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 atmospheric regions are determined by examining changes in temperature with respect to altitude, as different layers exhibit distinct vertical temperature profiles. The stratosphere has a smaller total mass than the troposphere, even though it is thicker, because the air density and pressure are lower in the stratosphere. The troposphere contains a significant proportion of the total atmospheric mass due to its higher air pressure and density at the Earth's surface. In contrast, the lower air density and pressure in the stratosphere result in air molecules being more scattered, leading to a smaller total mass.

Step-by-step solution

(a) Determining Boundaries Between Atmospheric Regions

Determining the boundaries between the regions of the atmosphere involves examining the changes in temperature with respect to altitude. Different atmospheric layers exhibit distinct vertical temperature profiles. The boundaries between these layers are marked by regions where the temperature gradient either changes significantly or remains constant.

(b) Understanding Atmospheric Pressure

Before we answer why the stratosphere has a smaller mass than the troposphere, even though it is thicker, we need to understand how atmospheric pressure works. Atmospheric pressure is the force exerted on a surface due to the weight of the air above it. As altitude increases, atmospheric pressure decreases because there is less air mass above that point.

(b) Density and Pressure Variation in the Atmosphere

Atmospheric density and pressure both decrease exponentially with increasing altitude. This is because, as we go higher up in the atmosphere, the gravitational force acting on the air molecules decreases, and they become more widely spaced apart. This decrease in density causes a corresponding decrease in atmospheric pressure.

(b) Comparing Mass of Troposphere and Stratosphere

Now, we will compare the masses of the troposphere and stratosphere. As mentioned earlier, atmospheric pressure decreases with increasing altitude. Since the base of the troposphere is at the Earth's surface, where the atmospheric pressure is at its highest, the troposphere contains a significant proportion of the total atmospheric mass. The troposphere is also more densely packed with air molecules compared to the stratosphere. The stratosphere, on the other hand, starts at a higher altitude where the atmospheric pressure and density are both lower than in the troposphere. Even though it extends up to 35 km, the total mass of the stratosphere is less than that of the troposphere because the air density and pressure are lower in this layer, making the air molecules more scattered. In conclusion, even though the stratosphere is physically thicker than the troposphere, its total mass is smaller due to the lower air density and pressure in this atmospheric layer.

Key concepts

Troposphere

The troposphere is the lowest layer of Earth's atmosphere and is where we live. It extends from the Earth's surface up to about 12 km (7.5 miles) above sea level. One key characteristic of the troposphere is that the temperature decreases with altitude.
This is because the troposphere is heated mainly from the ground up, as the Earth's surface absorbs sunlight and re-emits it as heat.

• The troposphere is where most of our weather occurs, including clouds, rain, and storms.
• It contains about 75% of the atmosphere's total mass and 99% of its water vapor.

Because of its proximity to the Earth's surface, the troposphere has the highest air density and pressure.

Stratosphere

Above the troposphere lies the stratosphere, which extends from about 12 km to roughly 50 km (31 miles) above the Earth's surface. This layer is quite different from the troposphere. In the stratosphere, the temperature actually increases with altitude, a phenomenon not commonly seen in other layers.
This temperature change is due to the presence of the ozone layer. The ozone molecules absorb and scatter the sun's ultraviolet radiation, which warms this part of the atmosphere.

• The stratosphere contains most of the ozone layer, which protects life on Earth by blocking harmful UV radiation.
• Commercial airplanes often fly in the lower stratosphere to escape the turbulence and weather patterns present in the troposphere.

Though physically thicker than the troposphere, the stratosphere has a smaller overall mass due to lower air density and pressure.

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of air above a surface. It is greatest at sea level because that's where the weight of the air column is the highest. As you move higher up in the atmosphere, the atmospheric pressure decreases. This occurs because there are fewer air molecules above you exerting weight.

• Pressure is typically measured in units like millibars or inches of mercury.
• At higher altitudes, both air pressure and density are lower, affecting breathing and weather dynamics.

Understanding atmospheric pressure is essential to explain why the lower layers of the atmosphere, such as the troposphere, have more mass compared to the higher layers like the stratosphere.

Temperature Gradient

A temperature gradient refers to how temperature changes with altitude in the different layers of the atmosphere. This gradient plays a key role in defining the boundaries of atmospheric layers. In the troposphere, the temperature generally decreases as altitude increases. However, in the stratosphere, the temperature rises with altitude due to the absorption of solar radiation by the ozone layer.

• The temperature gradient is crucial for weather patterns and wind formation in the troposphere.
• Inversion layers, where temperature increases with altitude, can trap pollutants and impact air quality.

Comparing temperature gradients across atmospheric layers helps us understand phenomena like jet streams and the formation of weather patterns.