Hygrometer
Also called: Psychrometer · Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 7, Ch 2 "Understanding the Weather"
Meaning
A hygrometer is the instrument that measures humidity, which is the water vapour in the air. The common type is the wet-and-dry bulb hygrometer, also called a psychrometer. It has two thermometers, and one of them is wrapped in wet cloth. Water evaporates from the cloth and cools the wet bulb. Dry air takes up water faster, so the wet bulb cools more. A big gap between the two readings means low relative humidity. A small gap means the air is humid. Other hygrometers use human hair, which lengthens in moist air, or electronic sensors.
Example
On a dry summer day in Jaipur, Rajasthan, the wet bulb reads well below the dry bulb, which shows low humidity. On a muggy day in Kochi, Kerala, the two readings are close together.
Don't confuse with
- Rain gauge: it measures how much rain has fallen, in mm. It does not measure the vapour in the air.