By Megan Shoop Updated Mar 24, 2022
Humidity is the amount of water vapor present in the air. While a hygrometer provides the most precise reading, many people don't have one at home. Fortunately, meteorologists and engineers have long used a simple wet‑bulb/dry‑bulb experiment to estimate relative humidity. The method requires only two standard thermometers, a cotton ball, and a bit of patience, yet it delivers a trustworthy approximation.
Shake both mercury thermometers so that the mercury is as close to the bulb as possible. Soak a cotton ball in room‑temperature water and tape it around the bulb of one thermometer. Place this wet‑bulb thermometer and a second, dry‑bulb thermometer side by side in the same environment—inside the room or outdoors.
Leave the thermometers in place for several hours, or overnight if you prefer a more stable reading. When you check them, note the temperatures displayed by each.
Subtract the wet‑bulb temperature from the dry‑bulb temperature. The result gives a rough estimate of relative humidity. For example, if the dry bulb reads 75 °F and the wet bulb reads 40 °F, the air is approximately 35 % humid. The greater the evaporation from the wet bulb, the cooler its reading, indicating drier air.
Try the experiment in rooms known to be humid (e.g., a bathroom after a shower) and dry (e.g., a hallway with air conditioning) to see the variation in readings.
Note: This DIY method provides an estimate. For critical applications—such as HVAC design or sensitive electronics—use a calibrated hygrometer.
