A hygrometer is a device used to measure the humidity, or amount of water vapor in the air.[1] The more water vapor in the air, the higher the humidity. High humidity can make a hot day feel even hotter because our bodies cannot sweat as efficiently with the extra moisture in the air.[2] You may have noticed that your hair becomes frizzy on a humid day. Hair expands when there is more moisture in the air.[3] Using this principle you can create a basic hygrometer with your hair to measure relative humidity.