Chapter 8 Composition And Structure Of Atmosphere

The air we breathe is fundamental to life on Earth. While we can survive without food and water for some time, access to air is constantly required. This highlights the critical importance of understanding the Earth's atmosphere, the gaseous layer that envelops our planet.

The atmosphere is a dynamic mixture primarily composed of various gases, along with varying amounts of water vapour and solid particles. It is an integral part of the Earth's total mass. A significant portion of the atmospheric mass, about 99 percent, is concentrated within the first 32 kilometers ($32 \, km$) above the Earth's surface. Air itself is invisible, lacking colour and odour; its presence is only directly perceived when it moves as wind.

The atmosphere plays a vital protective role. For example, the presence of the ozone layer shields life from harmful ultraviolet radiation from the sun.

Composition Of The Atmosphere

The atmosphere is composed of gases, water vapour, and dust particles. The relative proportions of these components change significantly with altitude.

Gases

The atmosphere is a mixture of numerous gases. The majority of the atmosphere is composed of two gases:

• Nitrogen ($N_2$): Approximately 78% by volume. Nitrogen is relatively inert but essential for life through the nitrogen cycle.
• Oxygen ($O_2$): Approximately 21% by volume. Oxygen is vital for respiration by most living organisms. Its concentration decreases rapidly with height, becoming negligible above 120 kilometers ($120 \, km$).

Other gases are present in much smaller quantities (trace gases). Some of these, despite their low concentration, are meteorologically very important:

• Argon ($Ar$): Approximately 0.93% by volume. Inert gas.
• Carbon Dioxide ($CO_2$): A small but crucial component, currently around 0.04% (400 ppm) of the atmosphere by volume. It is transparent to incoming shortwave solar radiation but absorbs and re-emits outgoing longwave terrestrial radiation. This property makes it a primary greenhouse gas, significantly contributing to the Earth's greenhouse effect and influencing global temperatures. While historically a minor component, its volume has been increasing substantially over recent decades due to human activities, particularly the burning of fossil fuels, leading to concerns about rising air temperatures and climate change. Carbon dioxide is mainly concentrated in the lower atmosphere, found only up to about 90 kilometers ($90 \, km$) from the surface.
• Ozone ($O_3$): A gas concentrated primarily in the layer between 10 and 50 kilometers ($10 - 50 \, km$) above the surface, forming the ozone layer. Ozone is critical because it absorbs harmful ultraviolet (UV) radiation from the sun, preventing most of it from reaching the Earth's surface. Without this protection, UV radiation would be extremely damaging to life.

Other trace gases include Neon ($Ne$), Helium ($He$), Krypton ($Kr$), Xenon ($Xe$), Methane ($CH_4$), and Hydrogen ($H_2$), present in even smaller amounts.

Water Vapour

Water vapour ($H_2O$) is a highly variable component of the atmosphere, meaning its concentration varies significantly depending on location and altitude. It is most abundant in warm, wet tropical regions, where it can constitute up to 4 percent of the air volume. In contrast, in dry deserts or cold polar areas, its concentration is less than 1 percent. Generally, water vapour decreases with increasing altitude and decreases from the equator towards the poles.

Like carbon dioxide, water vapour is a powerful greenhouse gas. It absorbs some incoming solar radiation and, more importantly, absorbs a significant portion of the Earth's outgoing radiated heat. This absorption helps to regulate the Earth's temperature, preventing it from becoming excessively cold at night or too hot during the day, acting like a thermal blanket.

Water vapour is also fundamental to atmospheric processes that lead to clouds and precipitation. Its presence also influences the stability and instability of the air, affecting weather patterns.

Water vapour is also mainly concentrated in the lower atmosphere, extending up to about 90 kilometers ($90 \, km$) from the surface.

Dust Particles

The atmosphere contains a multitude of tiny solid particles originating from various natural and human-made sources. These include sea salt particles lifted by wind from ocean spray, fine soil particles, smoke and soot from fires, volcanic ash, pollen grains, and fragments from disintegrated meteors.

Dust particles are typically more concentrated in the lower layers of the atmosphere, especially near their source regions. However, strong upward movements of air (convection currents) can lift them to higher altitudes. Subtropical and temperate regions, often characterized by dry conditions and winds, tend to have higher concentrations of dust compared to the cleaner air found near the equator or the poles.

Many of these dust and salt particles play a crucial role in cloud formation. They act as hygroscopic nuclei, providing surfaces upon which water vapour can condense to form tiny water droplets or ice crystals, which are the building blocks of clouds.

Structure Of The Atmosphere

The atmosphere is not uniform from bottom to top but is structured into distinct layers based on significant changes in temperature and density with altitude. Density is highest at sea level due to the weight of the overlying air column and decreases rapidly as altitude increases.

Based on temperature variations, the atmosphere is divided into five principal layers (from bottom to top):

1. Troposphere: The lowest layer, extending from the Earth's surface up to an average height of about 13 kilometers ($13 \, km$). Its thickness varies, being thinner at the poles (about 8 km) and thicker at the equator (about 18 km). The greater thickness at the equator is due to stronger convection currents that transport heat to higher altitudes. This layer contains almost all the atmospheric water vapour and dust particles. All weather phenomena and climate changes occur within the troposphere. Temperature generally decreases with increasing altitude in the troposphere at an average rate of approximately $1^\circ C$ for every 165 meters ($165 \, m$) of ascent (this is called the standard lapse rate). The troposphere is the layer most crucial for supporting biological activity on Earth's surface.
2. Stratosphere: Located above the troposphere, extending up to an altitude of about 50 kilometers ($50 \, km$). A transition zone called the tropopause separates the troposphere and stratosphere, where the temperature inversion begins. Temperature remains nearly constant for a short height at the base of the stratosphere (the tropopause level, around $-80^\circ C$ over the equator and $-45^\circ C$ over the poles) and then gradually increases with height throughout the stratosphere. The key feature of the stratosphere is the presence of the ozone layer, which absorbs most of the sun's harmful ultraviolet radiation, causing the temperature increase in this layer.
3. Mesosphere: Situated above the stratosphere, extending up to a height of about 80 kilometers ($80 \, km$). In the mesosphere, temperature again decreases with increasing altitude, reaching its coldest point in the atmosphere (down to about $-100^\circ C$) at its upper boundary. This upper boundary is called the mesopause. Meteors typically burn up in the mesosphere upon entering the atmosphere.
4. Thermosphere: Located above the mesosphere, extending from about 80 kilometers ($80 \, km$) up to around 400 kilometers ($400 \, km$) or more. This layer contains electrically charged particles called ions, leading it to be sometimes referred to as the ionosphere. The ionosphere is important because it reflects radio waves, enabling long-distance radio communication. Temperature increases sharply with height in the thermosphere due to the absorption of high-energy solar radiation by the sparse gases. Although temperatures are technically very high, the air is so thin that there is little heat energy.
5. Exosphere: The outermost layer of the atmosphere, located above the thermosphere, gradually transitioning into outer space. It extends from around 400 kilometers ($400 \, km$) upwards. The air is extremely thin or rarefied in this layer, consisting mostly of widely dispersed hydrogen and helium atoms. Very little is known about the exosphere, and it represents the boundary where the Earth's atmosphere merges with the vacuum of space.

While all atmospheric layers interact and influence the Earth system, geographers often focus particularly on the lower two layers, the troposphere and stratosphere, as they are most directly related to weather, climate, and surface processes.

Diagram illustrating the Earth's atmosphere divided into layers (Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere) with their approximate heights and temperature changes with altitude.

Elements Of Weather And Climate

Weather refers to the state of the atmosphere at a particular place and time, while climate describes the average weather conditions over a longer period in a region. The key elements that define both weather and climate, and which are constantly changing in the atmosphere, are:

• Temperature
• Atmospheric Pressure
• Winds
• Humidity
• Clouds
• Precipitation

These elements are measured and studied to understand the atmospheric conditions and their influence on human life and the Earth's surface.

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