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To construct this hygrometer, you will need two thermometers, some cotton gauze, a rubber band, a small container, some water, and a piece of cardboard large enough to hang the two thermometers. Moisten the gauze with some water and wrap it around the bulb (bottom tip) of the thermometer. Secure the gauze in place with the rubber band. In order for this hygrometer to work long term, the gauze must stay moist during readings. Fill the small container with water and make sure part of the gauze is in contact with the water. The gauze will stay wet as long as it is touching the water in the container. Align the thermometers so that they are sitting next to each other with their tops and bottoms in the same place. Secure them in place using tape or hot glue. Use caution when working with hot glue. This hygrometer allows you to determine a relative humidity level of the air. Look at each thermometer at eye level and record the degree at which the red liquid sits. Write down these temperatures. Make multiple recordings throughout the day and take note of the weather as well. Is it sunny? Is it raining? Obtain a wet/dry humidity table like the one found here. The left side of the table (y-axis) is the temperature reading of the dry thermometer. The horizontal (x-axis) of the table is the difference between the dry thermometer and the wet thermometer. Subtract the dry thermometer temperature from the wet thermometer (wet dry = difference). Find the temperature of the dry bulb on the vertical axis and then find the corresponding difference on the horizontal axis. Where those two meet on the graph tells you the relative humidity. Be sure that your table matches the scale you are using. If you're using Fahrenheit, use a Fahrenheit table. If you're measuring in Celsius, use a Celsius table.

Summary:
Gather the necessary materials. Cover one thermometer with wet gauze. Attach the two thermometers to the cardboard. Record the temperatures on each thermometer. Determine relative humidity using a wet/dry humidity table.