1.6: Layers of the Atmosphere

In the previous sections, we talked about how both air pressure and density decrease with height, because most of the atmosphere is held close to the Earth’s surface. This change in pressure and density occurs quickly at first, but slows down at higher altitudes. Unfortunately the change in temperature with altitude is not nearly as straightforward. When we look at a vertical profile of the atmosphere, we see that it can be divided into different layers in a number of ways. We can define layers by how the air temperature varies with height, by the gas composition, and even by the electrical properties of each layer.

The vertical temperature structure of Earth’s atmosphere (Public Domain) in kilometers (left axis) and miles (right axis). The layers are labeled as “spheres” and the boundaries between the layers are labeled as “pauses”. For example the troposphere is the lowest layer and the tropopause is the boundary between the troposphere and the stratosphere. The yellow line shows temperature in °C, and the cloud indicates where most of the weather occurs.

As can be seen in the above figure, the air temperature decreases with height up to the tropopause, 11 km in this example. This occurs because radiation from the sun warms the Earth’s surface, and the surface warms the air just above it. This will be discussed in further detail in later chapters.

Troposphere

The layer of the atmosphere that we are most familiar with is the troposphere, which extends from the surface to around 11 km. All of the every day weather that we experience on Earth happens within the troposphere, which is characterized by frequent rising and sinking vertical air motions. The troposphere gets its name from the root word “tropo” in Greek, which means turning.

At the top of the troposphere, the air temperature stops decreasing with height in a region known as the tropopause, which separates the troposphere and the stratosphere above. The height of the tropopause varies depending on the season and location. In warmer areas near the equator, the tropopause tends to be higher (around 17 km), while in colder polar regions the tropopause is lower (around 9 km) because warm layers of air are thicker than layers of cold air. For the same reason, the tropopause is found at higher elevations in the summer, and at lower elevations in the winter. Aircraft fly at the tropopause height.

Atmospheric Boundary Layer

The atmospheric boundary layer (ABL) is located within the lowest 0.3 to 3 km of the troposphere and is affected in several ways due to its close proximity with the Earth’s surface. Airflow closest to the surface slows down due to the effect of friction, which can be amplified due to the type of terrain or amount of vegetation. Because of this, the boundary layer experiences the greatest amount of turbulence in the atmosphere. In addition, the boundary layer is warmed by the Earth’s surface during the daytime, so it is the layer that is most affected by the diurnal (day-night) heating cycle.

The change in temperature with altitude in the troposphere which is the lowest layer of Earth’s atmosphere. The turbulent boundary layer near the surface is also seen (shaded tan), along with the standard atmospheric lapse rate (-6.5 K·km^-1), and near-surface day (red) / night (blue) temperature differences (CC BY-NC-SA 4.0).

Stratosphere

In the stratosphere, the air temperature increases with height, causing a temperature inversion. This inversion layer tends to keep rising and sinking tropospheric air from mixing with stratospheric air. It also prevents rising and sinking from happening in the stratosphere itself. Because of this, it is called a stratified layer. The root word “strato” means layered, or spreading out, which is a good way to describe the many layers of the stratosphere. The temperature of the stratosphere increases with height because of the presence of a gas called ozone (\(\ce{O3}\)), which heats the air through the absorption of ultraviolet (UV) radiation from the sun.

At around 50 km, where the stratosphere is warmest (due to most of the UV radiation absorption occurring in the uppermost parts of the layer). The boundary known as the stratopause separates the stratosphere below from the mesosphere above.

Mesosphere

In the mesosphere, the air temperature once again decreases with height and the air is extremely thin with a low density. The average atmospheric pressure in this layer is about 1 mb, which means that about 99.9% of all air molecules are located below this level and only about one thousandth of the atmospheric mass is located above. You would not survive very long in the mesosphere, however, due to the extreme thinness of the air, freezing temperatures, and direct exposure to ultraviolet radiation. In addition, your blood would begin to boil at normal body temperatures if it were directly exposed to the low air pressure. The reason that temperatures decrease with height in the mesosphere is due partially to the fact that there is not a lot of ozone to absorb UV radiation. Because of this, the air molecules in the mesosphere lose more energy than they absorb.

At about 85 km near the mesopause, the atmosphere is at its coldest at about -90°C. The mesopause separates the mesosphere from the thermosphere above.

Thermosphere

In the thermosphere, the air density is extremely low so even a small amount of radiation absorption can lead to a large increase in temperature. Molecules can travel for entire kilometers without colliding into another molecule. This is the layer where auroras occur as a result of interactions between charged solar particles and air molecules.

At the top of the thermosphere, at above 500 km above the Earth’s surface, molecules can actually escape the gravitational pull of the Earth. This region is known as the exosphere, and represents the upper limit of the atmosphere.

This figure illustrates the relative heights of different phenomena in the various vertical layers of the atmosphere. The Kármán Line at 62 miles (100 km) designates the boundary between Earth’s atmosphere and outer space. Image by NOAA Satellites (Public Domain).

The previous figure shows the levels in Earth’s atmosphere and the phenomena and activities that occur there. For example, commercial jet airplanes fly at the base of the stratosphere, while the International Space Station is positioned in the thermosphere.

Chapter 1 contained a vast array of topics, from defining temperature and pressure, to describing atmospheric vertical structure and components. As a reminder, these were our learning goals:

1. Convert between temperature units of Fahrenheit, Celsius, and Kelvin
2. Use mathematical formulas to define atmospheric temperature, pressure, and density
3. Compute pressure and density changes with altitude
4. Describe the vertical structure of Earth’s atmosphere
5. Define and apply the ideal gas law
6. Describe hydrostatic balance
7. Discuss the difference between weather and climate
8. Note the location of terminology, coordinate systems, and units for future reference

The next section will discuss weather vs. climate and various terms and coordinate systems that are useful to review.