Chapter 7 Composition And Structure Of Atmosphere

Survival for any organism, especially humans, is intricately linked to the availability of air. Unlike food or water, which we consume periodically, breathing air is a constant, second-by-second necessity. This fundamental dependence highlights the critical importance of understanding the Earth's atmosphere in detail.

The atmosphere is the gaseous envelope surrounding the Earth. It's a dynamic **mixture of different gases**, along with water vapour and dust particles. This atmospheric layer is held to the Earth by gravity and is an integral part of the planet's mass. Approximately 99% of the atmosphere's total mass is concentrated within the lowest 32 kilometers from the surface. Air itself is typically invisible and odorless; its presence becomes noticeable only when it is in motion as wind.

The atmosphere is vital for life, containing essential gases such as **oxygen** (required by animals and humans for respiration) and **carbon dioxide** (needed by plants for photosynthesis). Furthermore, layers like the ozone layer play a crucial protective role. Without the ozone layer, harmful ultraviolet (UV) radiation from the sun would reach the surface in much higher intensities, posing a severe threat to life.

Composition Of The Atmosphere

The atmosphere is composed primarily of **gases, water vapour, and dust particles**. The proportions of these components vary, particularly with altitude.

Gases

The major gases in the dry atmosphere are **Nitrogen ($N_2$)** and **Oxygen ($O_2$)**. Other gases like Argon, Carbon Dioxide, Neon, Helium, Krypton, Xenon, Hydrogen, Methane, Nitrous Oxide, and Ozone are present in smaller or trace amounts.

The concentration of some gases changes significantly at higher altitudes. For instance, oxygen becomes negligible above approximately 120 km. Carbon dioxide and water vapour are mainly confined to the lower atmosphere, up to about 90 km.

While Nitrogen (approx. 78%) and Oxygen (approx. 21%) make up the bulk of the atmosphere and their proportions are relatively constant in the lower layers, some trace gases are meteorologically very important:

• **Carbon Dioxide ($CO_2$):** Although a small percentage (around 0.03-0.04%), $CO_2$ is crucial because it behaves differently towards incoming and outgoing radiation. It is relatively **transparent to incoming solar radiation** from the sun, allowing it to pass through and heat the Earth's surface. However, it is largely **opaque to outgoing terrestrial radiation** (heat radiated by the Earth). $CO_2$ absorbs some of this outgoing heat and re-emits it back towards the surface, contributing significantly to the **greenhouse effect**. This natural effect helps warm the planet and makes it habitable. In recent decades, the volume of $CO_2$ in the atmosphere has been increasing, primarily due to human activities like burning fossil fuels (coal, oil, natural gas). This increase is linked to a rise in global air temperatures.
• **Ozone ($O_3$):** This gas is found mainly in the layer between 10 km and 50 km above the Earth's surface (the stratosphere). The ozone layer acts as a vital natural shield, absorbing most of the Sun's harmful **ultraviolet (UV) radiation**. This absorption prevents the majority of damaging UV rays from reaching the Earth's surface, protecting living organisms from severe health impacts.

Water Vapour

**Water vapour ($H_2O$)** is a highly variable component of the atmosphere, meaning its concentration differs greatly depending on location, altitude, and time. Its amount decreases significantly with increasing height.

In warm, humid regions near the equator, water vapour can constitute up to 4% of the air volume. In contrast, in dry desert or cold polar regions, it may be less than 1%. The concentration of water vapour also generally decreases from the equator towards the poles.

Like carbon dioxide, water vapour is a greenhouse gas. It absorbs both incoming solar radiation and, more significantly, outgoing terrestrial radiation. This absorption helps regulate Earth's temperature, acting somewhat like a blanket to prevent extreme cooling at night or excessive heating during the day. Water vapour also plays a critical role in atmospheric processes related to weather, influencing the stability and instability of air masses, which drives phenomena like cloud formation and precipitation.

Dust Particles

The atmosphere contains numerous **solid particles**, suspended in the air, which originate from various sources. These include natural sources like **sea salts** (from ocean spray), fine **soil particles**, volcanic **ash**, **pollen** (from plants), and fragments of disintegrated **meteors**. Human activities also contribute particles like **smoke** and **soot**.

Dust particles are typically more concentrated in the lower layers of the atmosphere. However, strong upward air currents (convection) can transport them to higher altitudes. Subtropical and temperate regions, especially those with dry winds, often have higher concentrations of dust particles compared to the cleaner air found in equatorial or polar areas.

These dust and salt particles have a crucial meteorological role: they often act as **hygroscopic nuclei**. This means that water vapour in the air readily condenses around these particles, providing surfaces for the formation of tiny water droplets or ice crystals, which are the building blocks of **clouds**.

Structure Of The Atmosphere

The atmosphere is not uniform but is structured into distinct layers based on differences in **density** and **temperature** characteristics with increasing altitude. Density is highest closest to the Earth's surface because the weight of the overlying air compresses the layers below. Density decreases rapidly as altitude increases.

Scientists have divided the atmosphere into five main layers based on how temperature changes with height. Starting from the Earth's surface and moving upwards, these layers are: **Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere**.

Troposphere

This is the **lowermost layer** and is the part of the atmosphere we live in and interact with most directly. Its average height is about 13 km, but this varies significantly with latitude and season. It is thinner at the poles (around 8 km) and thickest at the equator (about 18 km). The greater thickness at the equator is due to strong upward convection currents driven by intense solar heating, which transport heat and air to higher levels.

The troposphere contains almost all of the atmosphere's **water vapour and dust particles**. Significantly, **all weather phenomena** (clouds, rain, storms, winds, etc.) occur within this layer. Temperature in the troposphere generally **decreases with increasing height** at an average rate of about 1°C for every 165 meters. This temperature decrease is known as the environmental lapse rate. The troposphere is the most crucial atmospheric layer for all forms of biological activity, as it contains the air, water, and weather systems necessary for life.

The boundary zone between the troposphere and the layer above (the stratosphere) is called the **Tropopause**. At the tropopause, the temperature stops decreasing with height and remains relatively constant. Temperatures at the tropopause are extremely cold, averaging around -80°C over the equator and -45°C over the poles.

Stratosphere

Located directly above the tropopause, the **Stratosphere** extends upwards to a height of approximately 50 km. A distinguishing feature of the stratosphere is the presence of the **Ozone Layer**. This layer is vital because it absorbs harmful ultraviolet (UV) radiation from the sun. This absorption warms the stratosphere, causing temperature to generally **increase with height** in this layer, contrasting with the troposphere below. The ozone layer effectively shields life on Earth from the damaging effects of intense UV radiation.

The boundary at the top of the stratosphere, where temperature begins to decrease again, is called the **Stratopause**, located at about 50 km altitude.

Mesosphere

Above the stratosphere lies the **Mesosphere**, extending from the stratopause (50 km) up to about 80 km altitude. In this layer, temperature once again **decreases with increasing height**, reaching the coldest temperatures in the atmosphere (as low as -100°C) at its upper boundary.

The top boundary of the mesosphere, at around 80 km, is called the **Mesopause**. This marks the level of minimum temperature in the vertical profile of the atmosphere. Meteors entering the Earth's atmosphere typically burn up in the mesosphere, creating visible streaks of light.

Thermosphere

Above the mesopause and extending upwards is the **Thermosphere**. This layer is located approximately between 80 km and 400 km altitude, although its upper boundary is less precisely defined. In the thermosphere, temperature begins to **increase rapidly with height** due to the absorption of high-energy solar radiation (like X-rays and extreme UV) by the sparse atmospheric gases. Although the temperature is very high, the particles are so spread out that it wouldn't feel hot to us. The lower part of the thermosphere, roughly between 80 km and 400 km, contains a region called the **Ionosphere**.

The **Ionosphere** is characterized by the presence of electrically charged particles (ions and free electrons), formed when solar radiation knocks electrons off atoms. This ionized layer is important for radio communication, as it reflects radio waves transmitted from the Earth back down to the surface, enabling long-distance radio communication.

Exosphere

The **Exosphere** is the **uppermost layer** of the atmosphere, situated above the thermosphere. It is the thinnest and most rarefied layer, where atmospheric gases gradually thin out and merge into outer space. Very little is known about the exact conditions or composition of the exosphere. The particles here are extremely spread out, and many atoms and molecules escape into space. There is no clear boundary between the exosphere and the vacuum of space.

While all layers of the atmosphere influence the Earth system, geographers often focus particularly on the **troposphere** and **stratosphere** as these layers are most directly relevant to human activities, weather, climate, and the biosphere.

Elements Of Weather And Climate

Within the atmosphere, particularly the troposphere, several key properties are constantly changing and influencing conditions on Earth. These are known as the elements of weather and climate.

The main elements are:

• Temperature: The degree of hotness or coldness of the air.
• Pressure: The weight of the column of air above a given point.
• Winds: The horizontal movement of air, driven by pressure differences.
• Humidity: The amount of water vapour in the air.
• Clouds: Visible masses of condensed water vapour or ice crystals.
• Precipitation: Water falling from the atmosphere to the surface in various forms (rain, snow, hail, etc.).

These elements are interconnected and their dynamics determine the weather at any given moment and the climate patterns over longer periods and larger areas. A detailed discussion of these elements and their influence is typically covered in subsequent chapters.

Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In About 150 Words

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Project Work

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