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How to Calculate Grains of Humidity

Humidity is the amount of water vapor present in the atmosphere. Meteorologists express humidity in percentages of the total air volume. However, scientists express the value of humidity in grains. Understanding both indoor and outdoor humidity affects building, agriculture and health. Humidity varies in different temperatures and pressure. Higher-pressure areas force water vapor together, raising the humidity. Colder temperatures slow the movement of water molecules, causing condensation. The intersection where temperature and humidity cause condensation is called the dew point. Knowing the value of both temperature and pressure in your area is key to your calculations.

Things You'll Need

Psychrometer with wet and dry bulb values
Barometer

Instructions

- 1
  Turn on the psychrometer and note the wet bulb and dry bulb temperatures. Subtract the wet bulb temperature from the dry bulb temperature. You can also record these numbers from a thermometer with a dry bulb and a second with a wet bulb. The wet bulb thermometer has a small cotton tube that wraps around the bulb of one thermometer. The cotton tube rests in a bowl of water and wicks the fluid up to the bulb. Simply subtract the temperature of the bulb with the wet cotton from the temperature of the dry thermometer. This is the difference.
- 2
  Multiply the difference by the specific heat of dry air, which is approximately 1.005.
- 3
  Divide the atmospheric pressure (the reading on the barometer) by the atmospheric pressure at sea level, which is 1013. Multiply this number by the specific heat of water vapor, which is approximately 4.186.
- 4
  Subtract the number in Step 3 from the number in Step 2. This is the top of your humidity ratio.
- 5
  Multiply the difference from Step 1 by the specific heat of water vapor, or 4.186. Multiply this number by -1 (negative 1).
- 6
  Subtract the latent heat of vaporization, which is approximately 2,500. The result is negative.
- 7
  Divide the number in Step 4 by the number in Step 6. This gives you the relative humidity in grains.

Insects & Science