The hygrometer is used for measuring

1. Analyze the question The question asks what a hygrometer measures. This is a question about ecological and weather instruments.

2. Explain the concept The name comes from the Greek hygros, meaning "wet" or "moist". A hygrometer measures the amount of water vapour in the air, which is called humidity. In ecology and biology, it is usually used to find the relative humidity, the amount of water vapour in the air compared with the maximum the air can hold at that temperature, given as a percentage.

3. Go through the options

• A. Relative humidity: Correct. This is the main job of a hygrometer. One example is the wet-and-dry bulb hygrometer, where the difference in temperature between the two thermometers is used to find relative humidity.
• B. Specific gravity: This is measured with a hydrometer (for example, to test the density of a liquid or battery acid). The names are easy to confuse.
• C. Rainfall: This is measured with a rain gauge.
• D. Salinity: This is measured with a salinometer (or a hydrometer-based salt meter).

4. Answer The hygrometer measures relative humidity, which is option A.

5. Exam tip Match the first part of each instrument's name to what it measures:

• Hygro- = moisture, so hygrometer measures humidity
• Hydro- = water or liquid density, so hydrometer measures specific gravity
• Anemo- = wind, so anemometer measures wind speed
• Baro- = pressure, so barometer measures atmospheric pressure