in industrial applications.
The rate of evaporation depends on:
- Temperature difference: Greater difference between dry and wet bulb allows more cooling
- Humidity: Lower humidity allows faster evaporation
- Air movement: Ventilation increases evaporation rate
- Surface area: More surface area increases evaporation
Human Body Cooling
Our bodies maintain temperature through sweating. When sweat evaporates, it cools the skin. However, when wet bulb temperature exceeds 35°C (95°F), evaporation becomes ineffective, and the body cannot cool itself. This creates life-threatening conditions where core body temperature rises uncontrollably, leading to heat stroke and potentially death.
Wet Bulb Globe Temperature (WBGT)
Wet Bulb Globe Temperature (WBGT) is a composite temperature index that estimates the effect of temperature, humidity, wind speed, and solar radiation on humans. It is widely used by athletes, military personnel, and industrial workers to prevent heat-related illnesses.
WBGT is calculated differently for indoor and outdoor conditions:
- Indoor WBGT: WBGT = 0.7 × Tw + 0.3 × T (without solar radiation)
- Outdoor WBGT: WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × T (with solar radiation)
Where Tw is wet bulb temperature, T is dry bulb temperature, and Tg is globe temperature (measured in a black globe to account for solar radiation).
Heat Stress Categories
| Wet Bulb Range | Category | Risk Level | Description |
|---|---|---|---|
| < 25°C | Safe | Low | Safe conditions for physical activity |
| 25-27°C | Caution | Moderate | Fatigue possible with prolonged exposure |
| 27-30°C | Extreme Caution | High | Heat stroke, heat cramps, or heat exhaustion possible |
| 30-32°C | Danger | Very High | Heat cramps or heat exhaustion likely, heat stroke possible |
| > 32°C | Extreme Danger | Extreme | Heat stroke highly likely - life-threatening conditions |
Applications
Heat Safety
Critical for assessing heat stress risk in outdoor work, athletics, military operations, and emergency response. Wet bulb temperature above 35°C (95°F) represents the human survivability limit.
Cooling Towers
Essential for designing and operating industrial cooling towers. Wet bulb temperature determines the theoretical minimum cooling water temperature achievable.
HVAC Systems
Used in designing evaporative cooling systems, determining cooling loads, and optimizing energy efficiency in air conditioning systems.
Swamp Coolers
Determines the effectiveness of evaporative coolers (swamp coolers). The difference between dry bulb and wet bulb temperature indicates cooling potential.
Meteorology
Important for weather forecasting, understanding air mass properties, and predicting heat wave severity and duration.
Athletics & Sports
Used to determine safe practice and competition conditions, modify activity levels, and prevent heat-related illnesses in athletes.
Formula Explanations
Stull Formula
The Stull formula is an empirical equation that accurately calculates wet bulb temperature from dry bulb temperature and relative humidity. It is accurate for relative humidities between 5% and 99% and temperatures between -20°C and 50°C.
Where T is dry bulb temperature in °C, RH is relative humidity in %, and Tw is wet bulb temperature in °C.
Wet Bulb Globe Temperature (WBGT)
WBGT is a composite index that accounts for temperature, humidity, wind speed, and solar radiation. It provides a more accurate assessment of heat stress than temperature alone.
Where Tw is wet bulb temperature, T is dry bulb temperature, and Tg is globe temperature.
Evaporative Cooling Potential
The evaporative cooling potential indicates how effective evaporative cooling can be. It is calculated as the percentage difference between dry bulb and wet bulb temperatures relative to dry bulb temperature.
Higher values indicate greater potential for evaporative cooling, which is most effective in hot, dry climates.
Frequently Asked Questions
What is wet bulb temperature and why is it important?
Wet bulb temperature is the lowest temperature achievable through evaporative cooling. It represents the theoretical limit of cooling and is critical for assessing human heat stress, designing HVAC systems, and evaluating evaporative cooling effectiveness. Unlike dry bulb temperature, wet bulb accounts for humidity and evaporation effects.
How does wet bulb temperature differ from heat index?
Wet bulb temperature is a direct physical measurement representing evaporative cooling potential, while heat index is a "feels like" temperature combining temperature and humidity. Wet bulb is more accurate for assessing physiological heat stress, especially at high temperatures where evaporation becomes critical for human survival.
What is the difference between wet bulb temperature and WBGT?
Wet Bulb Globe Temperature (WBGT) is a composite index that combines wet bulb temperature (70%), globe temperature (20%), and dry bulb temperature (10%) to account for solar radiation and other factors. WBGT provides a more comprehensive heat stress assessment for outdoor activities, while wet bulb alone is sufficient for indoor or shaded conditions.
At what wet bulb temperature does heat become dangerous?
Wet bulb temperatures above 25°C (77°F) require caution. Above 27°C (81°F) poses high risk. Above 30°C (86°F) is dangerous, and above 32°C (90°F) is extremely dangerous. The critical threshold is 35°C (95°F), where the human body cannot cool itself through sweating, leading to potentially fatal heat stroke.
How is wet bulb temperature used in HVAC design?
Wet bulb temperature determines the minimum achievable cooling water temperature in cooling towers, the effectiveness of evaporative coolers (swamp coolers), and cooling load calculations. HVAC engineers use psychrometric charts based on wet bulb temperature to design efficient air conditioning and ventilation systems.
Can I use wet bulb temperature to evaluate swamp cooler effectiveness?
Yes! The difference between dry bulb and wet bulb temperature indicates evaporative cooling potential. Larger differences (hot, dry conditions) mean swamp coolers work well. When wet bulb approaches dry bulb (high humidity), evaporative cooling becomes ineffective. The calculator shows cooling potential percentage and efficiency rating.
How accurate is the Stull formula for calculating wet bulb temperature?
The Stull formula is accurate within ±0.5°C for relative humidities between 5% and 99% and temperatures between -20°C and 50°C. It's widely used in meteorology and HVAC applications. For extreme conditions or high-altitude applications, iterative psychrometric calculations may be more accurate.
What factors affect wet bulb temperature measurements?
Wet bulb temperature depends on dry bulb temperature, relative humidity, atmospheric pressure, and air movement. Accurate measurement requires proper ventilation (aspirated psychrometer), clean wick, distilled water, and standard pressure. Altitude affects readings due to pressure variations.
Official Data Sources
Wet Bulb Temperature Calculator
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