Also known as a wet-and-dry-bulb hygrometer, the psychrometer is a device containing two mercury thermometers. One thermometer has its bulb wrapped in a moist cloth, while the other thermometer's bulb is dry. The gradual evaporation of moisture from the wet bulb keeps its temperature lower than that of the dry bulb. The larger the temperature difference between the two thermometers, the higher the relative humidity of the air.
Invented in 1783, the mechanical hygrometer is the least accurate device for measuring humidity. However, it remains a common project for school science fairs. Its main component is a strand of hair, or another humidity-absorbing material like wood or paper. As the hair absorbs moisture from the air, it changes in length, and this change appears on a scale or dial as an estimate of relative humidity.
The chilled mirror hygrometer is one of the most precise devices for measuring relative humidity. This type of hygrometer, which dates back to the 1960s, uses a chilled metallic "mirror" and an electronic photodetector to identify the dew point, which is the temperature at which ambient water vapor will condense on the mirror. To maintain its accuracy, a chilled mirror hygrometer requires frequent mirror calibrations and cleanings.
Because they can measure atmospheric humidity on a regional or global scale, weather satellites are essential for monitoring climate trends and forecasting weather changes. Satellite-based instruments such as the microwave humidity sounder (MHS) detect microwave radiation diffused from the Earth's surface, which varies depending on the amount of humidity in the atmosphere. These satellites play an important role in the prediction of storms and other destructive weather events.