Relative Humidity Calculator

The relative humidity calculator allows you to determine the relative humidity from the air temperature and dew point.

Remember those hot and humid months during summer and how it feels? The warm and muggy feeling can be attributed to high relative humidity that limits the amount of sweat evaporated from our skin. Less sweat evaporation means we feel much hotter than the actual temperature. To know more about relative humidity and how it affects us, continue reading this article. You will find answers to questions like:

• What is relative humidity?
• How to calculate relative humidity.
• How to measure relative humidity.

We have also included an example of calculating relative humidity using our calculator.

The relative humidity is a measure of how moist the air is. We can define relative humidity as the actual amount of water vapor (moisture) present in the air compared to the total amount of water vapor that the air can hold at a given temperature.

To express relative humidity as the ratio of vapor pressure $P_w$ to the saturation vapor pressure $P_{ws}$ at a given temperature, we can use the equation:

Note that we always express relative humidity as a percentage.

Another concept that we should be familiar with while talking about relative humidity is absolute humidity. It is the mass of water vapor present in a unit volume of air. We can write the formula for absolute humidity as:

where:

• $H$ – Absolute humidity usually expressed in g/m³;
• $m$ – Mass of water vapor; and
• $V$ – Volume of air and water vapor mixture.

We can express the relation between dew point, air temperature, and relative humidity as:

where:

• $D_p$ – Dew point temperature (in °C) (to learn more about it, visit the dew point calculator);
• $RH$ – Relative humidity;
• $T$ – Air temperature; and
• $\beta = 17.625$ and $\lambda = 243.04 \text{\degree C}$ – Revised Magnus coefficients as recommended by Alduchov and Eskridge.

Substituting the values and rearranging the above equation, we can write the formula for relative humidity as:

If you want to learn how to calculate relative humidity from the actual and saturation mixing ratios of air, we recommend checking our mixing ratio of air calculator.

Now let's see how we can use the relative humidity (RH) calculator to find out the relative humidity when the dew point is 70 °F and the air temperature is 95 °F.

1. Enter the air temperature as 95 °F (35 °C). The tool has an inbuilt temperature conversion option that allows you to input the temperature in other units.

2. Type the dew point temperature as 70 °F (~21.1 °C).

3. The RH calculator will display the relative humidity as 44.48%.

4. You can also use this tool to calculate air temperature and dew point.

To measure relative humidity, we use hygrometers. Some commonly used hygrometers are:

1. Psychrometer: The psychrometers measure the relative humidity by measuring the difference between dry bulb temperature and wet bulb temperature.
2. Hair hygrometer: This device relies on measuring the relationship between the relative humidity and length of hair.
3. Capacitive type electronic hygrometer: These hygrometers detect the change in a sensor's capacitance when it absorbs moisture and thus measures the humidity.
4. Chilled-mirror dew point hygrometer: As the name suggests, these hygrometers measure the dew point temperature, which we can then use to calculate relative humidity.

In general, we feel more comfortable when the relative humidity level is between 30-50%. Lower humidity levels can lead to dryness, irritation, and itching.

Levels higher than 50% are uncomfortable as our sweat won't evaporate efficiently, and we will "feel" a higher temperature. Higher humidity levels also cause mold growth which can lead to various health issues.