A hygrometer measures relative humidity, the ratio of the partial pressure of water vapor in the air to the partial pressure if the water vapor was concentrated enough to precipitate. Relative humidity is therefore a measure of how close the weather is to precipitation conditions. If you know the temperatures with and without a damp cloth on the thermometer bulb, you can look up the relative humidity in tabular form. This is called the “wet-dry method.”
Buy a sling psychrometer at a lab or educational supply outlet. Dampen the little piece of cloth attached to the bulb of one of its two thermometers.
Swing the two thermometers around by the psychrometer’s handle for five minutes. This brings down the temperature of the dampened thermometer as water evaporates.
Enter the two temperatures into your psychrometer’s slide scale to read off the relative humidity. If the psychrometer doesn’t have a slide scale, use an online table or calculator.