The humidity of air measured in percentage is called :

1. Atmospheric Moisture & The Hydrological Cycle (basic)

When we talk about moisture in the atmosphere, we are essentially discussing water vapor—the invisible, gaseous form of water. Even on a clear day, the air around you contains a certain amount of this vapor, which is fundamental to weather patterns and the hydrological cycle. To understand how "sticky" or "dry" the air feels, meteorologists use different ways to measure this moisture, often referred to as humidity.

The most direct measure is Absolute Humidity, which tells us the actual weight of water vapor in a specific volume of air, usually measured in grams per cubic meter (g/m³) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 10, p. 86. However, absolute humidity can be tricky because if the air expands or contracts due to pressure changes, the density changes even if the amount of water stays the same. To solve this, we use Specific Humidity, which measures the weight of water vapor per unit weight of air (grams per kilogram). This measure is more stable because it doesn't change with changes in air pressure or volume Physical Geography by PMF IAS, Chapter 24, p. 328.

The concept you will encounter most often in daily life and geography exams is Relative Humidity (RH). This is not a measurement of weight, but a percentage representing how much water vapor the air is currently holding compared to the maximum amount it could hold at that specific temperature FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 10, p. 86. Think of a sponge: if a sponge can hold 100ml of water but currently has 50ml, its "relative wetness" is 50%. Crucially, the air's capacity to hold water depends entirely on temperature; warm air can hold significantly more water vapor than cold air. Therefore, if you take a parcel of air and cool it down without adding any water, its Relative Humidity will increase because its total capacity has shrunk.

Type of Humidity	Definition	Unit of Measurement
Absolute Humidity	Actual mass of water vapor per volume of air.	Grams per cubic meter (g/m³)
Specific Humidity	Mass of water vapor per mass of air.	Grams per kilogram (g/kg)
Relative Humidity	Ratio of current vapor to max capacity at a given temperature.	Percentage (%)

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 10: Water in the Atmosphere, p.86; Physical Geography by PMF IAS, Chapter 24: Hydrological Cycle (Water Cycle), p.328

2. Evaporation, Condensation, and Latent Heat (basic)

To understand the atmosphere, we must first understand how water changes its form. Evaporation is the process where liquid water transforms into water vapor. Think of it as water molecules gaining enough energy to "break free" from the surface of a water body. This energy doesn't just disappear; it is stored within the vapor as Latent Heat of Vaporization. Because this heat is used to change the state (from liquid to gas) rather than raising the temperature of the water itself, the temperature of the system remains constant during the process Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. This is why evaporation is a cooling process—it takes heat away from the surface.

Several factors dictate how fast water evaporates. It is not just about heat; it is about the "thirst" of the air and the movement of molecules. For instance, air with low relative humidity has more capacity to hold moisture, significantly increasing the evaporation rate Physical Geography by PMF IAS, Hydrological Cycle, p.328. Similarly, strong winds help by sweeping away the saturated air near the surface and replacing it with drier air. Interestingly, the composition of the water matters too—ocean water evaporates about 5% slower than fresh water because salinity reduces the ability of water molecules to "bounce off" the surface Physical Geography by PMF IAS, Hydrological Cycle, p.329.

The reverse process is Condensation, where water vapor turns back into liquid. This is the most critical step for weather because the "hidden" energy stored during evaporation is now released as Latent Heat of Condensation. This release of heat is the fundamental "fuel" that powers the atmosphere. It provides the energy necessary for the formation of massive cumulonimbus clouds and the devastating strength of tropical cyclones Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. Without this transfer of energy, our weather would be far less dynamic.

Process	Phase Change	Energy Interaction	Atmospheric Impact
Evaporation	Liquid to Gas	Absorbs Heat (Cooling)	Adds moisture to the air
Condensation	Gas to Liquid	Releases Heat (Warming)	Fuels storms and cyclones

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294-295; Physical Geography by PMF IAS, Hydrological Cycle, p.328-329

3. Air Saturation and Dew Point (intermediate)

To understand the atmosphere, we must first understand that air acts like a container for water vapor. However, unlike a glass jar, the "size" of this air container changes with temperature. Saturation is the state where a parcel of air is holding the maximum amount of water vapor possible at its current temperature. At this specific point, the air can no longer "absorb" any more moisture, and any additional vapor would have to condense into liquid. In technical terms, when air is saturated, its Relative Humidity (RH) is exactly 100% Physical Geography by PMF IAS, Hydrological Cycle, p.326. If the air is only holding half of what it could potentially carry, we say it is unsaturated, with an RH of 50%.

The Dew Point is the critical temperature threshold at which a given sample of air becomes 100% saturated Physical Geography by PMF IAS, Hydrological Cycle, p.327. Imagine a warm parcel of air that is currently unsaturated. As that air cools down, its capacity to hold water vapor shrinks. Eventually, it reaches a temperature where its current water content exactly matches its maximum capacity. That temperature is the Dew Point. It is a vital concept because once air temperature drops below the dew point, condensation begins, leading to the formation of dew, frost, fog, or clouds FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.87.

There are two primary ways to drive an air parcel toward saturation and its dew point:

• Cooling the air: By lowering the temperature, you decrease the air's holding capacity until it matches the existing moisture level. This is the most common cause of cloud formation.
• Adding moisture: By evaporating more water into the air, you fill the "container" until it reaches its limit Physical Geography by PMF IAS, Hydrological Cycle, p.327.

Scenario	Effect on Capacity	Effect on Relative Humidity
Temperature Increases	Increases (Air can hold more)	Decreases (Air becomes "drier")
Temperature Decreases	Decreases (Air can hold less)	Increases (Air moves toward Dew Point)

Sources: Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326-327; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.87

4. Cloud Classification and Formation (intermediate)

When moist air rises and cools below its dew point, water vapor condenses around tiny particles like dust or sea salt (hygroscopic nuclei). This collective mass of water droplets or ice crystals suspended in the atmosphere is what we call a cloud. To understand the sky, meteorologists classify clouds based on two primary factors: their physical form and their altitude Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), Weather, p.124.

At the foundational level, we look at four basic shapes. Cirrus clouds are thin, feathery, and fibrous, often appearing like wisps of hair. Cumulus clouds look like heaps of cotton wool with flat bases, indicating vertical air movement. Stratus clouds form uniform, gray layers covering large parts of the sky, while Nimbus clouds are the heavy, dark, shapeless masses that bring rain FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.88. Interestingly, because high-altitude clouds exist in freezing temperatures, clouds like Cirrus are composed entirely of tiny ice crystals rather than water droplets Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Hydrological Cycle (Water Cycle), p.335.

By combining these forms with their height, we create a more precise hierarchy. We use prefixes like 'Cirro-' for high clouds and 'Alto-' for middle clouds. For example, Altostratus is a middle-layered cloud, while Cirrocumulus is a high, heaped cloud. We also measure the amount of sky covered by these clouds in units called oktas, where 0 oktas is a clear sky and 8 oktas is completely overcast Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), Weather, p.124.

Altitude Category	Approx. Height	Common Cloud Types
High Clouds	6,000m - 12,000m	Cirrus, Cirrocumulus, Cirrostratus
Middle Clouds	2,000m - 6,000m	Altocumulus, Altostratus
Low Clouds	Below 2,000m	Stratocumulus, Nimbostratus
Vertical Development	Varies (can span all levels)	Cumulus, Cumulonimbus (Thunderstorm clouds)

Sources: Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), Weather, p.124; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.88; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Hydrological Cycle (Water Cycle), p.335

5. Forms of Precipitation and Rainfall Types (intermediate)

Once the moisture in the air condenses around nuclei, the resulting droplets or ice crystals must grow large enough to overcome atmospheric resistance and fall to the ground under the influence of gravity. This process is called precipitation. It is important to distinguish between the form the moisture takes (like rain or snow) and the mechanism that causes the air to rise and cool in the first place (the rainfall type).

Forms of Precipitation are primarily determined by the temperature profile of the atmosphere through which the moisture falls. While Rainfall consists of water drops larger than 0.5 mm, Drizzle is characterized by very small, light drops (< 0.5 mm) that seem to float Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.338. If the temperature is below 0°C, moisture falls as Snow in the form of fine flakes or crystals. A more complex form is Sleet, which occurs when raindrops fall through a sub-freezing layer of air near the ground, causing them to solidify into small pellets of ice FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Water in the Atmosphere, p.88. In contrast, Hail consists of hard, rounded pellets (5 to 50 mm) formed by intense vertical currents in convective clouds that force ice pellets to move up and down, adding concentric layers of ice Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.338.

Types of Rainfall are classified based on the mechanism that triggers the ascent of air:

• Convectional Rainfall: Common in equatorial regions, where intense solar heating causes air to expand and rise vertically, leading to heavy, short-duration afternoon showers.
• Orographic (Relief) Rainfall: This occurs when moisture-laden air is forced to ascend a physical barrier, such as a mountain range. The windward side receives heavy rainfall due to cooling by expansion, while the leeward side (rain-shadow area) remains dry as the air descends and warms Certificate Physical and Human Geography, GC Leong, Climate, p.136.
• Cyclonic (Frontal) Rainfall: This happens when two air masses of different temperatures and densities meet, forcing the warmer, lighter air to rise over the colder, denser air.

Feature	Convectional	Orographic
Trigger	Thermal heating of the surface	Physical barrier (Mountains)
Location	Mostly tropical/equatorial	Windward slopes of mountains
Characteristic	Short, intense, accompanied by lightning	Permanent wet and dry (shadow) zones

Sources: Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.338; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Water in the Atmosphere, p.88; Certificate Physical and Human Geography, GC Leong, Climate, p.136

6. Comparing Humidity: Absolute, Specific, and Relative (exam-level)

To understand how the atmosphere holds moisture, we must distinguish between three key ways of measuring it: Absolute, Specific, and Relative Humidity. While they all describe 'dampness,' they serve different purposes in weather science.

Absolute Humidity is the most straightforward: it is the actual weight of water vapor present in a unit volume of air, usually expressed in grams per cubic meter (g/m³). Because it depends on volume, it is quite sensitive to temperature changes. For instance, if a parcel of air expands as it warms, the volume increases, and the absolute humidity drops even if the amount of water remains the same Physical Geography by PMF IAS, Chapter 24, p.326. In contrast, Specific Humidity is the weight of water vapor per unit weight of air (expressed as grams per kilogram). This is a more stable measure because the mass of the air doesn't change when the air expands or contracts due to temperature or pressure shifts Physical Geography by PMF IAS, Chapter 24, p.328.

The most critical concept for weather forecasting, however, is Relative Humidity (RH). This is not an absolute measure of 'how much' water is there, but a percentage showing how close the air is to being 'full' (saturated) at a specific temperature FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 10, p.86. For example, in equatorial regions, RH often exceeds 80%, meaning the air is holding 4/5ths of its total capacity Certificate Physical and Human Geography, Chapter 13, p.120.

Type of Humidity	Definition	Units	Sensitivity
Absolute	Weight of vapor per unit volume of air.	g/m³	Changes with air expansion/contraction.
Specific	Weight of vapor per unit weight of air.	g/kg	Stable; stays constant despite temp/pressure changes.
Relative	Ratio of actual vapor to maximum capacity at a given temp.	Percentage (%)	Highly sensitive to temperature changes.

Crucially, Relative Humidity is inversely related to temperature. If you keep the moisture constant but increase the temperature, the air's capacity to hold water increases, and the Relative Humidity drops. Conversely, cooling the air brings it closer to saturation (100% RH), which eventually leads to condensation and rainfall Physical Geography by PMF IAS, Chapter 24, p.327.

Sources: Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326-328; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water in the Atmosphere, p.86; Certificate Physical and Human Geography, Weather, p.120

7. Solving the Original PYQ (exam-level)

You have just explored how moisture enters our atmosphere through evaporation and how air's capacity to hold water vapor is strictly dependent on temperature. This question brings those building blocks together by asking you to identify the specific metric we use to describe the air's degree of saturation. As you learned from FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT Class XI, the atmosphere's moisture content isn't just measured by weight; it is often measured as a ratio of what is currently present versus the maximum amount the air can hold. This relationship is what determines the likelihood of clouds, fog, or rain.

To arrive at the correct answer, you must focus on the phrase "measured in percentage." While Absolute Humidity represents the actual mass of water vapor in a specific volume of air (expressed in $g/m^3$) and Specific Humidity measures the mass of vapor against the mass of air (expressed in $g/kg$), neither of these represents a proportion. Only Relative Humidity compares the actual water vapor content to the air's total capacity at a given temperature, multiplying that ratio by 100 to yield a percentage. This is why a reading of 100% signifies that the air is fully saturated and cannot hold any more moisture, a concept detailed in Physical Geography by PMF IAS.

UPSC often uses "all of the above" as a distractor trap to tempt students who recognize that all the options are valid types of humidity. However, you must be precise with your units. According to Certificate Physical and Human Geography, GC Leong, Absolute Humidity is problematic for meteorologists because it changes when air volume changes (due to pressure or temperature) even if no moisture is added. Because the question specifically asks for the metric defined by a percentage, Relative Humidity is the only scientifically accurate choice. Always look for the specific unit mentioned in the question stem to avoid these common classification traps.