Which of the following instruments is used to measure relative humidity?

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.

2. Explain the concept Each instrument is named for what it measures:

• Hydrometer: measures the density (specific gravity) of liquids, such as battery acid or milk.
• Thermometer: measures temperature.
• Hygrometer: measures humidity (moisture in the air). The prefix "hygro-" comes from the Greek word for moisture or wetness.
• Anemometer: measures wind speed.

3. Solution

• Relative humidity concerns moisture in the air, so we need an instrument whose name relates to moisture: "hygro-".
• A is wrong because it deals with liquid density.
• B is wrong because it deals with temperature.
• D is wrong because it deals with wind speed.
• The wet-and-dry bulb hygrometer, which you may have seen in school laboratories, uses two thermometers. The difference between their readings is used with a table to find the relative humidity.

4. Answer The correct option is C. Hygrometer.

5. Exam tip Use the prefix to match the instrument to its job:

• Hygro = humidity
• Hydro = heavy liquids (density)
• Anemo = air movement (wind)
• Thermo = temperature

Questions on weather instruments also commonly ask about the barometer (atmospheric pressure) and the rain gauge (rainfall), so learn those as well.