1.3: Layers of the Atmosphere

Vertical Profile of the Atmosphere

In the previous sections, we discussed how both air pressure and density decrease with height because most of the atmosphere is held close to 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 layers in several 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.

Figure \(\PageIndex{1}\): The vertical temperature structure of Earth’s atmosphere 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. (Public Domain; NOAA, via Wikimedia Commons)

As illustrated in Figure \(\PageIndex{1}\), 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. The layers of the atmosphere are described in the following sections.

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 the weather we experience on Earth occurs 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 with 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 proximity to the Earth’s surface. Airflow closest to the surface slows due to friction, which can be amplified by terrain or vegetation. As a result, the boundary layer experiences the most significant 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. Near the top of the atmospheric boundary layer is a section where temperature briefly increases with height, called an inversion. Inversions act like a lid, preventing air from above from mixing into the boundary layer. This can have profound implications on days when pollution is particularly problematic.

Stratosphere

In the stratosphere, the air temperature increases with height, causing a temperature inversion. This inversion layer tends to prevent tropospheric air from rising and 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, air temperature decreases with height, and the air is extremely thin and low-density. The average atmospheric pressure in this layer is about 1 mb, which means that about 99.9% of all air molecules are below this level, and only about one thousandth of the atmospheric mass is above it. 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 temperatures decrease with height in the mesosphere is that there is not much ozone to absorb UV radiation. As a result, 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, around -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 significant increase in temperature. Molecules can travel for entire kilometers without colliding with another molecule. This is the layer where auroras form due to interactions between charged solar particles and air molecules.

At the top of the thermosphere, 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.

Figure \(\PageIndex{2}\): 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. Alternative text in the paragraph below. (Public Domain; NOAA Satellite and Information Service via Flickr).

Figure \(\PageIndex{2}\) is an illustration showing the layers of Earth’s atmosphere with examples of objects and phenomena found in each. From lowest to highest: the troposphere contains clouds and a jet airplane; the stratosphere contains a commercial jet airplane; the mesosphere shows meteors; the thermosphere shows auroras, a space shuttle, and the International Space Station; and the exosphere shows satellites. The Troposhere extends to an approximate altitude of 7 miles, while the Stratosphere extends to an approximate altitude of 31 miles. The Mesosphere extends to an approximate altitude of 50 miles while the Thermosphere extends to an altitude of 440 miles, and the Exosphere to an altitude above 6,200 miles. The Kármán line, commonly considered the boundary between the atmosphere and outer space is marked at 62 miles.