A Short History of Nearly Everything: Chapter 17 Summary & Analysis

Without the atmosphere, Earth would be frozen over. Its “gaseous padding” also protects us from deadly objects in space, and without its drag, raindrops would knock us unconscious. Surprisingly, the atmosphere isn’t all that vast. If Earth were the size of a desktop globe, the atmosphere would be as thick as a couple coats of varnish.  The atmosphere has four layers: troposphere, stratosphere, mesosphere, ionosphere (or thermosphere). The troposphere is the most vital to us. It rises from ground level up to 7-10 miles high, and it contains all the water and weather we need. Beyond that, Bryson says, is “oblivion.”

The stratosphere comes next: its boundary is marked by clouds flattening out. In 1902, Léon-Philippe Teisserenc de Bort discovers the stratosphere while traveling in an air balloon, though humans can’t survive up there because it is oxygen-deprived and -70°F. Beyond the troposphere, the temperature rises again (due to ozone) and then plummets to -130°F, before rising to 2,700°F in the thermosphere. Temperature marks the density of molecules in the atmosphere: it’s warmer at sea level because there are lots of molecules bumping into each other and creating heat. Higher up, the molecules are sparser. When “hopelessly ground-hugging” humans ascend too far, the sparseness of molecules can cause confusion, nausea, hypothermia, and frostbite.

Mountaineers call the area above 25,000 feet the “Death Zone,” though altitude sickness sets in above 15,000 feet. Humans can’t tolerate living above 18,000 feet (fetuses, for example, can’t survive pregnancies at those altitudes due to oxygen deprivation). In the 1780s, experimental balloon ascenders assume it’s hotter higher up because they’ll be closer to the sun, but it turns out that the sparsity of molecules actually makes it colder. On a hot day, when one feels the sun on one’s back, this isn’t actually the sun’s heat—rather, the heat is generated by excited molecules that are moving faster and colliding more. 

Air seems weightless, but actually it’s quite heavy. For instance, changes in air pressure can pile an extra half-ton of air on a sleeping person, but they won’t feel it because their body presses back on the weight and finds equilibrium. Storms expose air’s mass more adequately—when air picks up speed, it can destroy everything in its way. Particles in storm clouds pickup electric charges. Scientists don’t know why, but lighter particles become positively-charged and heavier particles (at the base of clouds) become negatively-charged. Negatively-charged particles are drawn toward the positively-charged Earth, which is what creates lightning. A bolt of lightning travels at 270,000 miles per hour and heats the air around it to 50,000°F, which is hotter than the sun’s surface.

Humans didn’t know much about the atmosphere until aviation took off. Clear-air turbulence, for example, is a random and undetectable pocket of lively air cells in an otherwise calm sky. We have no idea what causes it. More generally, convection causes air currents. Moist, warm air rises until it hits the top of the troposphere and spreads out, cooling as it goes. The cooling makes the air sink, and at ground level it spreads out into the low-pressure spaces that hot air has just left before warming and rising again. At the Equator, weather is the most stable and predictable; elsewhere, it’s far more erratic. 

The sun’s heat is unevenly distributed, causing differences in air pressure. The atmosphere tries to create equilibrium, which causes wind. Halley first predicts this mechanism in the 1600s, though it takes until 1835 for an engineer named Gustav-Gaspard de Coriolis to work out the details (which is why we now call the mechanism the “Coriolis effect”). Meteorology only takes off as a science in the 19th century, after Dutch instrument maker Daniel Gabriel Fahrenheit invents the first thermometer in 1717 and devises the Fahrenheit scale (which puts freezing at 32° and boiling at 212°). In 1742, a Swedish man named Anders Celsius devises a competing (and more intuitive scale) running from zero to 100.

The “father of meteorology,” however, is an English pharmacist named Luke Howard, who names the cloud types in 1803. His system divides clouds into three groups: stratus (layered clouds), cumulous (fluffy clouds), and cirrus (high, thin, feathery clouds). A fourth term, “nimbus” identifies rain clouds. Bryson thinks “the beauty of Howard’s system” is that the terms can be combined to account for every type of cloud, such as “cirrostratus” or “cumulonimbus.” Though Howard’s system has been expanded upon over the years, many of the newer cloud names (like “mammatus” and “mediocris”) haven’t caught on outside esoteric scientific circles. Incidentally, the expression “on cloud nine” comes from cumulonimbus, the ninth and fluffiest cloud in the system.

About 60 percent of rainwater returns to the atmosphere within a of couple days through evaporation, though some goes down into ground water and may take thousands of years to return to the sky. Water molecules live in lakes for about a decade before evaporating and for about 100 years in the ocean. Evaporation itself is a fast process. About six million years ago, continental movement closed the Strait of Gibraltar, and the entire Mediterranean dried up in just 1,000 years. The evaporated water fell as freshwater rain in other oceans, reducing their salinity and making them freeze more, triggering an ice age. As Bryson puts it, “a little change in Earth’s dynamics can have repercussions beyond our imagining.”

Oceans are also important in meteorology, since rising and falling water molecules (saltier water is denser and falls) create ocean currents, moving water around that subsequently heats and rises as air, with huge effects on Earth’s climate. Changes in salinity, for example, can trigger ice ages. Oceans are also essential for creating stability on Earth. The sun burns 25 percent brighter than it did when the solar system was young, and it should have had “catastrophic” effects on Earth—but tiny marine organisms capture carbon dioxide in rainfall and use it to make their shells, preventing the element from rising back up into the atmosphere and creating a worse greenhouse effect that would warm the planet to disastrous effects.

Before industrial activity, the atmosphere’s carbon dioxide level is 280 parts per million. By 1958, it rises to 315 parts per million, and today it’s over 360 parts per million. Scientists predict that by the end of the 21st century, it will be 560 parts per million. Earth’s oceans (and forests, which also absorb carbon) are effectively saving us from ourselves, but if we cross over a certain threshold, plants will die and release their carbon, rendering life unsustainable. Luckily, Earth is remarkably good at returning itself to a stable state. The last time this happened, it took only 60,000 years.