1.2: Overview of Earth’s Atmosphere

Everything that happens on Earth is caused in some way by radiation from the sun, which is 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 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

Pie chart showing the gases that make up Earth’s atmosphere (CC BY-SA 3.0).

Earth’s atmosphere is primarily composed of nitrogen (\(\ce{N}\)) and oxygen (\(\ce{O}\)), with smaller quantities of other gases, as shown in the figure above. Nitrogen makes up around 78%, oxygen makes up around 21%, and Argon almost 1% of the total dry air volume in the atmosphere. Other gases (such as water vapor, the gaseous form of \(\ce{H2O}\)) 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 (\(\ce{O2}\)) and releases carbon dioxide (CO₂). When plants photosynthesize, they take in carbon dioxide and release oxygen. When water (\(\ce{H2O}\)) evaporates from the ocean, additional water vapor (\(\ce{H2O}\)) is added to the atmosphere. When the Kilauea volcano degasses, sulfur dioxide (\(\ce{SO2}\)) is added to the atmosphere.

Water Vapor

Water vapor concentrations vary greatly from location to location, and from time to time. In warm, moist, tropical regions, water vapor can make up nearly 4% of the atmospheric composition, while in 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 of water vapor changing phase to liquid is called condensation. When liquid water becomes a gaseous vapor, it is known as 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 major 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-releases a portion of the Earth’s outgoing radiation, which allows our planet to 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.

Hawaiian Focus Box

The air composition of Hawaiʻi occasionally faces a unique threat due to the emissions from Kīlauea, a volcano located along the southern shore of Hawaiʻi’s Big Island. Kīlauea has been continuously erupting since 1983 and is the most active of the volcanoes that make up the Big Island. In the first few years after 1983, Kīlauea emitted up to 30,000 tons of sulfur dioxide (\(\ce{SO2}\)) a day, but that number has stabilized recently to around 5,000 tons per day. At present day with the 2018 Kilauea eruptions currently occurring, \(\ce{SO2}\) emissions are again elevated. These emissions are known as volcanic smog, or more commonly, “vog”. The sulfate particles in vog are very small, so they can effectively infiltrate human airways. In environments with high relative humidity, such as inside the human body, these particles will hydrolyze and expand, which irritates lungs and obstructs airways. Vog has been linked to respiratory disease, sinusitus, and even lung cancer. Therefore, Hawaiʻi’s residents need to take precautions.

Water vapor and other gases escaping from the Kīlauea volcano at the Halemaʻumaʻu vent (CC BY-SA 4.0).

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 the gravitational force, which pulls downward on the atmosphere. 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 has an effect on 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 have no weight 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 (IS) of Units. 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 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 the surface of Earth and decreases with height, because there are more molecules held tightly together at the surface by gravity than there are 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.

Density (\(\rho\), kg·m^-3) vs. height (\(z\), km) in the atmosphere (CC BY-NC-SA 4.0).

The image above 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. The exponential equation below approximates the distribution of density with height

\[\rho(z)=\rho_0 \times \exp (-z / H) \nonumber \]

\[H=\frac{R_d \times T}{g} \nonumber \]

where \(\rho\) (kg m^-3) is density, \(\rho_0\) (kg m^-3) is the density at the Earth’s surface, \(z\) (m) is altitude, \(T\) (K) is temperature (assumed to be constant through the atmosphere), \(R_d\) = 287.053 J·K^-1·kg^-1 (gas constant for dry air), and \(g\) is the acceleration due to gravity (m·s^–2).

Note

Remember that the equations you see in this text are good approximations. They all involve assumptions of some kind. In the case of the exponential distribution of density with altitude, the primary issue is the assumption that temperature is constant throughout the atmosphere. This will become clear later.

“Scale height” is denoted by \(H\) and represents an e-folding distance for the drop off of density in the atmosphere. Typically the value of \(H\) is around 8000 m. This means that density at 8 km is approximately 1/e (e=2.71828) of the value at the surface. A back of the envelope calculation gives the density of the atmosphere at 8 km as 1.2 kg·m^-3 divided by 3 and is approximately equal to 0.4 kg·m^-3. Checking the image above, it is clear that this provides a relatively good estimate for density.

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 billions of 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.

\[\text { Pressure }=\frac{\text { Force }}{\text { Area }} \nonumber \]

\[P=\frac{F}{A} \nonumber \]

A Newton (N) is the unit for force and m² is the unit for area, in the International System of Units (SI). Therefore, the standard unit for pressure is in Newtons per square meter — or Pascals (Pa), which are defined as 1 N·m^-2.

Pro Tip: You will generally find air pressure expressed in units of millibars or (mb) or hectopascals (hPa) on weather maps. These two units are equivalent to one another. In the Practical Meteorology: An Algebra-based Survey of Atmospheric Science textbook by Roland Stull off of which this OER is based, kPa are often used. In the aviation field, Inches of mercury (inHg) are also 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.

In future calculations, you will usually need to express pressure in Pascals (Pa) for your units to cancel out. Always be mindful of the units you are given and be sure you are able to convert from one type of unit to another.

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 the surface of the Earth and less above, air pressure is maximized at the surface and decreases with height nearly exponentially, analogous to air density.

Height (z, km) vs. pressure (P, kPa) in the atmosphere (CC BY-NC-SA 4.0).

The image above 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. The exponential equation below approximates the distribution of pressure with height

\[P(z)=P_0 \times \exp \left(\frac{-z}{H}\right) \nonumber \]

\[H=\frac{R_d \times T}{g} \nonumber \]

where \(P\) (Pa) is pressure, \(P_0\) (Pa) is the pressure at the Earth’s surface, \(z\) (m) is altitude, \(T\) (K) is temperature (assumed to be constant through the atmosphere), \(R_d\) = 287.053 J·K^-1·kg^-1 (gas constant for dry air), and \(g\) is the acceleration due to gravity (m·s^–2).

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.

Air Temperature

It is commonly thought that air temperature is a measure of how cold or hot an object is. We instinctively know that the higher an object’s temperature, the hotter it will be, and the lower its temperature, the colder it will be. But what does temperature really measure? The answer lies in the motion of the molecules.

Air molecules are in constant motion, colliding with objects and one another. As we increase the temperature of a volume of air, the air molecules speed up, and collisions become more frequent. As we decrease the air temperature, the molecules slow down, and collisions occur less frequently. What is going on here? Simply put, air temperature is a relative measure of the average kinetic energy (kinetic meaning it relates to motion) of the molecules of a system.

Because there are more air molecules close to the Earth’s surface, density and air pressure are maximized at the surface and decrease with height. Based on this, do you think air temperature will decrease or increase as you move further away from the surface?

Within the lowest 10-12 km of the atmosphere, temperature tends to decrease with height, primarily because the Earth’s surface is warmed by sunlight, which then warms the layer of air directly above it, which warms the air above that, and so on. While this is true in the lowest surface layer of the atmosphere, this is not true throughout the rest of the atmosphere. The vertical temperature profile is more complicated than the vertical profile of pressure or density. Temperature is different for each of the different layers of the atmosphere, which will be discussed later.

Temperature Scales

Surface air temperature is commonly given in degrees Fahrenheit (°F) in the United States. If you were born and raised in the US, this is the temperature scale that you probably use in your daily life, and what you’ll see given when you look at the daily weather. You’ll know that 32°F is the temperature at which water freezes, and 212°F is the temperature at which water boils.

Degrees Celsius (°C) is also commonly used, especially internationally. Celsius is a convenient scale to use because water freezes at 0°C and boils at 100°C. A difference in 1°C is larger than a difference of 1°F by about 1.8 times. To convert between the two, use the following equations.

\[{ }^{\circ} C=\frac{5}{9} \times \left({ }^{\circ} F-32\right) \nonumber \]

\[{ }^{\circ} F=\left(\frac{9}{5} \times { }^{\circ} C\right)+32 \nonumber \]

The Kelvin scale (K) is almost always used as a temperature unit in scientific equations and is convenient in that it contains no negative numbers. The Kelvin scale begins at 0 K, or absolute zero, where atoms and molecules would theoretically be thermally motionless. The Kelvin scale is also sometimes known as the absolute temperature scale. The lowest possible temperature is 0 K but it does not occur naturally. The coolest naturally existing place known in the universe is the Boomerang Nebula, located in the Centaurus constellation about 5,000 light-years away from Earth. The temperature is measured at 1 K, only 1°C above absolute zero. A difference in 1 K is the same as a difference of 1°C, so a conversion is linear and simple.

\[K={ }^{\circ} C+273.15 \nonumber \]

Based on this, absolute zero is -273.15 °C. Keep in mind degrees Celsius (°C) and degrees Fahrenheit (°F) always have the degree ° symbol in front, but Kelvin (K) never uses this symbol. Typical values of temperature on Earth’s surface on the Kelvin scale are values around 260-310 K.

Within atmospheric sciences, the Kelvin and Celsius scale are used. This chapter is one of the only times you’ll see a discussion of Fahrenheit within the course.