Atmospheric humidity can be measured using a psychrometer, a dry bulb and wet bulb hygrometer or an electrical hygrometer.
The psychrometer and the dry bulb and wet bulb hygrometer use two thermometers to measure relative humidity; one measures the dry-bulb temperature and the other measures the wet-bulb temperature. The wet-bulb thermometer contains water in the base that evaporates and absorbs heat which decreases the temperature reading. To determine the relative humidity, the temperatures are taken from the dry-bulb thermometer and the temperature difference between the wet and dry bulb thermometers.

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Note: "Anemometer" is used to measure wind speed.
"Nephoscope" is used to measure the movement of clouds.
"Transmissometer" is used to determine visibility and runway visual range.