Calculate Humidity Using Wet And Dry Bulb

Calculate Relative Humidity

Enter the dry bulb and wet bulb temperatures, and atmospheric pressure to calculate relative humidity.

Humidity Reference Table

Dry Bulb (°C)	Wet Bulb Depression (Td-Tw °C)	Relative Humidity (%)

Relative Humidity (%) at 1013.25 hPa for different dry bulb temperatures and wet bulb depressions.

Humidity Chart

Our Relative Humidity Calculator helps you determine the amount of moisture in the air based on wet and dry bulb thermometer readings and atmospheric pressure. This is a vital measurement in meteorology, HVAC, storage, and many industrial processes.

What is Relative Humidity?

Relative Humidity (RH) is the ratio, expressed as a percentage, of the amount of water vapor actually present in the air to the maximum amount of water vapor the air could hold at that same temperature and pressure. The Relative Humidity Calculator uses standard psychrometric principles to find this value.

Warm air can hold more moisture than cold air. So, if the amount of water vapor stays the same, relative humidity decreases as the temperature rises and increases as the temperature falls.

Who Should Use This Calculator?

• Meteorologists and weather enthusiasts
• HVAC technicians and building managers
• Farmers and agricultural scientists
• Museum curators and art conservators
• Industrial process managers (e.g., textiles, printing)
• Anyone needing to understand air moisture content

Common Misconceptions

• High humidity always means it feels “muggy”: While high RH contributes, the actual temperature is also a major factor in how “muggy” it feels (related to the Heat Index).
• 100% RH means it’s raining: 100% RH means the air is saturated with water vapor at the current temperature. Fog or dew can form, but rain requires more complex atmospheric conditions and the formation of droplets large enough to fall.
• A Relative Humidity Calculator is only for outdoor air: It can be used indoors or outdoors, wherever you can measure wet and dry bulb temperatures and know the pressure.

Relative Humidity Calculator Formula and Mathematical Explanation

The Relative Humidity Calculator uses the following principles:

1. Saturation Vapor Pressure (e_s): First, we calculate the saturation vapor pressure at the dry bulb temperature (T_d) and the wet bulb temperature (T_w). A common formula (like the Tetens or Magnus-Tetens equation) is used, for example:

e_s(T) = 6.112 * exp((17.67 * T) / (T + 243.5)) (T in °C, e_s in hPa)

So, we find e_s(T_d) and e_s(T_w) (also called e_w).

2. Actual Vapor Pressure (e): The actual vapor pressure ‘e’ is then found using the psychrometric formula:

e = e_w – A * P * (T_d – T_w)

Where:

• e_w is the saturation vapor pressure at the wet bulb temperature T_w.
• A is the psychrometric constant (approximately 0.000665 °C^-1 for a standard psychrometer with water, adjusted slightly if T_w is below freezing or for different ventilation speeds, and for units).
• P is the atmospheric pressure.
• T_d is the dry bulb temperature.
• T_w is the wet bulb temperature.

3. Relative Humidity (RH): Finally, relative humidity is calculated as:

RH = (e / e_s(T_d)) * 100%

4. Dew Point (T_dew): The dew point temperature can be derived from the actual vapor pressure ‘e’:

T_dew = (243.5 * ln(e / 6.112)) / (17.67 – ln(e / 6.112)) (for e in hPa, T_dew in °C)

Variables Table

Variable	Meaning	Unit	Typical Range
Td	Dry Bulb Temperature	°C or °F	-20 to 50 °C
Tw	Wet Bulb Temperature	°C or °F	-20 to Td °C
P	Atmospheric Pressure	hPa (mb) or inHg	950 to 1050 hPa
es(T)	Saturation Vapor Pressure at temp T	hPa	0.6 to 123 hPa
ew	Saturation Vapor Pressure at Tw	hPa	0.6 to es(Td) hPa
e	Actual Vapor Pressure	hPa	0 to es(Td) hPa
RH	Relative Humidity	%	0 to 100%
Tdew	Dew Point Temperature	°C or °F	-30 to Td °C

Practical Examples (Real-World Use Cases)

Example 1: Warm Day

Suppose on a warm day:

• Dry Bulb Temperature (T_d): 30°C
• Wet Bulb Temperature (T_w): 22°C
• Atmospheric Pressure (P): 1010 hPa

Using the Relative Humidity Calculator, we find:

• e_s(30°C) ≈ 42.45 hPa
• e_w = e_s(22°C) ≈ 26.44 hPa
• e ≈ 26.44 – 0.000665 * 1010 * (30 – 22) ≈ 21.08 hPa
• RH ≈ (21.08 / 42.45) * 100% ≈ 49.7%
• T_dew ≈ 18.4°C

The relative humidity is about 49.7%, which is moderately humid.

Example 2: Cool, Damp Day

On a cooler, damp day:

• Dry Bulb Temperature (T_d): 10°C
• Wet Bulb Temperature (T_w): 8°C
• Atmospheric Pressure (P): 1020 hPa

Using the Relative Humidity Calculator:

• e_s(10°C) ≈ 12.28 hPa
• e_w = e_s(8°C) ≈ 10.73 hPa
• e ≈ 10.73 – 0.000665 * 1020 * (10 – 8) ≈ 9.37 hPa
• RH ≈ (9.37 / 12.28) * 100% ≈ 76.3%
• T_dew ≈ 6.1°C

The relative humidity is higher at 76.3%, indicating more moisture relative to saturation.

How to Use This Relative Humidity Calculator

1. Enter Dry Bulb Temperature: Input the temperature measured by a standard thermometer (dry bulb). Select the units (°C or °F).
2. Enter Wet Bulb Temperature: Input the temperature from a thermometer whose bulb is covered with a wet wick (wet bulb). This value must be less than or equal to the dry bulb temperature. The units will match the dry bulb temperature.
3. Enter Atmospheric Pressure: Input the current barometric pressure. Select the units (hPa/mb or inHg).
4. View Results: The calculator automatically updates the Relative Humidity (primary result), Saturation Vapor Pressure, Actual Vapor Pressure, and Dew Point Temperature as you enter the values.
5. Check Table and Chart: The table and chart update to reflect conditions around the entered dry bulb temperature.

The Relative Humidity Calculator gives instant feedback, allowing you to see how changes in temperature or pressure affect humidity.

Key Factors That Affect Relative Humidity Calculator Results

1. Dry Bulb Temperature: Higher dry bulb temperature, with the same amount of moisture, lowers RH because warm air can hold more moisture.
2. Wet Bulb Temperature: The closer the wet bulb is to the dry bulb, the higher the RH (less evaporation occurs).
3. Wet Bulb Depression (Td – Tw): The difference between dry and wet bulb temperatures is crucial. A large depression means dry air, low RH. A small depression means moist air, high RH.
4. Atmospheric Pressure: Higher pressure slightly increases the evaporative cooling effect, which can minutely adjust the calculated ‘e’ and thus RH, though temperature differences are more significant. See our article on understanding air pressure.
5. Accuracy of Thermometers: The precision of your dry and wet bulb thermometers directly impacts the accuracy of the Relative Humidity Calculator results.
6. Ventilation around Wet Bulb: The rate of air movement over the wet bulb affects the evaporation rate and thus the wet bulb reading. A sling psychrometer provides good ventilation.
7. Purity of Water on Wet Bulb: Impurities can affect the vapor pressure of the water on the wick.

Frequently Asked Questions (FAQ)

1. What is the difference between dry bulb and wet bulb temperature?

Dry bulb temperature is the air temperature measured by a standard thermometer. Wet bulb temperature is measured by a thermometer with its bulb covered in a water-soaked wick, over which air is passed. Evaporation from the wick cools the bulb, so the wet bulb temperature is usually lower than the dry bulb, except at 100% RH when they are equal.

2. Why is wet bulb temperature lower than dry bulb?

Evaporation of water from the wick on the wet bulb requires heat, which is taken from the thermometer bulb, thus cooling it. The drier the air (lower RH), the more evaporation occurs, and the greater the difference between the two temperatures.

3. What happens if the wet bulb temperature is higher than the dry bulb?

This is physically impossible under normal atmospheric conditions as evaporation always causes cooling. If your readings show this, there’s likely an error in measurement.

4. How accurate is this Relative Humidity Calculator?

The calculator uses standard and widely accepted formulas. The accuracy of the result depends primarily on the accuracy of your input temperatures and pressure readings, and the psychrometric constant used.

5. Can I use this calculator for very low or high temperatures?

Yes, but ensure the water on the wet bulb is liquid (or ice if below freezing, though the constant ‘A’ changes slightly for ice). At very high temperatures, ensure the wick remains wet. The formulas are generally valid over a wide range.

6. What is dew point temperature?

The dew point is the temperature to which air must be cooled, at constant pressure and water content, to reach 100% relative humidity (saturation). Learn more about what is dew point.

7. How does atmospheric pressure affect relative humidity calculations?

Pressure appears in the psychrometric formula that relates wet bulb depression to vapor pressure. Higher pressure increases the effect of the wet bulb depression slightly.

8. Where can I get wet and dry bulb readings?

You can use a sling psychrometer or a stationary psychrometer, which are instruments with both types of thermometers. More advanced weather instruments might also provide these readings or calculate RH directly.

Related Tools and Internal Resources

• What is Dew Point? – An article explaining the concept of dew point and its relation to humidity.
• Understanding Air Pressure – Learn about atmospheric pressure and its effects.
• How to Use a Sling Psychrometer – Guide on using this instrument to get wet and dry bulb readings.
• Weather Instruments Guide – A look at various tools used in meteorology.
• Atmospheric Moisture – Deeper dive into water vapor in the air.
• Calculating Heat Index – See how temperature and humidity combine to affect how we feel heat.