Think about why a wet towel dries faster on a hot dry day than on a humid day, this is directly connected to the moisture content of the surrounding air, which is what relative humidity measures. To quantify this, scientists use an instrument called a hygrometer. One simple design compares two thermometer readings, a dry bulb thermometer showing the normal air temperature, and a wet bulb thermometer whose bulb is covered by a damp wick. Water evaporating from the wick cools the wet bulb, and the cooling is larger when the surrounding air is drier because dry air can absorb more moisture through evaporation. By reading the temperature difference between the wet and dry bulbs and comparing it with standard psychrometric tables, the relative humidity of the air can be found. So the instrument used for measuring relative humidity is the hygrometer.