Atmosphere Characteristics Chapter 17

Atmosphere Characteristics Key Concepts • • How does weather differ from climate? Why do seasonal changes occur? Vocabulary ◆ ozone ◆ troposphere ◆ stratosphere ◆ mesosphere ◆ thermosphere ◆ summer solstice ◆ winter solstice ◆ autumnal equinox ◆ spring equinox Earth’s atmosphere is unique. No other planet in our solar system has an atmosphere with the exact mixture of gases or the moisture conditions and heat needed to sustain life as we know it. The gases that make up Earth’s atmosphere and the controls to which they are subject are vital to our existence. In this chapter, you will begin to examine the ocean of air in which we live. The state of the atmosphere at a given time and place is known as weather. The combination of Earth’s motions and energy from the sun produce a variety of weather. As shown in Figure 1, weather strongly influences our everyday activities. Weather is constantly changing, and it refers to the state of the atmosphere at any given time and place. Climate,however, is based on observations of weather that have been collected over many years. Climate helps describe a place or region. Climate often is defined simply as “average weather,” but this is not a complete description. For example, farmers need to know not only the average rainfall during a growing season, but they also need to know the frequency of extremely wet and extremely dry years. The most important measurable properties of weather and climate are air temperature, humidity, type and amount of precipitation, air pressure, and the speed and direction of the wind. Composition of the Atmosphere The composition of the atmosphere has changed dramatically over Earth’s nearly 4.6 billion year history. The atmosphere is thought to have started as gases that were emitted during volcanic eruptions. Evidence indicates that oxygen did not start to accumulate in the atmosphere until about 2.5 billion years ago. The atmosphere continues to exchange material with the oceans and life on Earth’s surface. Major Components Sometimes the term air is used as if it were a specific gas, which it is not. Air is a mixture of different gases and particles, each with its own physical properties. The composition of air varies from time to time and from place to place. However, if the water vapor, dust, and other variable components were removed from the atmosphere, its makeup would be very stable worldwide up to an altitude of about 80 kilometers. Look at Figure 2. Two gases—nitrogen and oxygen—make up 99 percent of the volume of clean, dry air. Although these gases are the most common components of air, they don’t affect the weather much. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of a number of other gases. Carbon dioxide is present in only small amounts (approximately 0.039 percent), but it is an important component of air. Carbon dioxide is an active absorber of energy given off by Earth. Therefore, it plays a significant role in heating the atmosphere. Variable Components Important materials that vary in the air from time to time and place to place include water vapor, dust particles, and ozone. These components also can have significant effects on weather and climate. The amount of water vapor varies from almost none to about 4 percent by volume. Why is such a small quantity so significant? • Water vapor is the source of all clouds and precipitation. Like carbon dioxide, water vapor absorbs heat given off by Earth. It also absorbs some solar energy. Movements of the atmosphere allow a large quantity of solid and liquid particles to be suspended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay in the air for very long. Still, many particles are microscopic and remain suspended for longer periods of time. These particles include sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, and ash and dust from volcanic eruptions. Another important variable component of the atmosphere is ozone. Ozone is a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is not the same as the oxygen we breathe, which has two atoms per molecule (O2). There is very little ozone in the atmosphere, and it is not distributed evenly. It is concentrated in a layer located between 10 and 50 kilometers above Earth’s surface. In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet (UV) radiation emitted by the sun. Ozone is then produced when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This collision must happen in the presence of a third, neutral molecule that acts as a catalyst. A catalyst allows a reaction to take place without being consumed in the process. Ozone is concentrated 10 to 50 kilometers above Earth because the UV radiation from the sun is sufficient to produce single atoms of oxygen. In addition, there are enough gas molecules to bring about the required collisions. The ozone layer is crucial to life on Earth. Ozone absorbs potentially harmful UV radiation from the Sun. If ozone did not filter most UV radiation and all of the sun’s UV rays reached the surface of Earth, our planet would be uninhabitable for many living organisms. Human Influence Air pollutants are airborne particles and gases that occur in concentrations large enough to endanger the health of organisms. Primary pollutants, shown in Figure 3, are emitted directly from identifiable sources. Emissions from transportation vehicles account for nearly half the primary pollutants by weight. Secondary pollutants are not emitted directly into air. They form in the atmosphere when reactions take place among primary pollutants and other substances. For example, after the primary pollutant sulfur dioxide enters the atmosphere, it combines with oxygen to produce sulfur trioxide. Then the sulfur trioxide combines with water to create sulfuric acid, an irritating and corrosive substance. Reactions triggered by strong sunlight are called photochemical reactions. For instance, when nitrogen oxides absorb solar radiation, a chain of complex reactions begins. If certain volatile organic compounds are present, secondary products form that are reactive, irritating, and toxic. This noxious mixture of gases and particles is called photochemical smog. Height and Structure of the Atmosphere Where does the atmosphere end and outer space begin? There is no sharp boundary. The atmosphere thins as you travel away from Earth until there are too few gas molecules to detect. Pressure Changes To understand the vertical extent of the atmosphere, examine Figure 4, which shows changes in atmospheric pressure with height. Atmospheric pressure is simply the weight of the air above. At sea level, the average pressure is slightly more than 1000 millibars, or slightly more than 1 kilogram per square centimeter. One half of the atmosphere lies below an altitude of 5.6 kilometers. Above 100 kilometers, only 0.00003 percent of all the gases making up the atmosphere exist. Temperature Changes The pictures of snow capped mountains rising above snow-free valleys shown in Figure 5 might remind you that Earth’s atmosphere becomes colder as you climb higher. But not all layers of the atmosphere show this temperature pattern. THIS IS A KEY DIAGRAM! Note the 50%! 50% of the atmosphere lies below 5.6 Kilometers of altitude. Notice the red line indicating PRESSURE - the higher the altitude the lower the pressure! THIS IS A KEY DIAGRAM - SUGGEST YOU COPY IT INTO YOUR NOTES. YOU MUST KNOW THE NAMES OF THE LAYERS OF THE ATMOSPHERE. NOTE WHERE THE OZONE LAYER RESIDES The atmosphere can be divided vertically into four layers based on temperature. Figure 6 illustrates these layers. The bottom layer, where temperature decreases with an increase in altitude, is the troposphere. It is in this layer that essentially all important weather phenomena occur. The thickness of the troposphere is not the same everywhere. It varies with latitude and the season. On average, the temperature drop continues to a height of about 12 kilometers, where the outer boundary of the troposphere, called the tropopause, is located. Beyond the tropopause is the stratosphere. In the stratosphere, the temperature remains constant to a height of about 20 kilometers. It then begins a gradual increase in temperature that continues until the stratopause, at a height of nearly 50 kilometers above Earth’s surface. Temperatures increase in the stratosphere because the atmosphere’s ozone is concentrated here. Recall that ozone absorbs ultraviolet radiation from the sun. As a result, the stratosphere is heated. In the third layer, the mesosphere, temperatures again decrease with height until the mesopause. The mesopause is more than 80 kilometers above the surface and the temperatures approach 90°C. The fourth layer extends outward from the mesopause and has no well defined upper limit. It is the thermosphere, a layer that contains only a tiny fraction of the atmosphere’s mass. Temperatures increase in the thermosphere because oxygen and nitrogen absorb shortwave, high energy solar radiation. Earth-Sun Relationships [Important] Nearly all of the energy that drives Earth’s variable weather and climate comes from the sun. Earth absorbs only a tiny percentage of the energy given off by the sun—less than one twobillionth. This may seem insignificant, but the amount is several hundred thousand times the electrical-generating capacity of the United States. Solar energy is not distributed evenly over Earth’s surface. The amount of energy received varies with latitude, time of day, and season of the year. As you will see, the variations in solar heating are caused by the motions of Earth relative to the sun and by variations in Earth’s land and ocean surface. It is the unequal heating of Earth that creates winds and drives the ocean’s currents. These movements transport heat from the tropics toward the poles in an attempt to balance energy differences. The results of these processes are the phenomena we call weather. Earth’s Motions Earth has two principal motions—rotation and revolution. Rotation is the spinning of Earth about its axis. The axis is an imaginary line running through the north and south poles. Our planet rotates once every 24 hours, producing the daily cycle of daylight and darkness. Revolution is the movement of Earth in its orbit around the sun. Earth travels at nearly 113,000 kilometers per hour in an elliptical orbit about the sun. Earth’s Orientation We know that it is colder in the winter than in the summer. But why? Length of day and a gradual change in the angle of the noon sun above the horizon affect the amount of energy Earth receives. Seasonal changes occur because Earth’s position relative to the sun continually changes as it travels along its orbit. Earth’s axis is not perpendicular to the plane of its orbit around the sun. Instead it is tilted 23.5 degrees from the perpendicular, as shown in Figure 7. Because the axis remains pointed toward the North Star as Earth moves around the sun, the position of Earth’s axis to the sun’s rays is constantly changing. If the axis were not tilted, we would not have seasonal changes. The orientation of Earth relative to the sun and the constant movement of Earth cause the angle of the noon sun to vary by up to 47 degrees (23.5 degrees to 23.5 degrees) for many locations during the year. For example, a mid-latitude city like New York, located about 40 degrees north latitude, has a maximum noon sun angle of 73.5 degrees when the sun’s vertical rays reach their farthest northward location in June. Six months later, New York has a minimum noon sun angle of 26.5 degrees.