A hydrogen-inflated polythene balloon is released from the surface of the earth. As the balloon rises to an altitude up in the atmosphere, it will

1. Layers and Structure of the Atmosphere (basic)

The atmosphere is a protective envelope of gases, water vapour, and dust particles that surrounds our planet, held in place by gravity. It isn't a uniform slab of air; rather, it is highly stratified. The most fundamental thing to understand is that the atmosphere is densest near the surface and becomes increasingly 'thin' as you go up. In fact, most of the atmospheric mass is concentrated in the lower layers because gravity pulls air molecules downward. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65. While nitrogen and oxygen make up the bulk of the air, critical components like CO₂ and water vapour are only found up to about 90 km, and oxygen becomes negligible by 120 km altitude. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64.

To study it effectively, we divide the atmosphere into five distinct layers based on temperature variations. The lowermost layer, where we live and where all weather happens, is the Troposphere. Its height isn't uniform—it is about 18 km at the equator but only 8 km at the poles. Why? Because intense heating at the equator creates strong convectional currents that push the air higher. Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7. In this layer, temperature drops as you go higher at a steady rate of roughly 6.4°C per km, known as the Normal Lapse Rate.

Above the troposphere lies the Stratosphere, extending up to 50 km. Unlike the turbulent troposphere, this layer is calm and clear, making it the preferred zone for flying commercial jets. Physical Geography by PMF IAS, Earths Atmosphere, p.275. Crucially, the stratosphere contains the Ozone layer. Ozone absorbs harmful Ultraviolet (UV) radiation, which actually causes the temperature to increase with height in this layer—a reversal of what happens in the troposphere. Beyond this, we find the Mesosphere (where temperatures drop again), the Thermosphere (where temperatures soar), and finally the Exosphere, which gradually fades into outer space.

Layer	Height (Approx)	Temperature Trend	Key Feature
Troposphere	0–13 km (Avg)	Decreases with height	Weather phenomena & highest density
Stratosphere	13–50 km	Increases with height	Ozone layer & ideal for flying
Mesosphere	50–80 km	Decreases with height	Meteors burn up here

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64-66; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7; Physical Geography by PMF IAS, Earths Atmosphere, p.275

2. Atmospheric Pressure and Altitude (basic)

To understand how pressure changes with height, imagine the atmosphere as a giant stack of blankets. If you are lying at the bottom, you feel the weight of every single blanket above you. This is atmospheric pressure—defined as the weight of a column of air stretching from the ground to the top of the atmosphere Physical Geography by PMF IAS, Pressure Systems and Wind System, p.304. At sea level, this weight is substantial (about 1.03 kg per square centimetre), but as you climb a mountain or fly in a plane, there are fewer 'blankets' of air above you. Consequently, the pressure drops because the total weight of the overhead air column decreases. In the lower atmosphere, this decrease happens quite rapidly. On average, pressure drops by about 1 millibar (mb) for every 10 metres of ascent Fundamentals of Physical Geography, Atmospheric Circulation and Weather Systems, p.76. However, this rate isn't perfectly constant because air is compressible; air near the surface is squashed more tightly (higher density) than air higher up. By the time you reach the summit of Mt. Everest, the air pressure is roughly two-thirds less than it is at sea level Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305. This drop in pressure has a profound effect on the volume of gases. Using the principles of thermal physics (specifically the Ideal Gas Law), we know that if the temperature is relatively stable, pressure and volume are inversely related. When a flexible container, like a weather balloon, rises into thinner air, the external 'squeezing' force of the atmosphere weakens. This allows the gas molecules inside to push outward more effectively, causing the balloon to expand significantly. This is why high-altitude balloons are launched looking half-empty; they need that extra space to accommodate the massive expansion that occurs as they reach the low-pressure environment of the upper atmosphere.

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.304; Fundamentals of Physical Geography, Atmospheric Circulation and Weather Systems, p.76; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305

3. Archimedes' Principle and Buoyancy in Gases (intermediate)

While we often associate buoyancy with ships in water, the same physical laws apply to the "ocean of air" surrounding us. Archimedes' Principle states that any object, wholly or partially immersed in a fluid (which includes both liquids and gases), is buoyed up by a force equal to the weight of the fluid displaced by the object. For a balloon to rise, it must be filled with a gas like Helium or Hydrogen, which is significantly less dense than the heavy Nitrogen (78%) and Oxygen (21%) that constitute the bulk of our atmosphere Physical Geography by PMF IAS, Earths Atmosphere, p.271.

As a balloon ascends, it encounters a fundamental change in its environment: decreasing atmospheric pressure. In the lower layers of the atmosphere, like the Troposphere, the air is dense and exerts significant pressure Geography Class XI NCERT, Composition and Structure of Atmosphere, p.66. However, as the balloon climbs higher, the weight of the air above it decreases. According to the Ideal Gas Law (PV = nRT), for a fixed amount of gas, pressure and volume are inversely proportional. Therefore, as the external pressure (P) drops, the internal gas molecules push the balloon's skin outward, causing the balloon to expand in volume.

Factor	At Sea Level	At High Altitude
Atmospheric Pressure	High	Low
Balloon Volume	Contracted (Partial Fill)	Expanded (Full/Stretched)
Buoyant Force	Sufficient to lift mass	Changes as air density thins

This expansion is a critical design factor for weather balloons. They are typically made of high-stretch materials like polyethylene and are launched looking "limp" or only partially inflated. If they were fully inflated at the surface, the expansion during the ascent would cause them to burst almost immediately. They continue to grow in size until the material reaches its elastic limit or the internal pressure exceeds the film strength, at which point the balloon eventually bursts.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Geography Class XI NCERT, Composition and Structure of Atmosphere, p.66

4. Meteorological Tools and Weather Balloons (intermediate)

To understand how we predict weather, we must look at how meteorological balloons (weather balloons) behave as they journey through the atmosphere. As a balloon ascends, it moves from the dense air at sea level into increasingly rarified regions of the atmosphere Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6. The most fundamental principle at play here is the relationship between pressure and volume. According to the Ideal Gas Law (PV = nRT), for a fixed amount of gas, if the external atmospheric pressure decreases, the volume of the gas must increase to maintain equilibrium. Consequently, a weather balloon continuously expands as it rises because the squeezing force of the outside air weakens with altitude.

This expansion is not an accident but a core part of the balloon's design. These balloons are typically made of highly elastic materials like polyethylene or latex. When launched, they look limp and only partially inflated; this is because engineers know the balloon will grow to many times its original size as it reaches the upper troposphere and stratosphere. The troposphere itself varies in thickness, being thicker in the tropics where warm air expands and thinner at the poles Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.28. As the balloon climbs through these layers, it carries a radiosonde—a battery-powered instrument package that measures pressure, temperature, and humidity, transmitting this data back to scientists on the ground Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.75.

Eventually, every weather balloon reaches a critical point. As the ambient pressure continues to drop, the internal pressure of the helium or hydrogen gas causes the balloon's skin to stretch to its elastic limit. Once the material can no longer accommodate the increasing volume, the balloon bursts. This usually happens at altitudes between 20 km and 35 km. This process allows us to create a vertical profile of the atmosphere, which is essential for identifying phenomena like high-speed winds that are accompanied by reduced air pressure Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.89.

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6; Exploring Society: India and Beyond, Social Science-Class VII, Understanding the Weather, p.28; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.75; Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.89

5. Boyle’s Law: Pressure-Volume Relationship (intermediate)

To understand how gases behave under different conditions, we must look at the relationship between pressure and volume, a principle known as Boyle’s Law. Imagine a fixed amount of gas trapped in a flexible container. If you apply more pressure to that container, the gas particles are forced closer together, and the volume shrinks. Conversely, if you reduce the pressure, the gas particles spread out, and the volume increases. This inverse relationship—where one variable increases as the other decreases—is the heart of Boyle's Law, provided the temperature remains constant.

This law is mathematically expressed as P ∝ 1/V, or more commonly, PV = k (where k is a constant). In practical terms, if you double the pressure on a gas, its volume will be halved. This happens because gases are highly compressible; unlike solids or liquids where particles are already in close contact, gas particles have vast empty spaces between them Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148. When pressure is applied, these spaces are reduced, increasing the density of the gas.

In the context of our atmosphere, Boyle's Law explains why objects containing gas behave differently at high altitudes. As an object rises, the ambient atmospheric pressure falls because there is less air pressing down from above. With this decrease in external pressure, the gas inside a flexible container will naturally expand to occupy a larger volume Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.297. This fundamental principle of expansion under low pressure is a cornerstone of thermal physics and meteorology.

Scenario	External Pressure	Gas Volume	Gas Density
Descending (e.g., deep sea)	Increases	Decreases	Increases
Ascending (e.g., high altitude)	Decreases	Increases	Decreases

Sources: Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.297

6. Expansion and Material Limits in High-Altitude Balloons (exam-level)

When we launch a high-altitude weather balloon, we are witnessing a fascinating tug-of-war between internal gas pressure and the thinning atmosphere. To understand why these balloons expand as they rise, we must look at the relationship between pressure and volume. At the Earth's surface, the atmosphere is dense and exerts significant pressure on the balloon. However, as the balloon ascends, the ambient atmospheric pressure decreases because there is less air pressing down from above Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.297.

According to the Ideal Gas Law (PV = nRT), if the temperature remains relatively stable and the amount of gas (n) is fixed, a drop in external pressure (P) allows the internal gas to push outward more effectively, leading to a dramatic increase in volume (V). This is why a balloon that looks like a limp, half-empty bag on the ground eventually expands into a massive, perfect sphere miles above the Earth. In fact, air naturally moves from regions of high pressure to low pressure until an equilibrium is sought Science Class VIII NCERT, Pressure, Winds, Storms, and Cyclones, p.88; in a balloon, this "flow" manifests as the gas stretching the elastic material outward to match the dropping external pressure.

However, this expansion has a physical ceiling known as the material limit. Most high-altitude balloons are made of high-grade polyethylene film. While this material is incredibly light and flexible, it has a finite tensile strength. As the balloon rises and the volume increases, the film is stretched thinner and thinner. Eventually, one of two things must happen: either a venting valve releases excess gas to maintain structural integrity, or the material reaches its breaking point and the balloon bursts Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.296. This "burst altitude" is a critical calculation for scientists who need to know exactly where their equipment will begin its parachute-assisted descent.

Stage	Ambient Pressure	Balloon Volume	Material State
Launch	High (1 atm)	Low (Underfilled)	Relaxed/Limp
Ascent	Decreasing	Increasing	Stretching
Peak Altitude	Very Low	Maximum	At Tensile Limit (Burst Point)

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.296-297; Science Class VIII NCERT, Pressure, Winds, Storms, and Cyclones, p.88

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes the concepts of Atmospheric Pressure and the Ideal Gas Law (PV = nRT) that you have just mastered. In the UPSC General Science syllabus, understanding how matter behaves under varying environmental conditions is crucial. At the Earth's surface, the high density of air molecules exerts significant External Pressure on the balloon. However, as the balloon rises, the atmosphere becomes thinner and the ambient pressure drops. To maintain an equilibrium between the internal hydrogen pressure and the decreasing external pressure, the gas molecules inside must occupy a larger space, leading directly to the conclusion that the balloon will Increase in size.

When approaching this problem, think like a physicist: if Pressure (P) decreases while the Amount of Gas (n) remains constant, the Volume (V) must increase to balance the equation. This is why high-altitude balloons are often launched partially inflated; they require extra room to expand as they reach the upper atmosphere. This principle is documented in technical design standards such as those found in NASA's Mathematical Thinking: Designing a High Altitude Balloon, which notes that balloons are engineered to accommodate massive volume changes at low pressures.

UPSC often includes "distractor" options to test the depth of your conceptual clarity. Option (A) is a common trap for students who over-index on temperature; while temperatures drop at high altitudes (which would technically decrease volume), the drastic fall in pressure is the dominant factor that forces expansion. Option (B) is a distraction based on a misunderstanding of aerodynamics, as internal gas pressure acts equally in all directions, maintaining a spherical or tear-drop shape rather than flattening. Finally, Option (D) ignores the elastic property of polythene, which is specifically chosen because it can stretch and expand until it reaches its material limit or bursts.