There are different tools available to help measure humidity, including a hygrometer, a hygrograph and a sling psychrometer. Each separate tool that measures humidity does so on a different scale.
Humidity is the amount of water vapor in the air. The hygrometer measures this water vapor on more of a local scale, using two thermometers that have a dry and a wet bulb. This tool looks at the speed at which water evaporates from the moist bulb. Other tools and equipment, such as a sling psychrometer and a hygrograph, are used to measure humidity on a global scale. For weather forecasting, the Stevenson screen is used.