A thermometer measures temperature (measurand). By the same instrument-to-measurand relation, a hygrometer measures what?

Introduction / Context:
The mapping is instrument → quantity measured. As thermometer → temperature, so hygrometer → humidity (moisture content of air). This is a direct scientific vocabulary test.

Given Data / Assumptions:
We use standard instrument–measurand pairs.

Concept / Approach:
Recall common meteorological instruments: hygrometer measures relative humidity/absolute humidity/dew point via related devices; barometer measures pressure; densitometer measures density; “strains” relate to strain gauges, not hygrometers.

Step-by-Step Solution:

Verification / Alternative check:
Weather stations include hygrometers for humidity tracking.

Why Other Options Are Wrong:
They are measured by different instruments or are unrelated.

Common Pitfalls:
Confusing hygrometer with hydrometer (liquid density) — note the spelling difference.

Final Answer:
Humidity