This measures humidity.

This measures humidity.

• A. Hygrometer
• B. Psychrometer
• C. Barometer
• D. Thermometer

Answer: (B)

Humidity is the amount of water vapor in the air. A psychrometer measures humidity by using two thermometers: a dry-bulb thermometer and a wet-bulb thermometer. The wet-bulb has its bulb wrapped in a wick saturated with water. When air moves over the bulbs, water evaporates from the wet-bulb, cooling it. The rate of evaporation—and thus how far the wet-bulb’s temperature drops compared with the dry-bulb—depends on the air’s moisture. In a dry sky, evaporation happens quickly and the wet-bulb reads much cooler than the dry-bulb; in a very humid environment, evaporation slows, and the two readings are close together. This difference between the dry-bulb and wet-bulb temperatures is used to determine relative humidity (often with a chart or calculator). Other instruments either measure related quantities or the air temperature itself, but they don’t directly provide humidity the way a psychrometer does. So the device that directly gauges how much water vapor the air contains, via evaporative cooling, is the psychrometer.

Humidity is the amount of water vapor in the air. A psychrometer measures humidity by using two thermometers: a dry-bulb thermometer and a wet-bulb thermometer. The wet-bulb has its bulb wrapped in a wick saturated with water. When air moves over the bulbs, water evaporates from the wet-bulb, cooling it. The rate of evaporation—and thus how far the wet-bulb’s temperature drops compared with the dry-bulb—depends on the air’s moisture. In a dry sky, evaporation happens quickly and the wet-bulb reads much cooler than the dry-bulb; in a very humid environment, evaporation slows, and the two readings are close together. This difference between the dry-bulb and wet-bulb temperatures is used to determine relative humidity (often with a chart or calculator). Other instruments either measure related quantities or the air temperature itself, but they don’t directly provide humidity the way a psychrometer does. So the device that directly gauges how much water vapor the air contains, via evaporative cooling, is the psychrometer.