1.1: Overview of Earth’s Atmosphere

Energy in the Atmosphere

Everything that happens on Earth is, in some way, caused by radiation from the sun, an average-sized star located near the edge of the Milky Way galaxy. It provides radiant energy (also called radiation or solar insolation) by converting hydrogen into helium near its core, which provides most of the Earth’s warmth. Here is a classic song about the Sun heating the Earth.

The Earth only receives a tiny portion of the sun’s total energy output. It is this radiation that drives the atmosphere’s wind and weather patterns on the Earth’s surface. It is because of this solar input that the Earth can maintain an overall global average surface temperature of approximately 15°C (59°F).

Composition of the Atmosphere

Figure \(\PageIndex{1}\): Pie chart showing the gases that make up Earth’s atmosphere. A breakdown is provided in the text. (CC BY-SA 3.0; Life of Riley, via Wikimedia Commons). Alternative Description of Image.

Earth’s atmosphere is primarily composed of nitrogen (\(\ce{N2}\)) and oxygen (\(\ce{O2}\)), with smaller quantities of other gases, as shown in the figure above. Nitrogen makes up around 78%, oxygen makes up around 21%, and Argon makes up almost 1% of the total dry air volume in the atmosphere. Other gases (such as water vapor, the gaseous form of water) take up varying amounts depending on the location and atmospheric conditions. There is a balance of input and output of atmospheric gases at the Earth’s surface, both from life and surface processes. For example, when we breathe, the human body takes in oxygen and releases carbon dioxide (\(\ce{CO2}\)). When plants photosynthesize, they take in carbon dioxide and release oxygen. When water (\(\ce{H2O}\)) evaporates from the ocean, additional water vapor (gas) is added to the atmosphere. When a volcano erupts, sulfur dioxide (\(\ce{SO2}\)) is added to the atmosphere (Figure \(\PageIndex{2}\)).

Water Vapor

Water vapor concentrations vary significantly from location to location and from time to time. In warm, moist tropical regions, water vapor can make up nearly 4% of the atmosphere, while in the polar regions, it can be just a tiny fraction of a percent. Water vapor cannot be seen in its gaseous form, but you may see it as it condenses into water droplets on the side of a cool glass of water on a hot day or into water droplets that make up clouds. The process by which water vapor changes phase to liquid is called condensation. When liquid water becomes a gas, it is called evaporation. When water becomes ice or ice becomes water, these processes are known as freezing and melting, respectively. When ice transitions directly to water vapor, the process is known as sublimation. The transition from water vapor to ice is known as vapor deposition. These terms will all be used frequently in subsequent chapters.

Water vapor is one of the most important gases in our atmosphere because when it changes phase from vapor to liquid or ice, it releases enormous amounts of heat, called latent heat. Latent heat is a significant source of energy in the atmosphere, particularly for tropical storms and other types of convection. In addition, water vapor is an important greenhouse gas in the atmosphere, meaning it absorbs and re-radiates a portion of the Earth’s outgoing radiation, helping our planet remain warm.

If you don’t understand convection, latent heat, and greenhouse gases yet, don’t worry, all will be discussed in later chapters. The point is that despite its small fractional percentage, water vapor is arguably the most important gas in the atmosphere.

Vertical Structure of the Atmosphere

Although the atmosphere extends vertically for hundreds of kilometers, almost 99% of it is within approximately 30 km of the Earth’s surface. Air molecules are pulled toward Earth by gravity, which pulls the atmosphere downward. This causes the air molecules closer to Earth’s surface to be more tightly compressed, meaning that there are more air molecules together in a given volume (a higher density). The greater the number of air molecules that exist above a certain altitude, the greater the effect of this compression, because the molecules are all being pulled downward together. Just as gravity affects the weight of different objects (weight is the force that acts on an object due to gravity), it also gives weight to air.

\[\text { Weight }=\text { mass } \times \text { gravitational acceleration } \nonumber \]

\[\text { Weight }=m \times g \nonumber \]

Mass is the measure of how much matter exists in a given object or space. Mass is occasionally confused with weight. While weight increases with more mass, mass would not weigh at all without the pull of gravity. Similarly, an object’s weight depends on the gravitational force: an object weighs much less on the moon than it does on Earth. Mass is typically given in grams (g) or kilograms (kg).

Note

You may want to review the International System of Units (SI). The base units we will use include meters (m), kilograms (kg), seconds (s), and Kelvin (K). We’ll also use the prefixes “kilo” (1000), “hecto” (100), and “micro” (10^-6) along with some derived units like hertz, Newtons, pascals, joules, and watts.

Air Density

Air density is determined by the amount of mass (\(m\)) that exists in a given space, or volume (\(V\)). If you fit a lot of mass into a tiny volume, you will have a higher density. If you have only a small amount of mass spread out over a large volume, the density will be lower.

\[\text { Density }=\frac{\text { mass }}{\text { volume }} \nonumber\]

\[\rho=\frac{M}{V} \nonumber\]

Putting these ideas together, air density is highest near Earth's surface and decreases with height because there are more molecules held tightly together by gravity at the surface than above. The standard unit for density is kg·m^-3.

Pro Tip: Density is often denoted by the Greek lowercase letter \(\rho\) (rho). Be careful not to confuse this with p or P, which are both used to denote pressure. When writing on paper, it may be useful to write ρ with a curly tail like a backwards q and to write p with a straight tail in order to differentiate the two.

Figure \(\PageIndex{2}\): Density (\(\rho\), kg·m^-3) vs. height (\(z\), km) in the atmosphere (CC-BY-NC-SA 4.0; Roland Stull via Practical Meteorology). Alternative Text Description.

Figure \(\PageIndex{2}\) shows the distribution of density (\(\rho\)) with altitude. Atmospheric surface density is typically around 1.2 kg·m^-3. We call this value \(\rho_0\) because it is the initial value at the surface. As you rise in the atmosphere, density decreases exponentially, with a steep drop at first, followed by progressively smaller decreases as you go higher. Table 1.1.1 is a table of the data from Figure 1.1.2.

Table 1.1.1. Approximate density vs height.

z (km)	ρ (kg·m⁻³)
0	1.20
2	1.00
4	0.82
6	0.66
8	0.53
10	0.41
12	0.31
14	0.23
16	0.17
18	0.13
20	0.10
22	0.075
24	0.055
26	0.040
28	0.030
30	0.022
35	0.012
40	0.006
45	0.003
50	0.0015

Air Pressure

You will often hear meteorologists on TV discuss the air pressure, or you might see H’s and L’s on a map displayed on the Weather Channel, denoting high and low pressure areas. Because air molecules are in constant motion, they collide with one another and other objects up to several billion times a second. Each time an air molecule collides with an object, it exerts a tiny amount of force. Air pressure refers to the total force that air exerts against a given area of an object.

Pro Tip: You will generally find air pressure expressed in units of millibars (mb) or hectopascals (hPa) on weather maps. These two units are equivalent. In the aviation field, Inches of mercury (inHg) are also commonly used. In Engineering, the SI unit Pascal (Pa) is commonly used. At sea level, the global average for atmospheric pressure is:

101.325 kPa = 1013.25 mb = 1013.25 hPa = 29.92 in. Hg. = 1 atm (atmosphere) = 101325 Pa.

It's important to know that air pressure is commonly measured in millibars/hectopascals in meteorology.

You can think of the surface air pressure as the total weight of a column of air molecules extending from the surface to the top of the atmosphere. Because there are more air molecules at Earth's surface and fewer above, air pressure is greatest at the surface. It decreases with height nearly exponentially, analogous to air density.

Figure \(\PageIndex{3}\): Height (z, km) vs. pressure (P, kPa) in the atmosphere. (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology). Alternative Text Description

Figure \(\PageIndex{3}\) shows the distribution of pressure (\(P\)) with altitude. Atmospheric surface pressure is typically around 1013 hPa. We call this value \(P_0\) because it is the initial value at the surface. Like density, air pressure decreases exponentially with height. Data for Figure 1.1.3 is provided in Table 1.1.2.

Table 1.1.2: Air Pressure vs. Height

z (km)	P (kPa)
0	101
1	90
2	80
3	70
4	62
5	55
6	48
7	42
8	37
9	33
10	30
12	24
15	18
20	10
30	4
40	1.5
50	0.5

You can get a sense of atmospheric pressure if you’ve ever dived more than a few feet underwater at the pool or beach. As you dive deeper, the weight of the water above you increases, and you feel increased discomfort or pressure on your head. This is why divers require special equipment to reach greater depths in the ocean. If you think of yourself at the bottom of an ocean of air, you can imagine why air pressure is highest down here at the surface. If you’ve ever flown on an airplane, you can also feel the changes in atmospheric pressure in your ears as you ascend and descend. Still, while pressure decreases at high elevations in an airplane, cabins are pressurized to maintain similar pressure to Earth’s surface, so you are not experiencing the full pressure drop with altitude.

Hydrostatic Balance

We’ve learned that atmospheric pressure decreases with height. We’ve also learned that air moves from areas of high pressure to areas of low pressure due to the pressure gradient force. Knowing these two things, you might think that the air in the atmosphere would escape into space, because there is high pressure at the surface and low pressure aloft. Because it does not, there must be a downward force that balances the upward vertical pressure gradient force. This downward force is one you should be very familiar with: gravity.

In the atmosphere, when the vertical pressure gradient force balances gravity, the system is in hydrostatic balance. The word hydro means water or fluid, and static means stationary, so that the name can be interpreted as a stationary fluid balance. This balance holds in most atmospheric situations.