Which scientific instrument is commonly used to measure the humidity, that is, the amount of water vapour present in the atmosphere?

Introduction / Context:
This question is about basic weather instruments and their uses. Humidity is a measure of how much water vapour is present in the air, and it strongly influences comfort, weather patterns and processes like drying of clothes. Meteorologists and HVAC engineers need accurate humidity measurements, and they use a special instrument for this purpose. Knowing the correct name of the instrument is useful for both science and general knowledge exams.

Given Data / Assumptions:

• The quantity to be measured is humidity or moisture content in the air.
• Options include barometer, anemometer, thermometer and hygrometer.
• We assume standard meteorological instruments and their classical definitions.

Concept / Approach:
A hygrometer is the instrument designed to measure humidity. It can be a simple hair hygrometer, a wet and dry bulb psychrometer or an electronic humidity sensor, but the generic name is hygrometer. A barometer measures air pressure, an anemometer measures wind speed and a thermometer measures temperature. Therefore, hygrometer is the only option that directly corresponds to humidity measurement.

Step-by-Step Solution:
Step 1: Recall that humidity refers to the amount of water vapour in the air and is often expressed as relative humidity or absolute humidity. Step 2: Understand that an instrument used to measure humidity is called a hygrometer (hygro relating to moisture). Step 3: Recognise that barometers are used for measuring atmospheric pressure, not humidity. Step 4: Note that anemometers measure the speed of wind and are not related to moisture measurement. Step 5: Observe that thermometers are used to measure temperature, a different physical quantity. Step 6: Compare these functions with the requirement of measuring humidity. Step 7: Conclude that the correct instrument for humidity is the hygrometer.

Verification / Alternative check:
Weather reports often mention relative humidity in percentage, and weather stations use hygrometers or psychrometers to record these values. Many digital room thermometers now include a humidity readout and are sometimes labelled thermo hygrometers, indicating that hygrometer is the humidity measurement component. These practical devices confirm the name and role of a hygrometer as the instrument used to measure atmospheric humidity.

Why Other Options Are Wrong:
Barometer measures atmospheric pressure, which is commonly expressed in millibar or hectopascal, not humidity. Anemometer measures wind speed and sometimes wind direction, not moisture content in the air. Thermometer measures temperature, usually in degree Celsius or kelvin, and does not measure water vapour content.

Common Pitfalls:
Because all the instruments listed are used in weather observations, it is easy to mix up their purposes. A simple memory aid is to link hygro with moisture, baro with weight or pressure, anemo with wind and thermo with heat or temperature. Remembering these prefixes makes it easier to select hygrometer whenever the question involves humidity or moisture in the air.

Final Answer:
The instrument used to measure atmospheric humidity is the Hygrometer.