How to Calculate Relative Humidity Using a Hygrometer

A professional tool for accurately determining relative humidity from dry and wet bulb temperature readings.

What is Relative Humidity?

Relative humidity (RH) is a crucial metric in meteorology and HVAC that measures the amount of water vapor present in the air. It is expressed as a percentage of the maximum amount of water vapor the air could possibly hold at a given temperature. When the air is holding the maximum possible water vapor, it is “saturated,” and the relative humidity is 100%. Warmer air can hold more moisture than cooler air.

A common instrument to gather the data needed for a how to calculate relative humidity using hygrometer query is a psychrometer, which is a specific type of hygrometer. It consists of two thermometers: a dry-bulb thermometer that measures ambient air temperature, and a wet-bulb thermometer, which has its bulb covered in a water-soaked wick. As water evaporates from the wick, it cools the bulb, causing the wet-bulb thermometer to show a lower temperature. The difference between these two temperatures, known as the wet-bulb depression, is directly related to the air’s humidity. This calculator uses those inputs to find the relative humidity. Check our Air Density Calculator for related calculations.

How to Calculate Relative Humidity: Formula and Explanation

The calculation of relative humidity from psychrometric readings involves several steps. First, we determine the saturation vapor pressure at both the dry-bulb and wet-bulb temperatures. Then, we calculate the actual vapor pressure, and finally, the relative humidity.

1. Saturation Vapor Pressure (e_s): We use the August-Roche-Magnus formula to approximate the saturation vapor pressure (the pressure when air is saturated with water) at a given temperature.

es(T) = 0.61094 * exp((17.625 * T) / (T + 243.04)) (where T is in Celsius and result is in kPa)

2. Actual Vapor Pressure (e): The actual vapor pressure is calculated using the Sprung formula, which adjusts the wet-bulb saturation pressure based on the temperature difference and atmospheric pressure.

e = es(Twb) - A * P * (Tdb - Twb)

3. Relative Humidity (RH): Finally, relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the dry-bulb temperature, expressed as a percentage.

RH (%) = (e / es(Tdb)) * 100

Variables Used in Relative Humidity Calculation

Variable	Meaning	Unit	Typical Range
Tdb	Dry-Bulb Temperature	°C or °F	-20 to 50 °C
Twb	Wet-Bulb Temperature	°C or °F	-20 to 50 °C (must be ≤ Tdb)
P	Atmospheric Pressure	hPa or inHg	950 to 1050 hPa
es	Saturation Vapor Pressure	kPa or hPa	0.1 to 12.0 kPa
e	Actual Vapor Pressure	kPa or hPa	0.1 to 12.0 kPa (must be ≤ es)
A	Psychrometric Constant	°C^-1	~0.000662

Practical Examples

Example 1: A Warm, Humid Day

Let’s consider a summer day where the measurements from a hygrometer are:

• Inputs:
  • Dry Bulb Temperature (T_db): 30 °C
  • Wet Bulb Temperature (T_wb): 25 °C
  • Pressure: 1013.25 hPa
• Results:
  • Saturation Vapor Pressure (e_s): 42.45 hPa
  • Actual Vapor Pressure (e): 31.68 hPa
  • Relative Humidity: ~74.6%

Example 2: A Cool, Dry Day

Now, let’s see how to calculate relative humidity using hygrometer readings on a cooler, drier day:

• Inputs:
  • Dry Bulb Temperature (T_db): 15 °C
  • Wet Bulb Temperature (T_wb): 10 °C
  • Pressure: 1020 hPa
• Results:
  • Saturation Vapor Pressure (e_s): 17.05 hPa
  • Actual Vapor Pressure (e): 8.79 hPa
  • Relative Humidity: ~51.5%

For more climate-related calculations, explore our Climate Change Calculator.

How to Use This Relative Humidity Calculator

1. Enter Dry Bulb Temperature: Input the ambient air temperature in the first field.
2. Enter Wet Bulb Temperature: Input the temperature from the wet-bulb thermometer. This value must be less than or equal to the dry bulb temperature.
3. Enter Atmospheric Pressure: For highest accuracy, input the current barometric pressure. The standard sea-level pressure is provided as a default.
4. Select Units: Choose your preferred units for temperature (°C or °F) and pressure (hPa or inHg). The calculation will automatically convert them.
5. Interpret the Results: The calculator instantly displays the final Relative Humidity (%), along with intermediate values like saturation and actual vapor pressure, which are key to the how to calculate relative humidity using hygrometer process.

Key Factors That Affect Relative Humidity

• Temperature: This is the most significant factor. As temperature increases, the air’s capacity to hold water vapor increases, so if the amount of moisture stays the same, the relative humidity decreases.
• Water Vapor Content: The actual amount of moisture in the air. Adding moisture (e.g., from evaporation from lakes, rivers, or oceans) increases relative humidity if the temperature is constant.
• Atmospheric Pressure: A change in pressure can affect RH. Higher pressure can lead to a slight increase in relative humidity, assuming temperature and moisture content are constant.
• Proximity to Water Bodies: Areas near oceans or large lakes typically have higher average humidity due to the abundant source of water for evaporation.
• Wind and Air Movement: Wind can transport moist or dry air from one region to another, rapidly changing the local relative humidity.
• Vegetation: Plants release water vapor through a process called transpiration, which can increase local humidity levels, especially in densely forested areas.

Our Wind Chill Calculator can help you understand the combined effect of temperature and wind.

Frequently Asked Questions (FAQ)

1. Why is the wet-bulb temperature usually lower than the dry-bulb temperature?

Because of the cooling effect of evaporation. Water evaporating from the wick on the wet bulb requires energy, which it takes from the thermometer in the form of heat, thus lowering its temperature. The drier the air, the faster the evaporation and the larger the temperature drop.

2. What happens if the wet-bulb and dry-bulb temperatures are the same?

This indicates that the air is saturated, and the relative humidity is 100%. No evaporation can occur because the air cannot hold any more moisture.

3. Can relative humidity be over 100%?

In a state known as supersaturation, yes, but it is unstable. This typically happens in very clean air, free of condensation nuclei (like dust or pollen). In this state, water vapor needs a surface to condense upon. In most natural conditions, RH will not exceed 100% as condensation (dew, fog) will occur.

4. Does this calculator work for any altitude?

Yes, by allowing you to input the local atmospheric pressure, the calculator can adjust the psychrometric constant for better accuracy at different altitudes. Lower pressure at higher altitudes affects evaporation rates.

5. What is the difference between relative humidity and dew point?

Relative humidity is a ratio of how much moisture is in the air compared to how much it *could* hold at that temperature. The Dew Point is the actual temperature the air would need to cool to in order to reach 100% relative humidity. This calculator provides the dew point as an intermediate value.

6. How accurate is this calculation?

It is highly accurate for standard atmospheric conditions. The formulas used (like the Magnus and Sprung formulas) are well-established approximations used in meteorology. Accuracy depends on the precision of your input temperature and pressure readings.

7. Why is it important to know how to calculate relative humidity using a hygrometer?

It’s crucial for many fields, including agriculture (crop health), HVAC (indoor comfort and health), manufacturing (process control), and meteorology (weather forecasting). You can learn more about weather with our Weather Forecasting Tools.

8. What is a “psychrometric chart”?

It’s a graphical representation of the thermodynamic properties of moist air, allowing users to find all properties (like RH, dew point, enthalpy) by knowing just two. This calculator is essentially a digital version of a psychrometric chart.