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Guide to Reading

Chapter 20 directs your attention upward, into the envelope of gas that surrounds Earth’s solid and liquid body. The text has consistently shown that geology is the study of the entire Earth environment and interactions between all of its components. This interaction is especially well illustrated as this chapter opens with a discussion of Earth’s early atmosphere and the major changes it’s undergone over time. You learn that:

Its first components (hydrogen and helium) were driven off as the solid planet heated up.
Volcanoes supplied the ingredients of the next atmosphere (water vapor, carbon dioxide, sulfur dioxide, and nitrogen).
The atmosphere supplied water to fill the ocean basins.
The oceans absorbed carbon dioxide from the atmosphere and precipitated out large amounts as limestone.
The chemical weathering of rocks withdrew carbon dioxide from the atmosphere.
Photosynthetic organisms contributed oxygen to the atmosphere.
The ozone layer, which forms from oxygen and filters out harmful ultraviolet radiation, had to develop before life could safely leave the protection of seawater.
The atmosphere continues to change and in the future it could be very different from what it is today.

The chapter continues by presenting the composition and characteristics of the modern atmosphere. You read about:

its gaseous components (mainly nitrogen and oxygen)
its solid components (tiny particles called aerosols)
its atmospheric pollutants (including sulfates and nitrates that produce acid rain, the greenhouse gas carbon dioxide which contributes to global warming, and chlorofluorocarbons [CFCs] that break down ozone and create the ozone hole)
its air pressure at various elevations
its atmospheric heat and temperature, and how these are changed adiabatically
its water content (relative humidity and latent heat of condensation)
its layers (including the homosphere, heterosphere, ionosphere, mesosphere, mesopause, stratosphere, stratopause, thermosphere, troposphere, and tropopause)
the aurora borealis and the aurora australis

Next you look into how the atmosphere moves from one place to another (winds) and in doing so reacts with and changes the physical and biological Earth underneath. Even humans don’t escape the influence of winds. For example, mankind’s travels have been affected by winds from the days of sailing ships on the oceans to the days of flying planes in the jet stream. Your author examines many aspects of this topic, including both local and global circulation patterns. He begins by showing how incoming solar radiation (insolation) varies and is responsible for creating temperature differences and pressure gradients that set the stage for winds to blow. He continues with an explanation of the basic classification of both high altitude and surface winds and the geographic zones they flow through. There are divergence and convergence zones (including polar highs, polar fronts or subpolar lows, subtropical convergence zones or subtropical highs, and the intertropical convergence zone or equatorial low). Three large-scale, global-encircling convection cells exist in both the Northern and Southern Hemispheres (Hadley cells, Ferrel cells, and polar cells). Prevailing surface winds include the northeast and southeast tradewinds, doldrums, surface westerlies, horse latitudes, and polar easterlies. Even winds at high altitude (like the high-altitude westerlies and jet streams) have their effect on Earth below. Once again you read about the Coriolis effect, which deflects air flow in a manner analogous to its deflection of ocean waters (see Chapter 18).

The chapter continues with more topics that will remind you that the scope of geology is broad. You will read about atmospheric regions with recognizable physical characteristics (air masses), their boundaries (cold, warm, and occluded fronts), and their patterns of motion (cyclones and anticyclones). Weather phenomena (including various types of clouds and precipitation) are produced when lifting mechanisms cause air in the air masses to rise. Severe weather events, storms, can catastrophically change Earth’s surface. Your author briefly discusses thunderstorms, tornadoes, nor’easters, hurricanes, typhoons, and cyclones.

The chapter concludes by discussing climates, which are the average weather conditions in a region over a year. Several physical characteristics of Earth serve as controlling factors of climate. These include latitude, altitude, proximity to water, proximity to ocean currents, proximity to orographic barriers, and proximity to high- or low-pressure zones. The section ends with a brief discussion of two climate variations, monsoons and El Niño.

The next two chapters return you to Earth’s surface, and you take a look at deserts and glaciers. As you read about them, take a moment to realize that the existence of these special areas on Earth’s solid surface are the result of atmospheric conditions you’ve read about in this chapter.