An instrument used to measure relative humidity is the?

Step 1: Analyze the question The question asks which instrument measures relative humidity, which is the amount of water vapour in the air compared with the maximum it could hold at that temperature.

Step 2: Explain the concept Each option is an instrument whose name tells you what it measures:

• Hygrometer: "hygro-" comes from the Greek word for moisture or wetness. It measures humidity in the air. Examples are the wet-and-dry bulb hygrometer and the hair hygrometer.
• Hydrometer: "hydro-" means water or liquid. It measures the relative density (specific gravity) of liquids, such as acid in a car battery or milk.
• Pyrometer: "pyro-" means fire or heat. It measures very high temperatures, such as those in furnaces.
• Manometer: "mano-" relates to pressure. It measures the pressure of gases or liquids.

Step 3: Solve Relative humidity is about moisture in the air, so we need the instrument linked to moisture. That is the hygrometer. The wet-and-dry bulb hygrometer works by comparing the readings of two thermometers. The wet bulb cools by evaporation, and the difference between the readings is used with tables to find the relative humidity.

Step 4: Answer The correct option is A. hygrometer.

Tip: Look at the prefix.

• Hygro = moisture, so humidity
• Hydro = liquid, so liquid density
• Pyro = fire, so high temperature
• Mano = pressure

Hygrometer and hydrometer are often confused in exams. Remember "Hy-Gro = Gas moisture (humidity)", and hydrometer is for liquids.