Composition and Structure of the Atmosphere: Exhaustive Study Guide

Global Significance of the Atmosphere

• Air is a fundamental requirement for the survival of all living organisms. While humans can survive for a limited time without food or water, survival is impossible for more than a few minutes in the absence of air.

• The atmosphere is defined as a mixture of various gases that envelopes the earth from all sides.

• It provides essential life-giving gases, such as oxygen for humans and animals, and carbon dioxide for plants.

• The air is considered an integral part of the earth’s total mass. Approximately $99\%$ of the total mass of the atmosphere is concentrated within a height of $32\,km$ from the earth’s surface.

• Air is characterized as being both colorless and odorless; its presence is primarily perceived when it blows in the form of wind.

Composition of the Atmosphere

The atmosphere is composed of three primary categories of substances: gases, water vapour, and dust particles.

Atmospheric Gases

• The proportion of various gases changes significantly in the higher layers of the atmosphere. Oxygen becomes almost negligible in quantity at a height of approximately $120\,km$.

• Carbon dioxide and water vapour are found only up to an altitude of $90\,km$ from the surface of the earth.

Carbon Dioxide ($CO_2$)

• Carbon dioxide is meteorologically significant because it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.

• It absorbs a portion of the terrestrial radiation and reflects some of it back toward the surface of the earth, making it largely responsible for the greenhouse effect.

• While the volume of other gases remains constant, the volume of carbon dioxide has been increasing over the past few decades, primarily due to the burning of fossil fuels, which has led to a rise in global air temperatures.

Ozone ($O_3$)

• Ozone is located between $10\,km$ and $50\,km$ above the earth’s surface.

• It serves as a crucial filter by absorbing ultraviolet (UV) rays radiating from the sun, preventing them from reaching the surface and protecting life forms from harmful energy.

Water Vapour

• Water vapour is a variable gas whose concentration decreases with increasing altitude.

• In warm and wet tropical regions, water vapour may account for $4\%$ of the air by volume.

• In dry and cold regions, such as deserts and polar areas, it may account for less than $1\%$ of the air by volume.

• The concentration of water vapour generally decreases from the equator toward the poles.

• It functions like a blanket by absorbing parts of the insolation from the sun and preserving the earth’s radiated heat, ensuring the planet becomes neither too cold nor too hot.

• It is a primary contributor to the stability and instability of the air.

Dust Particles

• The atmosphere contains small solid particles originating from diverse sources: sea salts, fine soil, smoke-soot, ash, pollen, dust, and disintegrated particles from meteors.

• These particles are generally concentrated in the lower layers of the atmosphere, though convectional air currents can transport them to much higher altitudes.

• Subtropical and temperate regions exhibit higher concentrations of dust particles compared to equatorial and polar regions due to the presence of dry winds.

• Dust and salt particles function as hygroscopic nuclei, around which water vapour condenses to form clouds.

Structure of the Atmosphere

The atmosphere is organized into five distinct layers characterized by varying density and temperature conditions. Density is at its peak near the surface and decreases as altitude increases.

The Troposphere

• The troposphere is the lowermost layer and the most important for biological activity.

• Its average height is $13\,km$, extending to about $8\,km$ near the poles and approximately $18\,km$ at the equator.

• The thickness is greatest at the equator because heat is transported to significant heights by strong convectional currents.

• This layer contains water vapour and dust particles. All changes in weather and climate occur within the troposphere.

• The temperature in this layer decreases at a rate of $1^{\circ}C$ for every $165\,m$ of height.

The Tropopause

• The tropopause is the zone that separates the troposphere from the stratosphere.

• The air temperature at the tropopause is approximately $-80^{\circ}C$ over the equator and roughly $-45^{\circ}C$ over the poles.

• The temperature in this zone remains nearly constant.

The Stratosphere

• The stratosphere is located above the tropopause and extends up to a height of $50\,km$.

• Its defining feature is the presence of the ozone layer, which shields life on earth by absorbing intense and harmful ultraviolet radiation.

The Mesosphere

• The mesosphere lies above the stratosphere and extends to a height of $80\,km$.

• In this layer, temperature begins to decrease again with altitude, reaching as low as $-100^{\circ}C$ at the $80\,km$ mark.

• The upper boundary of this layer is called the mesopause.

The Ionosphere

• Located between $80\,km$ and $400\,km$ above the mesopause.

• It contains electrically charged particles known as ions.

• This layer reflects radio waves transmitted from the earth back to the surface.

• Temperature in the ionosphere begins to increase with height.

The Exosphere

• This is the uppermost layer of the atmosphere, situated above the thermosphere.

• It is the highest layer, and information regarding it is limited.

• The contents of this layer are extremely rarefied, and it gradually merges into outer space.

Elements of Weather and Climate

• The primary elements of the atmosphere that are subject to constant change and influence human life are:

  • Temperature

  • Pressure

  • Winds

  • Humidity

  • Clouds

  • Precipitation

Questions & Discussion

Multiple Choice Questions

• Which one of the following gases constitutes the major portion of the atmosphere?

  • (a) Oxygen (b) Nitrogen (c) Argon (d) Carbon dioxide

• Atmospheric layer important for human beings is:

  • (a) Stratosphere (b) Mesosphere (c) Troposphere (d) Ionosphere

• Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:

  • (a) Gases (b) Dust particles (c) Water vapour (d) Meteors

• Oxygen gas is in negligible quantity at the height of atmosphere:

  • (a) $90\,km$ (b) $120\,km$ (c) $100\,km$ (d) $150\,km$

• Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?

  • (a) Oxygen (b) Nitrogen (c) Helium (d) Carbon dioxide

Short Answer Discussion Prompts

• What is the definition of the atmosphere?

• What are the specific elements that constitute weather and climate?

• Describe the overall composition of the atmosphere.

• Why is the troposphere considered the most important of all atmospheric layers?