The higher the temperature, the greater amount of water the air can hold.
Humidity is caused by evaporation of water.
A hygrometer is an instrument that measures humidity. The wet bulb is the thermometer with the cotton, the dry bulb is the thermometer without cotton. Attach both thermometers to the side of the milk carton or box by securing it with the thread or string. Wet the cotton with clean room temperature water. Immediately take temperature readings of both thermometers.