Working through this chapter of the study guide will enable you to:

1. Describe the origin, characteristics, and composition of the air envelope, called the atmosphere, that surrounds Earth.
2. Distinguish among the different regions of the atmosphere as a function of altitude and tell how energy plays a key role in its dynamic nature.
3. Tell how scientists keep track of conditions and changes in our atmosphere, and what characteristics they measure to do so.
4. Distinguish between air currents and wind, and explain what forces are responsible for each.
5. Describe local winds, and show how the general wind circulation on Earth produces a predictable overall pattern.
6. Explain the formation of the jet streams and other upper atmospheric wind processes.
7. Tell how clouds are classified, and name the most general types.
8. Discuss the mechanisms by which clouds form and dissipate.

Discussion

The atmosphere makes it possible for life to exist on Earth. You should not need any more reason than this to study the atmosphere, but if you are not already impressed by its importance, consider the grandeur of atmospheric phenomena such as rainbows, sunsets, and auroras. An ozone layer in the atmosphere protects us from harmful ultraviolet radiation from the Sun. The atmosphere also redistributes the heat supplied to our planet by the Sun, thus providing temperate climate conditions over large areas of Earth's surface that would be inhospitable to human life without this heat exchange. Studying the composition and structure of the atmosphere helps us to understand how complicated and vulnerable this large ocean of air really is. Current interest in ecology demands that we take better care of our fragile atmosphere. If we do not, it is likely that we will severely curtail our long-term chances for survival as a species.


Section  19.1Composition and Structure

Our atmosphere is quite complex, yet its composition remains surprisingly consistent. By volume, 99% is made up of only two common gases, nitrogen (78%) and oxygen (21%). The remaining 1% is primarily argon and carbon dioxide, together with very small traces of several other gases.

As can also be seen by consulting Table 19.1 in the textbook, some components of the air vary from day to day and from location to location across Earth's surface. These components include such things as water vapor, carbon monoxide, dust, pollen, salt particles, and ammonia, some of which are considered pollutants if they are introduced into the atmosphere in large quantities by human activity.

The atmosphere retains its balance between oxygen and carbon dioxide in a series of life processes involving plants and animals. Plants use carbon dioxide in their growth cycles and release oxygen into the air as a by-product in a process called photosynthesis. Animals, on the other hand, breathe in oxygen and exhale carbon dioxide. This natural balance is very important in maintaining the proper percentages of these gases in our atmosphere. In addition, carbon dioxide is released into the atmosphere by various combustion processes. Humans must be careful not to upset the atmospheric balance by releasing too much carbon dioxide into the air from the various combustion processes that provide heat and energy for use in our daily lives. This balance can also be compromised by destroying too many trees and plants that naturally convert carbon dioxide back into oxygen.


Section  19.2Atmospheric Energy Content

The energy that Earth receives from the Sun at the top of the atmosphere is called insolation (incoming solar radiation). About one-third of the energy reaching Earth is reflected back into space by clouds, particles in the air, and the surface of Earth itself. The fraction of radiation reflected is known as Earth's albedo. Some scattering of insolation also occurs in the atmosphere, as demonstrated by the overall blue color of the sky. This coloration is caused by scattering of short wavelength blue light. Another 15% of solar insolation is absorbed directly by the atmosphere, mostly by the ozonosphere layer.

The insolation that makes it through the atmosphere heats Earth's surface, which in turn heats the air near the surface by conduction and also by the emission of infrared radiation, which is readily absorbed by water vapor and carbon dioxide in the atmosphere. This heated air rises and produces convection cells that are responsible for the circulation of the atmosphere and for other dynamic processes that we collectively refer to as weather. Water is evaporated into the air by incoming solar energy and can later condense into rain, snow, or fog, which releases energy back into the atmosphere, quite often at a great distance from where the evaporation originally took place. These processes help to disperse heat energy and keep our climate temperate, especially near large bodies of water such as oceans and lakes.

The atmosphere acts somewhat like a transparent filter that readily admits sunlight and allows the heated surface of Earth to distribute energy to the lower layers of the atmosphere by infrared emissions. This process is known as the greenhouse effect. (See Figure 1 in the chapter's second Highlight.) The amount of heat trapped by the greenhouse effect is influenced by the composition of the atmosphere, which in turn is greatly affected by pollution created by humans.


Section  19.3Atmospheric Measurements and Observations

Our lives are greatly influenced by the weather, and nearly everyone has a profound interest in the daily fluctuations of local atmospheric conditions. Meteorologists measure several quantities when predicting and reporting weather conditions, including temperature, atmospheric pressure, humidity, wind speed and direction, and the amount of recent precipitation. Such information is gathered by weather stations, analyzed by computer, and reported to the public in radio and television broadcasts.


Section  19.4Air Motion

The gravitational pull of Earth acts downward on molecules in the air, causing the density of air to increase as you descend into the atmosphere and to reach a maximum near Earth's surface. It is, however, pressure differences due to temperature variations across the surface of Earth that cause horizontal air movements. When encountering the unbalanced forces produced by these pressure differences, air always moves from a high-pressure region to a low-pressure one.

Localized heating of Earth's surface gives rise to convection cycles. These occur because heated air expands and rises, allowing cooler air to flow in horizontally to take its place. The convection cycles set up in this way are often referred to as thermal circulation. The horizontally moving air, or wind, is acted on by speed-dependent forces such as friction and the Coriolis force. The Coriolis force is the result of the rotation of Earth beneath the moving air, which causes it to appear to be deflected as it moves. Friction also causes deflection and retardation of moving air. As air flows outward from a high-pressure region in the Northern Hemisphere, it is deflected into clockwise rotation around the high by the Coriolis force. Air moving toward a low-pressure region is deflected into counterclockwise rotation around the low. Counterclockwise rotation around a low-pressure cell is called a cyclone, and clockwise rotation around a high is referred to as an anticyclone. These rotational patterns are reversed in the Southern Hemisphere.

Horizontal air movement is referred to as wind. The speed of moving air determines the strength of the wind. Wind speed is usually measured in miles per hour or kilometers per hour. Wind direction is determined by the direction from which the wind is blowing. For example, an east wind blows from east to west. The speed of the wind is measured with a device called an anemometer, and the direction of the wind is indicated by a wind vane.

Local heating and cooling often produce localized air movement. For example, when the ground is heated during the day, the air above it rises, and cool air flows in to replace the rising warm air. This often occurs near large bodies of water, and the resulting wind is called a sea breeze because the surface wind moves from the water toward the land. At night when the land cools off more rapidly than the adjacent water, the process is reversed and a land breeze is likely to occur. Other winds such as those associated with a monsoon occur because of similar localized heating of Earth's surface, although they are usually on a much larger scale.

The overall global circulation patterns are driven by differential heating that establishes basic north-south wind movement. Three basic north-south circulation patterns are set up in the Northern Hemisphere and three more in the Southern Hemisphere. (See Fig. 19.21 in the textbook.) The Coriolis force tends to turn these wind patterns as they move across Earth's surface. The results are six general regions of air circulation, three north of the equator and three south of the equator.

In the Northern Hemisphere the northeast trade winds are the first of these regions encountered. They blow primarily from the northeast, producing generally regular and steady winds between the equator and approximately 30^° N. Between 30^° N and 60^° N the pattern changes and is referred to as the westerlies because of the prevailing direction of air movement out of the west. North of 60^° the polar easterlies blow predominantly from the northeast. In the Southern Hemisphere these patterns are reversed. (See Fig. 19.21 in the textbook.) General wind patterns are responsible to a large extent for the climate in many parts of the world, and they also determined the trade and exploration routes used by early seafaring adventurers.

Prevailing wind patterns also exist in the upper atmosphere that greatly affect the climate and the severity of seasonal weather conditions. These fast-moving rivers of air are known as the jet streams. They are the result of pressure differences where great high-pressure and low-pressure areas meet. It is probable that jet streams also influence the formation of tornadoes and other severe atmospheric storms.


Section  19.5Clouds

Vertical air movement is responsible for the formation of clouds when water vapor is carried upward with rising warm air. This mixture often condenses into clouds after the rising air has cooled until it reaches its dew point. Clouds are classified by their appearance, their shape, and their altitude. High-altitude clouds make up the cirrus family, middle-altitude clouds are referred to by the prefix alto-, and low level clouds often include the prefix strato- in their names. Several descriptive names such as stratus (layered) and cumulus (heaped) are combined with the altitude designations to produce cloud names. (See this chapter's Spotlight feature.)

Fog near the surface of Earth also may be classified as a low-level cloud. Often huge clouds form that extend across the vertical altitude classifications. Such clouds are formed by large-scale, rising air masses. The lowest height of prevailing cloud cover is referred to as the "ceiling." This is often of great importance in determining the visibility available to pilots, especially for those who must fly without the use of electronic instrumentation.

Clouds consist of water droplets that have condensed in the air. This condensation can only occur when the temperature of the air falls below the dew point. As air rises, it cools, so the temperature of rising air is quite often lowered to its dew point, and cloud formation can begin at that altitude. The upward vertical motion stops when the temperature of the rising air reaches the temperature of the surrounding atmosphere. This is often referred to as a stable layer of the atmosphere. For this reason clouds often show very abrupt stops in their vertical development, producing surprisingly flat-topped clouds that can look almost solid enough to walk on if you see them from above while flying in an airplane.

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