Question

(a) How are the boundaries between the regions of the atmosphere determined? (b) Explain why the stratosphere, which is about 35 \(\mathrm{km}\) thick, has a smaller total mass than the troposphere, which is about 12 \(\mathrm{km}\) thick.

Short answer

The boundaries between regions of the atmosphere are determined based on differences in temperature, pressure, and atmospheric composition. The tropopause forms the boundary between the troposphere and the stratosphere. Despite being thicker than the troposphere, the stratosphere has a smaller total mass due to its lower density and pressure. The troposphere contains approximately 80% of Earth's total atmospheric mass, as it is located closer to the Earth's surface and its air is more compressed by gravity.

Step-by-step solution

Determining Boundaries in the Atmosphere

The boundaries between the regions of the atmosphere are determined based on differences in temperature, pressure, and atmospheric composition. These factors, combined with height above the Earth's surface, create distinct regions within the Earth's atmosphere. The specific regions in question, the stratosphere and troposphere, have some key differences. The lower region, the troposphere, is where weather occurs, and temperatures generally decrease with altitude. The stratosphere, located above the troposphere, has a more stable temperature distribution, where temperature increases with altitude due to the absorption of solar energy by the ozone layer. The boundary between these two layers is called the tropopause, and its altitude varies depending on latitude and season.

Properties of Stratosphere and Troposphere

The stratosphere is located above the troposphere at a height of about 10 to 50 km, with the thickness of around 35 km. It contains the ozone layer, which protects the Earth from harmful ultraviolet radiation. The ozone layer causes the temperature to increase with altitude in the stratosphere, creating a more stable and less turbulent environment than in the troposphere. The troposphere is the lowest layer of the Earth's atmosphere, extending from the surface to about 8 to 15 km in altitude, with an average thickness of around 12 km. Most weather phenomena, such as clouds and storms, occur in this layer. The troposphere has a higher pressure and greater temperature variation than the stratosphere, with temperatures generally decreasing with altitude.

Comparing the Mass of Stratosphere and Troposphere

The total mass of the atmosphere is not equally distributed throughout its layers. In fact, the troposphere, despite being thinner than the stratosphere, has a greater mass. This difference in mass can be attributed to the density and pressure differences between the two layers. In the troposphere, the air pressure and density are higher than in the stratosphere. Approximately 80% of the Earth's total atmospheric mass is located in the troposphere, which is compressed closer to the Earth's surface due to gravity. Although the stratosphere is thicker, its density is much lower than the troposphere. The atmospheric density decreases with altitude, and thus, the mass of the stratosphere is less than that of the troposphere. In conclusion, the key factors that determine the boundaries between atmospheric layers are temperature, pressure, and atmospheric composition, while the difference in mass between the stratosphere and troposphere is mainly due to the differences in density and pressure between these two regions. Even though the stratosphere is thicker than the troposphere, its lower density results in a smaller total mass.

Key concepts

Troposphere properties

Starting from the ground and stretching upward to about 8 to 15 kilometers, the troposphere is the layer of Earth's atmosphere closest to the surface. It's a dynamic and bustling layer where the majority of our weather patterns form. Within the troposphere, the temperature generally decreases with altitude at an average lapse rate of 6.5°C per kilometer.

The troposphere is home to various meteorological phenomena such as rain, snow, storms, and winds. This layer also holds about 80% of the Earth’s atmospheric mass, due to the higher air pressure and density near the surface. The dense composition of gases like nitrogen, oxygen, and carbon dioxide, combined with water vapor, makes it the perfect environment for life as we know it.

Air Pressure and Weather

Air pressure in the troposphere is highest at sea level and decreases with altitude. This gradient is what drives wind and helps form weather systems due to the movement of air masses. Temperature changes also influence weather; warmer air expands and rises, while cooler air contracts and sinks. This constant movement of air translates into the weather patterns we experience daily.

Stratosphere properties

Above the troposphere lies the stratosphere, which spans from approximately 10 to 50 kilometers above Earth's surface. A defining feature of the stratosphere is the presence of the ozone layer at 15 to 35 kilometers high. This layer absorbs and scatters the solar ultraviolet radiation, warming the air and giving the stratosphere a unique temperature profile where it increases with altitude — a factor that contributes to its relative stability compared to the layer below.

Ozone Layer and Temperature Inversion

The ozone layer is crucial not only for temperature inversion but also for protecting life on Earth from harmful UV rays. Because of this protective layer, the stratosphere lacks the weather activity found in the troposphere. Air movement is horizontal, leading to the formation of jet streams that significantly impact aviation and global weather patterns.

While this layer is broader in terms of kilometers, it’s much less dense and has a lower pressure than the troposphere. That's why, despite being thicker, the stratosphere contains far less mass, resulting in fewer air molecules and a much lower overall density.

Atmospheric composition

The atmosphere's composition varies slightly between its layers but is primarily made up of nitrogen (about 78%) and oxygen (about 21%). The remaining 1% consists of argon, carbon dioxide, neon, helium, methane, krypton, hydrogen, xenon, and ozone, as well as water vapor and other gases in trace amounts.

In the troposphere, the composition is more influenced by weather and life; water vapor content can widely vary and is higher here due to the presence of evaporation and precipitation. Carbon dioxide levels also fluctuate in this layer due to plant respiration and human activities. Conversely, the stratosphere contains less water vapor and ozone becomes a more dominant constituent, particularly where the ozone layer is located.

Trace Gases and Human Impact

While nitrogen and oxygen dominate, trace gases like carbon dioxide and methane are potent greenhouse gases that trap heat in the atmosphere. Human activities have significantly increased their concentrations, enhancing the natural greenhouse effect, and contributing to climate change. Monitoring atmospheric composition remains essential for understanding and predicting climate phenomena.