12.1: Earth’s Atmosphere

Our atmosphere exists as a shell of gases held close to Earth’s surface by the force of gravity. If our robotross could turn back time and visit Earth soon after it formed some 4.56 billion years ago, we would find an atmosphere much different than the one we experience today. Though the thickness and composition of Earth’s early atmosphere remains controversial (e.g., Emspak 2016), scientists are fairly certain that it lacked oxygen (O₂). However, owing to the evolution of oxygenic photosynthesis some 2.5 billion years ago, atmospheric concentrations of O₂ began to rise. This momentous occasion in Earth’s history has been named the Great Oxidation Event. Fortunately for us (and every other aerobic organism), the modern-day concentration of atmospheric O₂ hovers around 21 percent (by volume).

The most abundant gas in Earth’s modern atmosphere is nitrogen (N₂), at a concentration of about 78 percent. This is an important element for agriculture and oceanic phytoplankton—but one that is not easily obtained. Turning nitrogen gas into industrial fertilizer takes tremendous amounts of energy. Natural conversion of N₂ into biologically available forms (e.g., ammonium, nitrate, nitrite) is carried out by a special group of organisms: the nitrogen-fixing bacteria. So though N₂ is quite abundant in the atmosphere, it tends to be a limiting factor for biological processes, especially in the ocean.

Argon (Ar), a noble gas left over from Earth’s formation that is resupplied through the radioactive decay of potassium (K), ranks third among the gases present in Earth’s atmosphere. Because it is non-reactive, welders use Ar to prevent oxygen from reacting with the metals being welded. Otherwise, most people give it no thought.

The other gases in our atmosphere (possibly hundreds) exist only in trace amounts, but a few of them exert a profound effect on our planet. From Chapter 10, you may recall ozone, naturally abundant in the stratosphere and produced via photochemical reactions with car exhaust at Earth’s surface. Stratospheric ozone acts as a shield to protect organisms from mutation-causing UV radiation. We also learned about manufactured chlorofluorocarbons that eat away at stratospheric ozone and increase the intensity of harmful UV radiation reaching Earth’s surface, especially at Earth’s poles.

Of course, greenhouse gases (Chapter 12) keep Earth warm by absorbing longwave radiation from Earth’s surface. Greenhouse gases, especially water vapor, belong to a category known as variable gases, substances whose concentrations vary in the atmosphere. Among the variable gases, water vapor reigns supreme, accounting for up to 4 percent of the atmospheric gases at times. Of course, invisible water vapor and its visible cousins, liquid water and ice, make up the trio of forms responsible for precipitation and other important processes on our planet.

Earth’s Geophysical Fluids

As every robotross knows (or soon learns), it’s easier to fly through air than water. That’s because the atmosphere is 800 times less dense than the ocean. This difference in density explains the difference in the response of the atmosphere and ocean to forces that set them in motion. Atmospheric flows behave like Aesop’s speedy hare, whereas the ocean moves more like the slow, but steady, tortoise. Light transmits much farther in the atmosphere too, permitting us to see for miles and miles. By contrast, visibility in even the clearest ocean water rarely exceeds a couple hundred feet. Sound, however, can be transmitted for hundreds of miles in the ocean; in the atmosphere only something like a violent volcanic explosion can be heard that far away.

Despite their differences, the atmosphere and ocean belong to what scientists call Earth’s geophysical fluids—the air, water, and molten rock found on our planet. The properties and motions of these fluids occupy the attention of numerous disciplines of science, including meteorology, climatology, oceanography, geophysics, volcanology, engineering, and many more. But their study principally falls to a field of science called geophysical fluid dynamics—the study of fluid flows in nature. Our understanding of geophysical fluid dynamics extends well beyond academics—these fluids determine the habitability of our planet. As Vallis puts it (2016): “Geophysical fluid dynamics plays an enormous role in the development of our understanding of the natural world.”

Weather vs. Climate

A simple explanation for the difference between weather and climate comes from a 19th-century school child (whose name, unfortunately, was never recorded): “Climate lasts all the time and weather only a few times” (Le Row 1887). That’s a pretty good way to think about it. In Chapter 1, you learned weather represents what is happening in the atmosphere right now at a given location. Air temperature, precipitation, humidity, cloud cover, barometric pressure, wind speed and direction, and visibility are a few of the weather conditions that might be observed. Climate, on the other hand, is the long-term average of weather conditions at a given location or globally. Climate scientists typically use a 30-year period to compute the average climate for a location. When you hear the TV weatherperson talk about today’s temperatures being so many degrees above or below normal, the benchmark they are referring to is the 30-year average.

The terms weather and climate apply to the ocean as well (e.g., Carlowicz 2006; Pope 2021). Ocean weather may be expressed as a day when the water is cold or warm, with big waves or none, clear or murky water. Ocean weather, like atmospheric weather, changes day to day. Ocean climate refers to longer timescale changes in the ocean, such as El Niño/La Niña or the Pacific Decadal Oscillation.

The atmosphere and ocean work in concert to orchestrate Earth’s weather and climate. The air–sea boundary exchanges heat, water, gases, and chemicals between the atmosphere and the ocean. The atmosphere connects to the ocean; the ocean connects to the atmosphere. Together they drive much of what we experience on the surface of our planet.

Layers of the Atmosphere

If we send our robotross straight up from Earth’s surface into the sky, it would encounter different layers of the atmosphere. Generally speaking, the layers of the Earth’s atmosphere correspond to changes in temperature that occur with altitude. Meteorologists divide the atmosphere into five layers based on temperature or the presence of certain gases or other kinds of matter. The five layers of the atmosphere include (following NOAA/NESDIS 2016):

• The troposphere—the layer closest to Earth’s surface and the one in which we live. This is where we experience weather directly. Temperature here typically decreases with increasing altitude.
• The stratosphere—the layer above the troposphere and the one that produces the highest weather-related clouds, such as cirrus, cirrostratus, and cirrocumulus. Temperature increases with altitude in the stratosphere.
• The mesosphere—the layer above the stratosphere in the middle of Earth’s atmospheric layers, where meteors become visible as they are heated through friction with the gases present in this layer. The mesosphere produces noctilucent clouds, which are made of ice crystals that mysteriously glow at night. These are the highest clouds. In this layer temperature decreases with altitude.
• The thermosphere—the layer above the mesosphere named for its high temperature. That’s a bit misleading because despite the presence of gas molecules with high kinetic energy—the technical definition of high temperature—the molecules are so thin that you wouldn’t feel warmth were you to step out of an aircraft at this height. Temperature increases with altitude in the thermosphere.
• The exosphere—the layer above the thermosphere, so named because “exo” means outer, and this is the atmosphere’s outermost layer. The exosphere is really, really thin and really, really big. A recent analysis of decades-old data revealed that it extends beyond the Moon (Baliukin et al. 2019).

You may also notice at left a dashed line called the Karman Line. This line represents the official-but-not-quite-scientific boundary between our atmosphere and outer space. (Learn more at astronomy.com.)

Atmospheric Pressure

Like we did for the ocean, we can think of the atmosphere as a column—this time it’s a column of air. Meteorologists define an air column as an undefined cylindrical (or rectangular) volume of the atmosphere that extends from Earth’s surface to a given height. Similar to the concept of a water column, visualizing a column of air provides a convenient way to understand the nature of the forces that act upon the atmosphere.

Like the ocean, the atmosphere exerts pressure, the force exerted by a fluid on an object immersed within it. The weight of the atmosphere acting on a unit area of Earth’s surface (or us) represents atmospheric, or air, pressure. Air pressure at sea level exerts about 14.7 pounds per square inch (psi). Equivalent units in common use include millibars (1,013.25 mb) and inches of mercury (29.92 in. Hg; Ahrens and Henson 2018). Fortunately, gases in our bodies push back with an equal force so we don’t experience this crushing weight.

Unlike the ocean, however, air is compressible. Gravity pulls down on our atmosphere, causing air molecules to be packed more closely (compressed) at the Earth’s surface. Meteorologists refer to the packing of air molecules as air density—the number of molecules in a given volume. Fewer air molecules means less air pressure: the less air above you, the lower the pressure. So, in effect, as we climb higher into the atmosphere, both air density and air pressure diminish—rapidly at first, then slower as altitude increases. In mathematical terms they decrease exponentially.

The rapid decrease in air pressure with altitude explains why your ears pop when you travel into the mountains. Air pressure at higher elevations is less than air pressure at lower elevations, so your ears have to adjust. Your eustachian tube opens to equalize the air pressure inside your middle ear and the surrounding air, which occasionally leads to a pop, like a champagne cork releasing gases.

Changes in temperature also cause changes in pressure and density. Anyone familiar with a hot air balloon (or heating in a home) knows that hot air rises. As air warms, its molecules get farther apart. Fewer molecules in a space results in lower air density and pressure for that volume of warmed air. Because the warmed air has a lower pressure than the surrounding air, it rises. The surrounding air pushes the volume of warmed air upward until it reaches an altitude where the pressure of surrounding air and the pressure inside the balloon are equal. Similarly, cooling a body of air results in an increase in its density and pressure. As we know, cold air sinks. The changes in air pressure with temperature are key to understanding motions in the atmosphere.

Highs and Lows

Differences in the heating of Earth’s surface result from differences in latitude and seasonal variations in solar radiation. This and a number of other factors generate regions in our atmosphere with different air pressures. Meteorologists designate regions of high and low pressure. Places where the pressure in a particular area is higher than that of the surrounding region are called high-pressure regions, or highs. Places where the pressure is lower than the surrounding region are called low-pressure regions, or lows. On weather maps or the local news on TV, you’ll often see these regions symbolized with a red letter L for a low and a blue letter H for a high. Designating highs and lows is a kind of shorthand for illustrating regions with mild and stable weather, which typically occurs in high-pressure regions. Regions with unsettled and unpleasant weather generally occur in low-pressure regions.

Observations of air pressure at different locations (or altitudes) provide the basis for designating high- and low-pressure areas. A surface map of air pressure typically illustrates lines of equal pressure, or isobars. Looking at the center of a high-pressure system on an isobaric map, you can see that the pressure here is higher than the surrounding areas. Similarly, the center of a low-pressure system will have a lower pressure than the surrounding areas. While a complete understanding of isobaric maps is beyond the scope of this text, a general familiarity with them proves useful for understanding wind and ocean currents.