By Robin McDaniel | Updated Aug 30, 2022

Accurately gauging the moisture content of the air—known as relative humidity (RH)—is essential for weather‑related projects, scientific experiments, and everyday comfort. This guide walks you through a reliable method using basic equipment and straightforward calculations.

Step 1 – Gather Your Instruments

Acquire two bulb thermometers with round, cylindrical bulbs and Fahrenheit scales. Place them on a stable surface where they can freely stand and remain exposed to ambient air.

Step 2 – Measure the Dry‑Bulb Temperature

The dry‑bulb reading represents the actual air temperature. Because humidity increases with temperature, this measurement is critical. Set the thermometer in a neutral, dry spot away from direct sunlight to capture an accurate air temperature.

Step 3 – Measure the Wet‑Bulb Temperature

Wrap the bulb of a second thermometer in a wet muslin cloth and allow air to flow over it normally. As the moisture evaporates, the wet‑bulb temperature drops, revealing the air’s moisture level.

Step 4 – Determine the Dew Point

Use a psychrometric chart, slide rule, or calculator to plot the dry‑ and wet‑bulb temperatures. The intersection of their lines gives the dew point, a key value for computing RH.

Step 5 – Apply the Calculations

First, convert Fahrenheit to Celsius:
ÅC = (5/9) × (ÅF – 32). For the dew point, apply the same formula to the wet‑bulb reading.
Next, compute the actual vapor pressure (E_a) and saturated vapor pressure (E_s):
E_a = 6.11 × 10^{7.5 T_c/(237.7 + T_c)}
E_s = 6.11 × 10^{7.5 T_d/(237.7 + T_d)} (T_c = Celsius air temp, T_d = Celsius dew point).

Step 6 – Calculate Relative Humidity

Relative Humidity (%) = (E_a ÷ E_s) × 100. The result is the air’s moisture content expressed as a percentage.

Things Needed

• Bulb thermometers (dry and wet)
• Muslin cloth
• Water (for wetting the cloth)
• Psychrometric slide rule or digital calculator
• Psychrometric graph or software

Warning

Results may differ by 10% or more due to environmental variations and instrument accuracy. Use the procedure as a reliable estimate rather than an absolute measurement.