Atmospheric Stratification and Circulation Cells
On a clear day in Denver, a commercial jet cruises at about 11 km, where the outside air is near $-56\,^\circ\text{C}$. Down on the ground it is warm. Why does the air get colder as you climb, and why does it later start warming again? And why do storms in the United States nearly always arrive from the west? In this page you will answer both questions.
Thermal layers of the atmosphere
Scientists divide the atmosphere into layers based on how temperature changes with altitude. The temperature trend reverses at the boundaries between layers.
| Layer | Altitude | Temperature trend | Why |
|---|---|---|---|
| Troposphere | 0 to about 12 km | Decreases with height | Warmed from below by the ground |
| Stratosphere | about 12 to 50 km | Increases with height | Ozone absorbs UV |
| Mesosphere | about 50 to 85 km | Decreases with height | Little ozone to absorb energy |
| Thermosphere | above about 85 km | Increases with height | Thin gas absorbs high-energy solar radiation |
The troposphere holds about 75% of the atmosphere's mass and nearly all of its water vapor, so almost all weather happens here. Temperature falls at an average lapse rate of about $6.5\,^\circ\text{C}$ per km.
$$\text{lapse rate} = \frac{\text{temperature drop}}{\text{altitude gain}}$$The troposphere is warm at the bottom because sunlight mostly passes through air and heats the ground, and the ground then warms the air above it.
List the four main thermal layers of the atmosphere in order from Earth's surface upward. Then name the layer in which most weather occurs.
List the four main thermal layers of the atmosphere in order from Earth's surface upward. Then name the layer in which most weather occurs.
A weather balloon records these temperatures: | Altitude (km) | 0 | 12 | 20 | 50 | 85 | |---|---|---|---|---|---| | Temperature ($^\circ$C) | 15 | -56 | -56 | -2 | -90 |…
A weather balloon records these temperatures:
| Altitude (km) | 0 | 12 | 20 | 50 | 85 |
|---|---|---|---|---|---|
| Temperature ($^\circ$C) | 15 | -56 | -56 | -2 | -90 |
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Identify the layer between each pair of consecutive altitudes.
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Calculate the average lapse rate in the troposphere in $^\circ\text{C}$ per km.
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Explain what the temperature trend between 20 and 50 km tells you about the heat source for that layer.
The ozone layer and chemical stratification
Why does the stratosphere warm with height? Because it contains the ozone layer. Ultraviolet (UV) light from the Sun splits oxygen molecules, and the freed atoms combine with other oxygen molecules:
$$\text{O}_2 + \text{UV} \rightarrow 2\,\text{O}, \qquad \text{O} + \text{O}_2 \rightarrow \text{O}_3$$Ozone ($\text{O}_3$) then absorbs more UV and turns that energy into heat. Most UV is absorbed near the top of the stratosphere, so that region is warmest. This absorption protects living things: UV damages DNA and causes skin cancer and cataracts.
Thermal stratification is based on temperature. Chemical stratification is based on composition. Up to about 100 km, turbulence keeps the gases well mixed: about 78% nitrogen, 21% oxygen, and 0.9% argon. Above that, gases separate by molecular mass, with lighter gases on top.
Ozone is a trace gas, yet it matters hugely, depending on where it is:
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Stratospheric ozone is "good": it shields the surface from UV.
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Ground-level ozone is a pollutant in smog that irritates lungs and damages crops.
Chlorofluorocarbons (CFCs), once used in refrigerators and spray cans, drifted into the stratosphere. There UV broke them apart and released chlorine atoms, which destroy ozone molecules in a repeating chain reaction, so one chlorine atom can destroy thousands of ozone molecules. This created the Antarctic "ozone hole." The 1987 Montreal Protocol phased out CFCs, and the ozone layer is slowly recovering.
Name the atmospheric layer that contains the ozone layer. State what kind of radiation ozone absorbs and why this is important for living things.
Name the atmospheric layer that contains the ozone layer. State what kind of radiation ozone absorbs and why this is important for living things.
In the stratosphere, oxygen and ozone interact with ultraviolet (UV) light: $$\text{O}2 + \text{UV} \rightarrow 2\,\text{O}, \qquad \text{O} + \text{O}2 \rightarrow…
In the stratosphere, oxygen and ozone interact with ultraviolet (UV) light:
$$\text{O}_2 + \text{UV} \rightarrow 2\,\text{O}, \qquad \text{O} + \text{O}_2 \rightarrow \text{O}_3.$$-
Explain how these reactions form the ozone layer.
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Explain why the temperature in the stratosphere increases with altitude, unlike in the troposphere.
The atmosphere is made of about 78% nitrogen, 21% oxygen, and 0.9% argon by volume. These proportions stay nearly constant up to about 100 km, where the mixing gives way…
The atmosphere is made of about 78% nitrogen, 21% oxygen, and 0.9% argon by volume. These proportions stay nearly constant up to about 100 km, where the mixing gives way to separation by molecular mass. Meanwhile, the layers defined by temperature (troposphere, stratosphere, mesosphere, thermosphere) change at much lower altitudes.
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Compare chemical stratification with thermal stratification of the atmosphere.
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Explain why ozone, a trace gas, is essential in the stratosphere but a pollutant in the troposphere.
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Explain how chlorofluorocarbons (CFCs) damaged the ozone layer and why the international agreement to phase them out is an example of effective action.
Circulation cells
The Sun heats the equator more than the poles. Warm air rises at the equator, moves toward the poles, cools, and sinks. Earth's rotation bends this flow (the Coriolis effect), so instead of one big loop, each hemisphere has three cells.
| Cell | Latitudes | Air motion |
|---|---|---|
| Hadley | 0° to 30° | Air rises at the equator, sinks near 30° |
| Ferrel | 30° to 60° | Air sinks near 30°, rises near 60° |
| Polar | 60° to 90° | Air rises near 60°, sinks at the pole |
Surface winds are named for the direction they blow from: the trade winds (northeasterly, 0° to 30°), the westerlies (southwesterly, 30° to 60°), and the polar easterlies (60° to 90°). So at $15^\circ\text{N}$ surface winds blow from the northeast, and at $45^\circ\text{N}$ they blow from the southwest.
Rising air cools, and its water vapor condenses into clouds and rain, so the equator and $60^\circ$ are wet. Sinking air warms and dries, which is why many deserts sit near $30^\circ$: the Sahara and the Sonoran Desert. Storms across the United States (about $30^\circ$ to $50^\circ\text{N}$) are steered west to east by the westerlies. Rising air leaves lower pressure at the surface, so it forms low-pressure systems (cloudy, stormy weather). Sinking air forms high-pressure systems (clear, dry weather). Pressure is reported in hPa (millibars) or inches of mercury. A front is the boundary where two air masses meet: a cold front pushes cold air under warm air, and a warm front slides warm air over cold air. Storms form along fronts.
Describe the Hadley cell. State where air rises, where it sinks, and the approximate latitudes involved in each.
Describe the Hadley cell. State where air rises, where it sinks, and the approximate latitudes involved in each.
The three-cell model of global circulation has Hadley, Ferrel, and polar cells in each hemisphere. 1. Describe the direction of the surface winds at $15^\circ\text{N}$…
The three-cell model of global circulation has Hadley, Ferrel, and polar cells in each hemisphere.
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Describe the direction of the surface winds at $15^\circ\text{N}$ and at $45^\circ\text{N}$.
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Explain why many of Earth's major deserts, such as the Sahara and the Sonoran Desert, are found near $30^\circ$ latitude.
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Explain why storms generally move from west to east across the United States.
The jet stream
Where cells meet near the tropopause, strong temperature contrasts create fast, narrow bands of wind called jet streams. The polar jet stream lies near $60^\circ\text{N}$ at the boundary between warm air to the south and cold air to the north. It blows west to east at over 100 km/h and steers storm systems.
The larger the north-south temperature difference, the larger the pressure difference, and the faster the jet stream. Winter has a larger contrast than summer, so the jet is stronger and shifts south. In summer it is weaker and shifts north.
The polar jet stream lies near the boundary between the Ferrel cell and the polar cell. Suppose that, at the altitude of the jet stream, the average temperature at…
The polar jet stream lies near the boundary between the Ferrel cell and the polar cell. Suppose that, at the altitude of the jet stream, the average temperature at $40^\circ\text{N}$ is $-45\,^\circ\text{C}$ in winter and the temperature at $60^\circ\text{N}$ is $-65\,^\circ\text{C}$, while in summer these temperatures are $-48\,^\circ\text{C}$ and $-55\,^\circ\text{C}$.
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Calculate the north-south temperature difference in each season.
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Explain how a larger temperature difference affects the strength of the jet stream.
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Predict how the jet stream and the typical storm track over the United States differ between winter and summer, and justify your answer.
Summary
Back to the start: air cools with height in the troposphere because the ground heats it from below, and the temperature rises again in the stratosphere because ozone absorbs UV. U.S. storms arrive from the west because the westerlies and the jet stream steer them.