Wet bulb calculator

This wet bulb calculator quickly estimates the wet bulb temperature and wet bulb depression using the current air temperature and relative humidity. It is a valuable tool for weather analysis, HVAC design, agriculture, and monitoring heat stress conditions.

Related calculators:

What is wet bulb temperature?

Wet bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It is measured by wrapping a wet cloth around the bulb of a thermometer and ventilating it. As water evaporates, it cools the bulb — the drier the air, the more evaporation, and the greater the cooling.

It reflects the combined effect of temperature and humidity and is a critical factor in meteorology, HVAC design, agriculture, and occupational safety.

How is wet bulb temperature measured?

There are two primary ways:

1. Psychrometric method:
   • Use two thermometers: one dry bulb (measures ambient air temperature), and one wet bulb (with a moistened wick).
   • Air is passed over both thermometers.
   • The difference between the two readings is used to calculate the wet bulb temperature using a psychrometric chart or formula.
2. Online calculator:
   • You use known values of:
     • Dry bulb temperature (ambient temperature)
     • Relative humidity
   • These are input into a formula or lookup algorithm that estimates the wet bulb temperature.

How is wet bulb temperature calculated?

Wet bulb temperature is not trivial to compute directly but can be estimated using empirical formulas or psychrometric equations. One widely used approximation is the Stull formula:

Tw≈T×atan[0.151977×\sqrt{RH+8.313659}] \\+atan(T+RH) \\−atan(RH−1.676331) \\+0.00391838×\sqrt{RH^3} \\×atan(0.023101×RH)\\−4.686035 

where:

• T_w = wet bulb temperature (°C)
• T= dry bulb temperature (°C)
• RH= relative humidity (%), such as 60 for 60%
• atan = arc tangent or inverse tangent

This formula is accurate for temperatures between -20 and –50°C and relative humidities from 5% to 99%.

What is wet bulb depression?

Wet bulb depression is the difference between the dry bulb and wet bulb temperatures:

\text{Wet bulb depression} = \\ \text{Dry bulb temperature} - \\ \text{Wet bulb temperature}

• A high depression indicates low humidity (air is dry; more evaporation).
• A low depression means high humidity (air is moist; less evaporation possible).

Why is wet bulb temperature important?

Human health and safety

• If the wet bulb temperature exceeds 35°C, humans can no longer effectively cool themselves by sweating.
• High wet bulb temperatures can lead to heat stress or other health issues — even in shaded, well-ventilated areas.
• Widely used by occupational safety organizations (e.g., WBGT index) to monitor work environments.

HVAC and cooling design

• Determines how efficiently evaporative cooling systems (like swamp coolers) will work.
• Helps calculate cooling loads for buildings.

Agriculture and livestock

• Influences plant transpiration rates and animal comfort.
• Critical for greenhouse management.

Meteorology and aviation

• Used in forecasting fog, dew, frost, and cloud formation.
• Important for runway safety due to condensation and icing.

Example

Suppose:

• Dry bulb temperature = 30°C
• Relative humidity = 60%

Using the calculator or formula:

• Wet bulb temperature ≈ 23.996°C
• Wet bulb depression = 30 – 23.996 = 6.004°C

This means the air still has the potential to cool by ~6°C via evaporation, indicating moderate humidity.

Summary