How to Determine Relative Humidity: Formulas & More

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Easily calculate relative and absolute humidity
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Humidity can be taken with a hygrometer or another specialized device, but it can also be calculated if you know the air temperature, the dew point, and a few simple and standard equations. [1] We’ve compiled the most important equations for determining relative humidity (and where to find the information you need), but if you're looking for a more hands-on method, you can also build a sling psychrometer, an instrument that can measure relative humidity , with some cheap supplies and simple tools.

Equation for Relative Humidity

Relative humidity is the actual vapor pressure divided by the saturation vapor pressure. To find the actual vapor pressure, use 6.11 x 10^(7.5 x dew point) / (237.3 + dew point) . To find the saturation vapor pressure, use 6.11 x 10^(7.5 x air temperature) / (237.3 + air temperature) . Then, divide the actual vapor pressure by the saturation pressure.

Section 1 of 4:

Calculating Relative Humidity From Dew Point

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  1. If your temperature isn’t in Celcius already, convert it with the formula , where = Celsius temperature and = Fahrenheit temperature. [2]
    • You can find the dew-point by checking the weather page of your local newspaper or a website that publishes weather information for your local area, such as weather.gov. [3]
    • For example, if the temperature is 100°F, you would first subtract 32 from 100, which gives you 68. Then, you would multiply 68 by 5/9, which gives a temperature of 37.778°C.
    • You can also use online temperature converters to simplify this step.
  2. After converting the air temperature to Celsius, use this formula to calculate the saturated vapor pressure: 6.11 x 10^(7.5 x air temperature) / (237.3 + air temperature) . [4]
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  3. You can use the same formula to find the actual vapor pressure. Just substitute the dew point for the air temperature in the formula. [5]
    • Actual vapor pressure = 6.11 x 10^(7.5 x dew point) / (237.3 + dew point)
  4. Once you’ve divided your values, multiply them by 100 to get a percentage: Relative humidity = actual vapor pressure / saturation vapor pressure. [6]
  5. If math is not your forte, there are several online calculators you can use to calculate relative humidity. Just punch in the temperature and dew point, and you can find out what the relative humidity is in a fraction of a second. [7]
    • You can use the National Weather Service’s relative humidity calculator here .
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Section 2 of 4:

Finding Relative Humidity with a Sling Psychrometer

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  1. A plastic student thermometer is a cheap tube thermometer affixed on a white plastic backing. They are typically around 2 inches (5.1 cm) wide and 6 inches (15 cm) long and have a hole through the top of the plastic backing and a bulb at the bottom of the tube.
    • The thermometers should also be mercury-free.
    • You should be able to find plastic student thermometers at most office supply stores and some major retailers.
  2. First, slide the washer onto the screw. Then put the screw through the hole of one of your two thermometers. It doesn’t matter which thermometer you first put on the screw.
  3. After the washer and thermometer are on the screw, cut a 0.5 in (1.3 cm) section of a plastic straw, and slide it on. Then add the other thermometer and another washer. [8]
    • You can use another type of plastic spacer if it’s about the same size.
  4. Use a screwdriver to fix the screw into the dowel. Tighten the screw just enough so that it is secure. Be sure to leave plenty of room for the two thermometers to swing. [9]
  5. Use enough cloth to fully cover the bulb (about 2 inches (5.1 cm)), then secure it with a piece of string or rubber band. This will be your “wet-bulb” thermometer. [10]
    • For example, a section of a hollow shoelace, slit down the middle to create a single layer of cloth, works well.
    • The thermometer without the cloth will be your dry-bulb thermometer.
  6. Allow the cloth to get completely soaked, then take it out and gently squeeze out the excess water. [11]
  7. Hold the dowel out from your body and spin it so that the two thermometers whirl around the screw. As you swing the thermometers, water evaporates from the wet cloth you attached to your wet-bulb thermometer. This evaporation cools the bulb and lowers the temperature recorded on the wet-bulb thermometer. [12]
    • The amount of water that will evaporate from your psychrometer depends on how much water the atmosphere around it can hold. In a very humid environment, there’s not as much space for the water to evaporate into , so your cloth will be wetter at the end of the experiment.
    • More evaporation happens when the air is dry because dry air has a higher capacity than moist air to absorb additional moisture.
    • Wear safety glasses during this step.
  8. Record the temperatures from the two thermometers, starting with the wet-bulb. The wet-bulb temperature should be lower than the dry bulb. [13] The drier the air, the lower the wet-bulb temperature will be relative to the dry-bulb temperature.
  9. From these readings, you can figure out the relative humidity. Just find where the dry-bulb temperature and the difference between the wet- and dry-bulb readings intersect on the chart. You can also find a decent relative humidity chart here .
    • While this method will not give you a 100% accurate reading, it will give you an approximate measurement of how much water vapor there is in the air.
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Section 3 of 4:

Other Humidity Equations

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  1. If you want to reverse the equation, multiply the relative humidity by the actual vapor pressure to get saturation vapor pressure, which is absolute humidity. [14]
    • Absolute humidity = relative humidity * actual vapor pressure
  2. If you have the temperature, relative humidity percentage, and saturation rate, you can find out the mass of water in the air. [15] The saturation rate is how many grams of water air can hold at a certain temperature per kilogram. Multiply the saturation rate at the temperature by the humidity percentage.
    • Use a table to find the saturation rate for your temperature.
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Section 4 of 4:

How to Measure Humidity With a Hygrometer

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  1. Turn on your hygrometer if it’s digital, then hold it about 1.5 metres (4.9 ft) off the ground in the middle of a room. [16] Since it has the most airflow, you’ll get the best reading from the very center.
    • If you can, close all the doors and windows before taking a reading.
  2. When you breathe out, the air you’re expelling can be up to 95% relative humidity. [17] To get the most accurate results, hold the hygrometer away from your body with your arm straight out.
  3. If your hygrometer settles on a percentage before 2 minutes are up, use that number. [18] To speed up the process, move the hygrometer slowly back and forth, about 2 times per second.
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  • Question
    If it is raining heavy (like in the outer parts of a hurricane), shouldn't the humidity level be 100%?
    Donagan
    Top Answerer
    Not necessarily. The presence of lots of rain doesn't tell us for sure what the humidity is. Humidity refers to the air's water content, which is sometimes invisible.
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  • Question
    What is the ideal humidity on a gauge?
    Community Answer
    You will have to use a thermo and calculator and follow the steps. First measure the temperature, then wind pressure, and then divide them.
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  • Question
    Is there any particular name for that temperature difference between wet and dry bulbs?
    Community Answer
    The difference between dry bulb and wet bulb temperature is called wet bulb depression. Likewise the difference between temperature and dewpoint is referred to as the dewpoint depression.
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      References

      1. ↑ https://www.weather.gov/lmk/humidity
      2. ↑ https://www.weather.gov/media/epz/wxcalc/tempConvert.pdf
      3. ↑ https://www.weather.gov
      4. ↑ https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf
      5. ↑ https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf
      6. ↑ https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf
      7. ↑ https://www.wpc.ncep.noaa.gov/html/dewrh.shtml
      8. ↑ https://www.flinnsci.com/api/library/Download/b5cd538932f24cf6a0f0cb965dfc2b9e
      9. ↑ https://www.flinnsci.com/api/library/Download/b5cd538932f24cf6a0f0cb965dfc2b9e
      1. ↑ http://www.atmo.arizona.edu/students/courselinks/spring08/atmo336s1/courses/fall19/atmo170a1s1/lecture_notes/humidity/measuring_humidity.html
      2. ↑ http://www.atmo.arizona.edu/students/courselinks/spring08/atmo336s1/courses/fall19/atmo170a1s1/lecture_notes/humidity/measuring_humidity.html
      3. ↑ https://youtu.be/QbcaCxuA1LI?t=52
      4. ↑ https://www.flinnsci.com/api/library/Download/b5cd538932f24cf6a0f0cb965dfc2b9e
      5. ↑ https://youtu.be/EXjbjIgTgsA?t=113
      6. ↑ https://brownell.co.uk/datasheets/basics_humidity.pdf
      7. ↑ https://buythermopro.com/how-to-use-hygrometer-and-how-it-work/
      8. ↑ https://www.loc.gov/everyday-mysteries/meteorology-climatology/item/why-do-i-see-my-breath-when-its-cold-outside/
      9. ↑ https://trdsf.com/blogs/news/hygrometer-definition-types-uses-and-procedures

      About This Article

      Article Summary X

      You can calculate the relative humidity, or the amount of moisture in the air, if you know the air temperature and dew point. While you can use complicated mathematical equations, if math is not your forte then simply look up a relative humidity calculator online. Just punch in the temperature and dew point, and you can find out the relative humidity in a fraction of a second. To learn how to find relative humidity with a sling psychrometer, keep reading!

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