Which one of the following instruments is used for measuring the humidity of the air?

1. Elements of Weather and Climate (basic)

Welcome to your journey into the atmosphere! To understand how moisture behaves, we must first distinguish between two terms we often use interchangeably: Weather and Climate. While they share the same DNA, they operate on very different timescales. Weather refers to the momentary, day-to-day state of the atmosphere—the sudden rain shower in the afternoon or a hot breeze at noon INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.28. In contrast, Climate is the long-term pattern or the average of these weather conditions recorded over a significant period, usually 30 years or more Exploring Society:India and Beyond, Social Science-Class VII, Climates of India, p.46.

Both weather and climate are described using five fundamental elements. These are the building blocks of our atmospheric experience:

• Temperature: The degree of hotness or coldness of the air.
• Atmospheric Pressure: The weight of the air column pressing down on the Earth.
• Wind: The movement of air from high-pressure to low-pressure areas.
• Humidity: The amount of water vapor present in the air.
• Precipitation: Water falling to the Earth in forms like rain, snow, or hail.

As we dive deeper into Atmospheric Moisture, our focus narrows to Humidity. To measure it accurately, scientists use a specific instrument called a Hygrometer, also known as a Wet and Dry Bulb Thermometer or a Psychrometer Certificate Physical and Human Geography, GC Leong, Weather, p.120. Unlike a standard thermometer that just tells you the temperature, this device uses the cooling effect of evaporation from a moist wick to calculate exactly how much moisture is hidden in the air around us Certificate Physical and Human Geography, GC Leong, Weather, p.121.

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.28; Exploring Society:India and Beyond, Social Science-Class VII, Climates of India, p.46; Certificate Physical and Human Geography, GC Leong, Weather, p.120-121

2. Temperature Measurement and Extremes (basic)

To understand the atmosphere, we must look beyond just how 'hot' it feels right now. Meteorologists focus on Temperature Extremes—the highest and lowest points reached in a day—to identify climatic patterns. These are measured using a Maximum and Minimum Thermometer. Often, these two are combined into a single U-shaped glass tube known as Six’s thermometer. In a maximum thermometer, as the temperature rises, the mercury expands and pushes a small metal indicator up the tube. When the temperature falls, the mercury contracts, but the indicator stays at the highest point reached, allowing us to record the day's peak heat Certificate Physical and Human Geography, Chapter 13, p.118.

Accuracy is critical in these measurements. If you left a thermometer on a dark pavement, it would absorb too much heat and give a false reading. Therefore, instruments are housed in a Stevenson Screen—a white-painted wooden box with louvered sides, raised about 1.2 metres above the ground. This setup ensures the thermometer measures the ambient air temperature by protecting it from direct sunlight while allowing air to circulate freely Certificate Physical and Human Geography, Chapter 13, p.118. Modern meteorology is increasingly shifting toward digital thermometers for higher precision and easier data logging Exploring Society: India and Beyond (NCERT Class VII), Chapter 2, p.32.

Once we have the maximum and minimum readings, we calculate two vital statistics that help define a region's climate:

Sources: Certificate Physical and Human Geography, Chapter 13: Weather, p.118; Exploring Society: India and Beyond (NCERT Class VII), Chapter 2: Understanding the Weather, p.32

3. Atmospheric Pressure and Wind Direction (intermediate)

At its simplest, atmospheric pressure is the weight of the column of air above a specific point on Earth. Imagine the atmosphere as a giant ocean of air; the deeper you are (at sea level), the more weight is pressing down on you. This pressure is measured using a barometer, typically in units called millibars (mb). While the normal atmospheric pressure at sea level is approximately 1013 mb, a reading below 1000 mb usually indicates a 'depression' or low-pressure system, which is often associated with stormy or unsettled weather Exploring Society: India and Beyond (NCERT Class VII), Chapter 2, p. 35.

As we ascend in the atmosphere, there is less air above us, which means the pressure decreases with altitude. This is a critical concept for geography and human safety; for instance, army personnel serving at high-altitude posts like Khardung La in Ladakh (over 5600 meters) must carefully acclimatize because the air pressure is significantly lower than at sea level Exploring Society: India and Beyond (NCERT Class VII), Chapter 2, p. 35. To measure these changes, scientists use two main types of instruments:

When there is a difference in pressure between two regions, air naturally moves from high-pressure areas to low-pressure areas, creating wind. We track wind using two specific metrics: direction and speed. A wind vane (or a 'wind sock' often seen at airports) indicates the direction the wind is blowing from, helping pilots during takeoff and landing Exploring Society: India and Beyond (NCERT Class VII), Chapter 2, p. 37. Meanwhile, an anemometer—which typically consists of three or four rotating metal cups—measures the wind speed in kilometers per hour (km/h) by counting the rotations of its shaft GC Leong, Chapter 13, p. 122.

Sources: Exploring Society: India and Beyond (NCERT Class VII), Chapter 2: Understanding the Weather, p.35; Exploring Society: India and Beyond (NCERT Class VII), Chapter 2: Understanding the Weather, p.37; Certificate Physical and Human Geography (GC Leong), Chapter 13: Weather, p.117; Certificate Physical and Human Geography (GC Leong), Chapter 13: Weather, p.122

4. Atmospheric Moisture: Evaporation and Condensation (intermediate)

To understand the dynamics of our atmosphere, we must look at how water changes its state. This isn't just a physical change; it is an energy exchange. When water evaporates from the ocean surface, it doesn't just disappear into the air; it absorbs energy to break its molecular bonds. This energy is called Latent Heat of Vaporization. Interestingly, during this phase change, the temperature of the water doesn't actually rise; the heat is "hidden" within the water vapor molecules Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. When that vapor eventually turns back into liquid (clouds or rain), it releases that stored energy as Latent Heat of Condensation, which actually warms the surrounding atmosphere Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295.

This release of latent heat is a massive engine for weather. For example, when a saturated air parcel rises, the vapor inside begins to condense. The heat released during this condensation counteracts the cooling caused by rising, which is why saturated (wet) air cools more slowly than dry air as it ascends Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299. We measure this moisture using a hygrometer (or psychrometer), specifically the wet and dry bulb thermometer. This instrument uses the cooling effect of evaporation: a wet wick cools one bulb, and the temperature difference between it and the dry bulb tells us exactly how humid the air is Certificate Physical and Human Geography, GC Leong, Weather, p.120.

Condensation typically occurs when the air temperature is reduced to its dew point. Depending on the temperature and location, moisture takes various forms:

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.87

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299; Certificate Physical and Human Geography, GC Leong, Weather, p.120; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.87

5. Understanding Humidity: Absolute, Specific, and Relative (exam-level)

When we talk about humidity, we are simply describing the amount of water vapor present in the air. However, for a geographer or a civil services aspirant, "moisture" isn't just one number; it is measured in three distinct ways depending on whether we care about its weight, its volume, or its proportion relative to what the air can actually hold.

Absolute Humidity is the most literal measurement: it is the actual weight of water vapor per unit volume of air, usually expressed in grams per cubic meter (g/m³) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.86. While it tells us exactly how much water is there, it has a flaw—if the air expands or contracts due to temperature or pressure changes, the "absolute" number changes even if no water was added or removed. To solve this, scientists use Specific Humidity, which measures the weight of water vapor per unit weight of air (grams per kilogram). Because it relies on mass rather than volume, specific humidity remains constant regardless of the air's expansion or contraction.

The concept you will encounter most frequently is Relative Humidity (RH). This isn't an amount, but a ratio. It is the percentage of moisture present in the atmosphere compared to its full capacity at a specific temperature Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326. Think of the air as a sponge: if the sponge can hold 100ml of water but currently holds 50ml, its relative humidity is 50%. Crucially, the air's "capacity" is tied to its temperature; warm air can hold significantly more moisture than cold air. This leads to a counter-intuitive rule: if you heat up a room without adding water, the Absolute Humidity stays the same, but the Relative Humidity drops because the air's capacity has increased.

To measure these values, we use a hygrometer (or psychrometer), which typically consists of a wet and dry bulb thermometer GC Leong, Physical and Human Geography, Weather, p.120. When the air reaches its maximum capacity (100% RH), we call it "saturated." The specific temperature at which this saturation occurs for a given sample of air is known as the Dew Point Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.327. If the temperature drops even slightly below the dew point, the air can no longer hold its water, leading to condensation into dew, fog, or clouds.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.86; Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326-327; GC Leong, Physical and Human Geography, Weather, p.120-121

6. Measuring Humidity: The Psychrometer and Hygrometer (exam-level)

To understand how we measure the invisible moisture in the air, we must look at the hygrometer. While there are many types, the most common and reliable version used in weather stations is the wet and dry bulb thermometer, often called a psychrometer. This instrument consists of two identical mercury thermometers mounted side-by-side in a Stevenson Screen—a white, louvered wooden box designed to shield instruments from direct sunlight and rain while allowing air to circulate freely. This ensures we measure the true shade temperature of the air Certificate Physical and Human Geography, Chapter 13, p.118.

The magic of the psychrometer lies in the principle of evaporative cooling. One thermometer (the dry bulb) simply measures the ambient air temperature. The other (the wet bulb) has its bulb wrapped in a muslin wick that stays moist by dipping into a reservoir of distilled water. As air passes over the wet wick, water evaporates. Because evaporation requires energy (latent heat), it takes that heat from the thermometer bulb, causing the temperature to drop Certificate Physical and Human Geography, Chapter 13, p.120. The speed of this evaporation depends entirely on how much moisture is already in the air: if the air is dry, evaporation is rapid and cooling is significant; if the air is damp, evaporation slows down Exploring Society: India and Beyond, Chapter 2, p.38.

By comparing the two readings, meteorologists calculate Relative Humidity and the Dew Point. If the air is completely saturated (100% humidity), no evaporation can occur from the wick, meaning both thermometers will show the exact same reading. However, a large difference (known as the "wet-bulb depression") indicates very dry air Physical Geography by PMF IAS, Hydrological Cycle, p.326. Unlike an aneroid barometer (which measures pressure) or a wind vane (which shows direction), the psychrometer specifically reveals the water vapor content of our atmosphere.

Sources: Certificate Physical and Human Geography, Weather, p.118, 120-121; Exploring Society: India and Beyond, Understanding the Weather, p.38; Physical Geography by PMF IAS, Hydrological Cycle, p.326

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental elements of the atmosphere—temperature, pressure, and moisture—this question tests your ability to identify the specific tools used to quantify them. In your previous lessons, you learned that humidity refers to the amount of water vapor in the air, and its measurement relies on the principle of evaporative cooling. The wet and dry bulb thermometer (often called a psychrometer or a type of hygrometer) uses two thermometers: one to record the actual air temperature and another wrapped in a wet wick. The reasoning is simple: if the air is dry, water evaporates quickly from the wick, cooling the wet bulb significantly; if the air is humid, evaporation is slow, and the temperature difference is small. Therefore, (C) Wet and dry bulb thermometer is the correct instrument for this task, as noted in Certificate Physical and Human Geography, GC Leong.

To avoid common UPSC traps, you must distinguish between instruments that measure different atmospheric properties. Option (A), the wind vane, is strictly for determining wind direction, while the aneroid barometer in option (B) is used to measure atmospheric pressure without the use of liquids. Finally, option (D), the maximum and minimum thermometer, is designed to record the highest and lowest temperatures over a 24-hour period rather than moisture content. As highlighted in Exploring Society: India and Beyond, NCERT Class VII, precision in identifying these tools is key, as UPSC frequently groups related weather instruments to test your clarity on their specific functions.