Key Concepts

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Key Concepts of Chapter 1:

1. "Meteorology" is the scientific study of the atmosphere. "Weather" refers to the state of the atmosphere at a given time and place. "Climate" is an aggregate of weather conditions, the sum of all statistical weather information that helps describe a place or region. The nature of both weather and climate is expressed in terms of the same basic elements; the most important elements are: (1) air temperature, (2) humidity, (3) type and amount of cloudiness, (4) type and amount of precipitation, (5) air pressure, and (6) the speed and direction of the wind.

2. All science is based on the assumption that the natural world behaves in a consistent and predictable manner. The process by which scientists gather facts through observation and careful measurement and formulate scientific hypotheses and theories is called the "scientific method" which includes the following activities: (1) collect facts, (2) develop a scientific hypothesis, (3) construct experiments to validate the hypothesis, and (4) accept, modify, or reject the hypothesis on the basis of extensive testing. Other discoveries represent purely theoretical ideas that have stood up to extensive examination.

3. Earth’s four spheres include the atmosphere (gaseous envelope), the geosphere (solid Earth), the hydrosphere (water portion), and the biosphere (life). Each sphere is composed of many interrelated parts and is intertwined with all other spheres. Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the "Earth System." "Earth system science" uses an interdisciplinary approach to integrate the knowledge of several academic fields in the study of our planet and its global environmental problems.

4. A "system" is a group of interacting parts that form a complex whole. Closed systems are those in which energy moves freely in and out, but matter does not enter or leave the system. In an open system, both energy and matter flow into and out of the system.

5. The two sources of energy that power the Earth system are (1) the Sun, which drives the external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface, and (2) heat from Earth’s interior that powers the internal processes that produce volcanoes, earthquakes, and mountains.

6. Air is a mixture of many discrete gases, and its composition varies from time to time and place to place. Two gases, nitrogen and oxygen, make up 99 percent of the volume of the clean, dry air. Carbon dioxide, although present in only minute amounts (0.038 percent), is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Due to the rising level of carbon dioxide in the atmosphere during the past century often attributed to the burning of ever-increasing quantities of fossil fuels, it is likely that a warming of the lower atmosphere is triggering global climate changes, although water vapor is an even more powerful greenhouse gas than carbon dioxide. The variable components of air include water vapor, dust particles, and ozone. In the atmosphere, when water vapor changes from one state to another, it absorbs or releases heat and transports this latent (“hidden”) heat from one place to another, thus providing the energy source that helps drive many storms. "Aerosols" (tiny solid and liquid particles) are meteorologically important because these often invisible particles act as surfaces on which water can condense and are also absorbers and reflectors of incoming solar radiation. "Ozone," a form of oxygen that combines three oxygen atoms into each molecule, is a gas concentrated in the 10- to 50- kilometer height range in the atmosphere that absorbs much of the potentially harmful ultraviolet (UV) radiation from the Sun.

7. No sharp boundary to the upper atmosphere exists. The atmosphere simply thins as one travels away from Earth until there are too few gas molecules to detect. The change that occurs in atmospheric pressure (the weight of the air above) depicts the vertical extent of the atmosphere. Traces of atmosphere extend for thousands of kilometers beyond Earth’s surface.

8. Using temperature as the basis, the atmosphere is divided into four layers.

The temperature decrease in the troposphere, the bottom layer in which we live, is called the "environmental lapse rate." Its average value is 6.5°C per kilometer, a figure known as the "normal lapse rate." A temperature "inversion," in which temperatures increase with height, is sometimes observed in shallow layers in the troposphere. The thickness of the troposphere is generally greater in the tropics than in polar regions. Essentially all important weather phenomena occur in the troposphere.

Beyond the troposphere lies the stratosphere; the boundary between the troposphere and stratosphere is known as the tropopause. In the stratosphere, the temperature at first remains constant to a height of about 20 kilometers (12 miles) before it begins a sharp increase due to the absorption of ultraviolet radiation from the Sun by ozone. The temperatures continue to increase until the stratopause is encountered at a height of about 50 kilometers (30 miles).

In the mesosphere, the third layer, temperatures again decrease with height until the mesopause, some 80 kilometers (50 miles) above the surface.

The fourth layer, the thermosphere, with no well-defined upper limit, consists of extremely rarefied air. Temperatures here increase with an increase in altitude.

9. Besides layers defined by vertical variations in temperature, the atmosphere is often divided into two layers based on composition. The homosphere (zone of homogeneous composition), from Earth’s surface to an altitude of about 80 kilometers (50 miles), consists of air that is uniform in terms of the proportions of its component gases. Above 80 kilometers, the heterosphere (zone of heterogenous composition) consists of gases arranged into four roughly spherical shells, each with a distinctive composition. The stratified nature of the gases in the heterosphere varies according to their weights.

10. Occurring in the altitude range between 80 and 400 kilometers (50 and 250 miles) is an electrically charged layer known as the ionosphere. Here, molecules of nitrogen and atoms of oxygen are readily ionized as they absorb high-energy, shortwave solar energy. Three layers of varying ion density make up the ionosphere. Auroras (the aurora borealis, northern lights, and its Southern Hemisphere counterpart the aurora australis, southern lights) occur within the ionosphere. Auroras form as clouds of protons and electrons ejected from the Sun during solar-flare activity enter the atmosphere near Earth’s magnetic poles and energize the atoms of oxygen and molecules of nitrogen, causing them to emit light—the glow of the auroras.

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