• Weather refers to the state of the atmosphere at a given time and place. It is constantly changing, sometimes from hour to hour and other times from day to day. 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, those quantities or properties measured regularly. 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.

  

• Earth's four spheres include the atmosphere (gaseous envelope), the lithosphere (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.

• 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.

• Most natural systems have mechanisms that tend to enhance change, called positive feedback mechanisms and other mechanisms, called negative feedback mechanisms, that tend to resist change and thus stabilize the system.

  

• 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.

  

• Air is a mixture of many discrete gases, and its composition varies from time to time and place to place. After water vapor, dust, and other variable components are removed, two gases, nitrogen and oxygen, make up 99 percent of the volume of the remaining clean, dry air. Carbon dioxide, although present in only minute amounts (0.037 percent), is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Because of the rising level of carbon dioxide in the atmosphere during the past century, attributed to the burning of ever increasing quantities of fossil fuels, most scientists believe that a warming of the lower atmosphere will trigger global climate change.

• The variable components of air include water vapor, dust particles, and ozone. Like carbon dioxide, water vapor can absorb heat given off by Earth as well as some solar energy. When water vapor changes from one state to another, it absorbs or releases heat. In the atmosphere, water vapor transports this latent ("hidden") heat from one region to another, and it is 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 (O₃), is an important gas concentrated in the 10 to 50 kilometer height in the atmosphere that absorbs the potentially harmful ultraviolet (UV) radiation from the Sun.

• Over the past half century, people have placed Earth's ozone layer in jeopardy by polluting the atmosphere with chlorofluorocarbons (CFCs), which remove some of the gas. Ozone concentrations take an especially sharp drop over Antarctica during the Southern Hemisphere spring (September and October). Furthermore, scientists have also discovered a similar but smaller thinning of ozone near the North Pole during spring and early summer. Because ultraviolet radiation is known to produce skin cancer, ozone depletion seriously affects human health, especially among fair-skinned people and those who spend considerable time in the Sun. In late 1987, the Montreal Protocol, which represents a positive international response to the ozone problem, was concluded under the auspices of the United Nations.

  

• Balloons play a significant role in the systematic investigation of the atmosphere by carrying radiosondes (lightweight packages of instruments that send back data on temperature, pressure, and relative humidity) into the lower atmosphere. Rockets, airplanes, satellites, and weather radar are also among the methods used to study the atmosphere.

  

• No sharp boundary to the upper atmosphere exists. The atmosphere simply thins as you travel 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) helps understand the vertical extent of the atmosphere. One-half of the atmosphere lies below an altitude of 5.6 kilometers (3.5 miles), and 90 percent lies below 16 kilometers (10 miles). However, traces of the atmosphere extend for thousands of kilometers beyond Earth's surface.

  

• 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. The environmental lapse rate is not a constant and must be regularly measured using radiosondes. A temperature inversion, where 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.

  

• 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. With increasing altitudes, the four layers consist of molecular nitrogen (N₂), atomic oxygen (O), helium (He) atoms, and hydrogen (H) atoms, respectively. The stratified nature of the gases in the heterosphere varies according to their weights, with the outermost gas, hydrogen, being the lightest.

• Occurring in the altitude range between 80 and 400 kilometers (50–250 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 (from top to bottom, the D, E, and F layers, respectively) 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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