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atmosphere [Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of which is called meteorology.
Components and Characteristics of the Earth's Atmosphere
Layers of the Earth's Atmosphere
The earth's atmosphere is composed of distinct layers. The troposphere extends upward from the earth to a height of about 5 mi (8.1 km) at the poles, to about 7 mi (11.3 km) in mid-latitudes, and to about 10 mi (16.1 km) at the equator. The air in the troposphere is in constant motion, with both horizontal and vertical air currents (see wind). Throughout the troposphere temperature decreases with altitude at an average rate of about 3.6℉ per 1,000 ft (2℃ per 305 m), reaching about −70℉ (−57℃) at its apex, the tropopause. Above the troposphere is an atmospheric ozone layer, which is also the lower layer of the stratosphere. Temperature changes little with altitude in the stratosphere, which extends upward to about 30 mi (50 km). Above this layer is the mesosphere which extends to about 50 mi (80 km above the earth); the temperature sharply decreases from around 20℉ (10℃) at the base of the mesosphere to −166℉ (−110℃) before it begins to rise at the top of the mesosphere. The next layer is the thermosphere, which extends upward from the mesosphere to about 400 mi (640 km); its temperature increases rapidly with altitude because of the absorption of shortwave radiation by ionization processes, although, because of the thinness of the air, little heat energy is available. The final layer is the exosphere, which gradually gets thinner as it reaches into the vacuum of space at around 435 mi (700 km) above the earth's surface; the atmosphere is so attenuated at this altitude that the average distance air molecules travel without colliding is equal to the radius of the earth. Although some gas molecules and particles out to about 40,000 mi (64,400 km) are trapped by the earth's gravitational and magnetic fields, the density of the atmosphere at an altitude of about 6,000 mi (9,700 km) is comparable to that of interplanetary space.
Certain layers of the atmosphere within the main regions exhibit characteristic properties. Aurorae (see aurora borealis), or northern and southern lights, appear in the thermosphere. The ionosphere is in the range (50–400 mi/80–640 km) that contains a high concentration of electrically charged particles (ions); these particles are responsible for reflecting radio signals important to telecommunications.
Role of the Earth's Atmosphere
The earth's atmosphere is the environment for most of its biological activity and exerts a considerable influence on the ocean and lake environment (see biosphere). Weather consists of the day-to-day fluctuations of environmental variables and includes the motion of wind and formation of weather systems such as hurricanes. Climate is the normal or long-term average state of the atmospheric environment (as determined in spans of about 50 years). The atmosphere protects earth's life forms from harmful radiation and cosmic debris. The ozone layer also protects the earth from the sun's harmful ultraviolet rays; seasonal “holes” in the ozone layer, the first detected above Antarctica and the Arctic in the 1980s, caused considerable alarm about the consequences of air pollution. Subsequently there has been increasing recognition of the role of air pollution and other aspects of human activity in global warming and climate change. Meteors strike the thermosphere and mesosphere and burn from the heat generated by air friction.
See also Van Allen radiation belts.
See O. Allen, Atmosphere, (1983); M. I. Budyko and A. B. Ronov, History of the Earth's Atmosphere, (1987).
atmosphereThe gaseous envelope that surrounds a star, planet, or some other celestial body. The ability of a planet or planetary satellite to maintain an appreciable atmosphere depends on the escape velocity at its surface and on its temperature. Small bodies, such as the Moon, have low escape velocities that may be exceeded comparatively easily by gas molecules traveling with the thermal speeds appropriate to their temperature and mass. The speed of a gas molecule increases with temperature and decreases with molecular weight; the lighter molecules, such as hydrogen, helium, methane, and ammonia, can therefore escape into space more readily than heavier ones such as nitrogen, oxygen, and carbon dioxide. Some bodies, such as Pluto, may be so cold that most potential atmospheric gases lie frozen on their surfaces. A body may gain an atmosphere by any of several processes, including a temporary acquisition by collision with an object containing frozen gases, such as a cometary nucleus.
The study of planetary atmospheres – their contents, meteorology, and evolution – has developed rapidly with the advent of planetary space probes in the 1960s. The planets Earth, Mars, Venus, Jupiter, Saturn, Uranus, and Neptune, and Saturn's satellite Titan, have been examined at close range by probes and much has been learned about their atmospheres. Mercury and Neptune's satellite Triton have been found to have only the most tenuous of atmospheres. Pluto is also thought to have an atmosphere. The deep hydrogen, helium, methane, and ammonia atmospheres of Jupiter, Saturn, and the other giant planets probably derive directly from the solar nebula and icy planetesimals from which the planets are now thought to have coalesced by accretion (see Solar System, origin). The terrestrial planets, however, were too small to capture the warm gases of the early nebula; the present atmospheres of Venus, Earth, and Mars are thought to have been released from volcanoes after being liberated from chemical combination in subsurface rocks by radioactive heating. The unusual abundance of free oxygen to be found in the Earth's atmosphere, accounting for about a fifth of its entire contents, is a direct result of the photosynthesizing action of living organisms.
Exploration of the Solar System has provided a unique opportunity to examine weather systems of planetary atmospheres. Venus: slowly rotating cloud over the planet with a huge greenhouse effect. Mars: a thin atmosphere where the weather systems are strongly influenced by the large topography of the surface. Jupiter and Saturn: large fluid atmospheres, rapidly rotating, with internal heating to assist in driving the weather phenomena. Each one of these examples is a natural laboratory for geophysical fluid dynamics that provides insight into general meteorological processes.
the gaseous envelope surrounding the earth. The atmosphere is taken to be the region around the earth in which the gaseous medium rotates as a unit with the earth. The weight of the atmosphere is approximately 5.15 x 1015 tons. The atmosphere makes life possible on the earth and has a great influence on various aspects of human existence.
Origin and function. The present-day earthly atmosphere is apparently of secondary origin and was formed from the gases emitted by the earth’s solid shell (the lithosphere) after the formation of the planet. In the course of geological history the atmosphere underwent major evolution owing to a number of factors—dissipation (evaporation) of atmospheric gases into outer space, emission of gases from the lithosphere as the result of volcanic activity, dissociation (decomposition) of molecules under the action of ultraviolet solar radiation, chemical reactions between components of the atmosphere and the rock which formed the earth’s crust, and accretion (capture) of interplanetary media such as meteor-itic material. The development of the atmosphere was closely linked with geological and geochemical processes and with the activity of living organisms. The atmospheric gases in turn had a great effect on the evolution of the lithosphere. For example, the enormous quantities of carbon dioxide entering the atmosphere from the lithosphere were then stored in carbonaceous rocks. The atmospheric oxygen and the water supplied from the atmosphere were important factors that acted on the rock. Throughout the entire history of the earth, the atmosphere played an important role in the erosion process. Part of this process was precipitation from the atmosphere, which formed the rivers and changed the earth’s surface. No less important was the action of the wind, which carried fine fragments of rock for long distances. Variations in the temperature and other atmospheric factors had a major effect on the breakdown of rock. In addition, the atmosphere protects the earth’s surface from the destructive action of falling meteorites, most of which are burned up upon entering the dense layers of the atmosphere.
The activity of living organisms, which has had a powerful influence on the development of the atmosphere, itself depends to a great extent on the atmospheric conditions. The atmosphere holds back most of the sun’s ultraviolet radiation, which has a fatal effect on many organisms. Atmospheric oxygen is utilized in the respiratory processes of animals and plants, and atmospheric carbon dioxide in the food processes of plants. Climatic factors, especially thermal and moisture conditions, affect the state of health and activity of man. Agriculture is particularly dependent on climatic conditions. In turn, human activity is having an ever-increasing effect on the composition of the atmosphere and on climatic conditions.
Structure. Numerous observations have demonstrated that the atmosphere has a clearly defined layered structure (see Figure 1). The main features of the atmosphere’s layered structure are primarily dependent on the characteristics of the vertical temperature distribution. In the lowest part of the atmosphere—the troposphere, where intensive turbulent agitation is observed—the temperature decreases with increasing height, amounting on the average to 6° K per km. The height of the troposphere varies from 8 to 10 km in the polar latitudes and reaches 16–18 km at the equator. In view of the fact that the air density decreases rapidly with height, about 80 percent of the atmosphere’s mass is concentrated in the troposphere. Above the troposphere there is a transition
layer—the tropopause, which has a temperature of l90°-220° K—and above this the stratosphere begins. In the lower part of the stratosphere the temperature reduction with height ceases, and the temperature remains nearly constant up to a height of 25 km. This is called the isothermal region (the lower stratosphere); the temperature begins to increase higher up, in the inversion region (the upper stratosphere). The temperature reaches a maximum of —270° K at the stratopause level, which is located at a height of about 55 km. The atmospheric layer at heights between 55 and 80 km, where the temperature again decreases with height, has been named the mesosphere. Above it is a transition layer (the mesopause), and above this is the thermo-sphere, where the temperature increases with height, reaching very high values (above 1000° K). Still higher (at heights of ~ 1,000 km or more) is the exosphere, from which atmospheric gases are dissipated into space by means of diffusion. A gradual transition from the atmosphere to interplanetary space occurs in the exosphere. Usually all the atmospheric layers above the troposphere are called the upper atmosphere although sometimes the stratosphere or its lower part is included in the lower atmospheric layers.
All the structural parameters of the atmosphere (temperature, pressure, and density) have significant space-time variability (latitudinal, annual, seasonal, diurnal, and so on). Consequently, the data in Figure 1 reflect only the average state of the atmosphere.
The layered structure of the atmosphere has many other varied manifestations. The chemical composition is not uniform with height. Up to heights as great as 90 km, where there is intensive agitation of the atmosphere, the relative composition of the permanent components remains practically unchanged. (This entire atmospheric layer has been named the homosphere.) However, above 90 km—in the heterosphere—there is a marked change in the chemical composition of the atmosphere with height in response to dissociation of the gaseous molecules by ultraviolet solar radiation. The typical characteristics of this part of the atmosphere are the ozone layer and the inherent atmospheric glow.
Atmospheric aerosols, which are solid particles of earthly and cosmic origin suspended in the atmosphere, are characterized by a complex layered structure. The most frequently encountered aerosol layers are under the tropopause and at a height of about 20 km. The vertical distribution of electrons and ions is layered; this is reflected in the existence of D, E, and F layers of the ionosphere.
Composition. Unlike the atmospheres of Jupiter and Saturn, which consist chiefly of hydrogen and helium, and the atmospheres of Mars and Venus, which have carbon dioxide as their principal component, the earth’s atmosphere consists mostly of nitrogen and oxygen. It also contains argon, carbon dioxide, neon, and other permanent and variable components. The relative concentrations by volume of the permanent gases and the average concentrations of the variable components (carbon dioxide, methane, nitrous oxide, and some others), which apply only to the lower layers of the atmosphere, are given in Table 1.
|Table 1. Chemical composition of dry atmospheric air at the surface of the earth|
|Gas||Volume concentration (percent)||Molecular weight’|
|1 Average molecular weight of dry air is equal to 28.9644|
|Nitrous oxide ..........||0.00005||44.0128|
|Sulfur dioxide..........||from 0 to 0.0001||64.0628|
|Ozone ..........||from 0 to 0.0000,07 in summer||47.9982|
|from 0 to 0.000002 in winter|
|Nitrogen dioxide .....||from 0 to 0.000002||46.0055|
|Carbon monoxide .....||trace||28.01055|
The most important variable constituent is water vapor. The space-time variability of its concentration fluctuates within wide limits—at the earth’s surface, from 3 percent in the tropics to 2x 10−5 percent in the antarctic. The main mass of the water vapor is concentrated in the troposphere, since its concentration diminishes rapidly with height. The average content of water vapor in a column of atmosphere at the temperate latitudes is a “layer of precipitated water” approximately 1.6 to 1.7 cm thick. (This would be the thickness of the layer of condensed water vapor.) Information on the relative content of water vapor in the stratosphere is conflicting. It has been proposed, for instance, that at heights between 20 and 30 km the specific humidity increases with height. However, subsequent measurements have indicated that the stratosphere is very dry. Apparently the specific humidity in the stratosphere depends very little on height and runs from 2 to 4 mg/kg.
The variability of the water vapor content in the troposphere is controlled by the interplay of evaporation, condensation, and horizontal transportation processes. As a result of water vapor condensation, clouds are formed and atmospheric precipitation occurs in the form of rain, hail, and snow. The phase transition processes of water take place mostly in the troposphere. As a result, clouds in the stratosphere (at heights of 20 to 30 km) and the mesosphere (near the mesopause), which are called nacreous and silvery, are seen rather rarely, whereas tropospheric clouds usually cover about 50 percent of the earth’s surface.
Ozone exerts an influence on the atmospheric processes, particularly on the thermal conditions in the stratosphere. It is mainly concentrated in the stratosphere, where it gives rise to the absorption of ultraviolet solar radiation, which is the primary factor in heating the air of the stratosphere. The monthly average values of the total ozone content vary as a function of the latitude and time of year over a range from 0.23 to 0.52 cm (the thickness of the ozone layer at ground-level pressure and temperature). The ozone content has been found to increase from the equator to the pole and to vary annually from a minimum in the autumn to a maximum in the spring.
An important variable component of the atmosphere is carbon dioxide, the content variability of which is associated with the vital activity of plants (the process of photosynthesis), industrial pollution, and the solubility of this gas in seawater (the gas exchange between the ocean and the atmosphere). Ordinarily the variations in carbon dioxide content are not great, but sometimes they can reach appreciable values. During recent decades an increase in the carbon dioxide content has been observed because of industrial pollution; the increase may have an influence on the climate because of the hothouse effect produced by the gas. It is assumed that on the average the concentration of carbon dioxide is constant throughout the entire thickness of the homosphere. Above 100 km it begins to dissociate under the action of ultraviolet solar radiation having wavelengths shorter than 1,690 angstroms.
One of the most active components optically is the atmospheric aerosol, which consists of particles suspended in the air and varying in size from several nanometers to several tens of microns; the particles, formed by the condensation of water vapor, enter the atmosphere from the earth’s surface as a result of industrial pollution and volcanic eruptions, and enter from space as well. Aerosol is found both in the troposphere and in the upper layers of the atmosphere. Its concentration diminishes rapidly with height, but superimposed on this trend are numerous secondary maxima that are associated with the presence of aerosol layers.
Upper atmospheric layers. Above 20–30 km the molecules of the atmosphere are decomposed as a result of dissociation to a greater or lesser degree into atoms, so that free atoms, as well as new, more complex molecules, appear in the atmosphere. Somewhat higher the ionization processes begin to be essential.
The most unstable region is the heterosphere, where ionization processes and dissociation produce many photochemical reactions that cause a change in the composition of the air associated with height. Here also the gravitational separation of gases takes place; this manifests itself in the gradual enrichment of the atmosphere with lighter gases as the height increases. According to data from rocket measurements, gravitational separation of the inert gases argon and nitrogen is observed above 105–110 km. The principal components of the atmosphere in the layer between 100 and 210 km are molecular nitrogen, molecular oxygen, and atomic oxygen. (The concentration of the latter at a level of 210 km reaches 77±20 percent of the concentration of molecular nitrogen.)
The upper part of the thermosphere consists chiefly of atomic oxygen and nitrogen. At a height of 500 km molecular oxygen is practically absent, but molecular nitrogen— although of greatly diminished relative concentration—still predominates over the atomic form.
An important role is played in the thermosphere by tidal movements, gravitational waves, photochemical processes, the increased mean free path of particles, and other factors. The results of retardation observations by satellites at heights of 200–700 km have led to the conclusion that there is a mutual connection between density, temperature, and solar activity and the existence of diurnal, semiannual, and annual variations of the structural parameters. It is possible that the diurnal variation is to a significant degree the result of atmospheric tides. During periods of solar flares, the temperature at a height of 200 km can reach 1700°-1900° C in the lower latitudes.
Above 600 km the predominant component is helium; still higher—at 2,000 to 20,000 km—is the earth’s hydrogen corona. At these heights the earth is surrounded with a shell of charged particles having temperatures up to several tens of thousands of degrees. It is here that the inner and outer radiation belts of the earth are located. The inner belt is mainly filled with protons having energies of several hundred million electron volts (MeV) and is found within the limits of 500–1,600 km at latitudes of 35M0° above the equator. The outer belt consists of electrons having energies in the order of hundreds of keV. Beyond the outer belt there exists an “outermost belt” in which the concentrations and fluxes of electrons are considerably greater. The incursion of solar corpuscular radiation (the solar wind) into the upper layers of the atmosphere produces the auroras. In response to this bombardment of the upper atmosphere with electrons and protons from the solar corona, the inherent atmospheric glow (earlier known as noctilucent clouds) is also excited. Upon interaction of the solar wind with the earth’s magnetic field, a zone called the earth’s magnetosphere—which the streams of solar plasma do not penetrate—is created.
In the upper atmospheric layers, the existence of strong winds that reach velocities of 100 to 200 m/sec is typical. The velocity and direction of the wind within the boundaries of the troposphere, the mesosphere, and the lower thermo-sphere have a large space-time variability. Although the mass of the upper atmospheric layers is insignificant compared to that of the lower layers and the energy of the atmospheric processes in the upper layers is relatively small, there is apparently some effect on the weather and climate of the troposphere.
Radiation, heat, and water balances. Practically, the sole source of energy for all the physical processes that develop in the atmosphere is solar radiation. The principal feature of the atmosphere’s radiation mode is the so-called hothouse effect—the atmosphere absorbs solar shortwave radiation poorly (most of it reaches the earth’s surface), but it holds back the long-wave (entirely infrared) thermal radiation of the earth’s surface, thus greatly reducing the earth’s heat emission to outer space and raising its temperature.
The solar radiation coming into the atmosphere is partly absorbed there—chiefly by water vapor, carbon dioxide, ozone, and aerosols—and is scattered by the aerosol particles and the density fluctuations in the atmosphere. As a result of the scattering of the sun’s radiant energy in the atmosphere, both direct radiation and scattered radiation are observed; together they form the total radiation. The total reaching the earth’s surface is partially reflected from it. The amount of reflected radiation is determined by the reflectivity of the underlying surface—the so-called albedo. Because of the absorbed radiation, the earth’s surface is heated and becomes a source of its own long-wave radiation directed to the atmosphere. In turn, the atmosphere also emits long-wave radiation directed toward the earth’s surface (the so-called counterradiation of the atmosphere) and toward outer space (the so-called outgoing radiation). The efficient heat exchange between the earth’s surface and the atmosphere is determined by the effective radiation—the difference between the inherent radiation of the earth’s surface and the atmosphere’s counterradiation absorbed by the earth. The difference between the shortwave radiation absorbed by the earth’s surface and the effective radiation is known as the radiation balance.
The conversion of solar radiation energy following its absorption by the earth’s surface and in the atmosphere makes up the heat balance of the earth. The principal source of heat for the atmosphere is the earth’s surface, which absorbs the major portion of the solar radiation. Since the absorption of solar radiation in the atmosphere is less than the heat losses from the atmosphere to outer space by long-wave radiation, the radiation discharge of heat is supplied by the inflow of heat to the atmosphere from the earth’s surface in the form of turbulent heat exchange and the arrival of heat as a result of water vapor condensation in the atmosphere. Since the total amount of condensation for the whole atmosphere is equal to the quantity of precipitation and also to the amount of evaporation from the earth’s surface, the input of condensation heat to the atmosphere is numerically equal to the dissipation of heat by evaporation at the earth’s surface.
A certain part of the solar radiation energy is expended in maintaining the overall circulation of the atmosphere and on other atmospheric processes, but this part is insignificant compared with the principal components of the heat balance.
Movement of the air. Because of the great mobility of air, winds are observed in the atmosphere at all altitudes. The air movements depend on many factors, the most important of which is the nonuniform heating of the atmosphere in various regions of the globe.
Especially large temperature contrasts exist on the earth’s surface between the equator and the poles because of the difference in the solar energy input at different latitudes. In addition to this, the arrangment of the continents and oceans affects the temperature distribution. Owing to the high heat capacity and thermal conductivity of ocean water, the oceans substantially reduce the temperature variations resulting from changes in the incidence of solar radiation during the year. For instance, at temperate and higher latitudes the air temperature over the oceans is markedly lower in summer than over the continents, and in winter it is higher.
The nonuniformity of atmospheric heating promotes the development of large-scale air flow systems—the so-called general atmospheric circulation, which creates a horizontal heat transfer in the atmosphere, thus materially smoothing out the differences in the heating of the air in the several regions of the atmosphere. In addition to this, the general circulation implements the hydrologic cycle in the atmosphere, carrying water vapor from the oceans to the dry land and providing moisture for the continents. The movement of the air in the system of general circulation is closely related to the distribution of atmospheric pressure and also depends on the rotation of the earth. At sea level the pressure distribution is characterized by a low at the equator, a high in the subtropics (a high pressure belt), and a low in the temperate and high latitudes. Over the continents at latitudes outside the tropics, the pressure in winter is ordinarily higher, and in summer it is lower.
Associated with the planetary distribution of pressure is a complicated system of air flows, some relatively stable and others continuously changing in space and time. Among the stable air flows are the trade winds, which are directed from the subtropical latitudes of both hemispheres toward the equator. The monsoons—air flows that develop between an ocean and a continent and have a seasonal character—are also relatively stable. In the temperate latitudes, westerly air flows (from west to east) predominate. These flows include large vortices—the cyclones and anticyclones—usually extending over hundreds and thousands of kilometers. Cyclones occur in the tropical latitudes, where they are characterized by small size but particularly high wind velocities, often reaching hurricane force (the so-called tropical cyclones). In the upper troposphere and lower stratosphere, the relatively narrow (hundreds of km) jet streams occur; they have sharply defined boundaries within which the wind achieves enormous velocities—up to 100–150 m/sec. Observations indicate that the characteristics of atmospheric circulation in the lower part of the stratosphere are controlled by the processes in the troposphere.
In the upper half of the stratosphere, where temperature increases with height, the wind velocity increases with height and easterly winds predominate during the summer and westerlies in the winter. The circulation here is determined by the supply of heat in the stratosphere that comes from the high absorption of ultraviolet solar radiation by ozone.
In the lower part of the mesosphere at temperate latitudes, the velocity of the westerly winter air transport rises to maximum values of about 80 m/sec, and the easterly summer air transport reaches 60 m/sec at a level on the order of 70 km. Studies made in recent years have clearly demonstrated that the characteristics of the temperature field in the mesosphere cannot be explained by the influence of radiation factors alone. The dynamic factors are of great importance (in particular, the warming or cooling when air sinks or rises), as well as the possibility of heat sources that occur as a result of photochemical reactions (for example, the recombination of atomic oxygen).
Above the cold layer of the mesopause (in the thermosphere), the air temperature begins to rise rapidly with height. In many respects this region of the atmosphere is similar to the lower half of the stratosphere. It is probable that the circulation in the lower part of the thermosphere is determined by processes in the mesosphere and that the dynamics of the upper layers of the thermosphere are dependent on the absorption here of solar radiation. However, it is difficult to investigate atmospheric movements at these heights because of their considerable complexity. Tidal movements (chiefly the solar semidiurnal and diurnal tides), under the effect of which wind velocity at altitudes above 80 km reaches 100–120 m/sec, are of great importance in the thermosphere. A characteristic feature of atmospheric tides is their great variability depending on latitude, time of year, height above sea level, and time of day. In the thermosphere substantial variations in wind velocity depending on height are also observed (chiefly near the 100 km level); they are attributable to the influence of gravitational waves. The so-called tur-bopause, which is located in the height range between 100 and 110 km, sharply separates the region above it from the zone of intense turbulent agitation.
In addition to large-scale air flows, there are in the lower atmospheric layers numerous local air circulations (breeze, bora, mountain-valley winds, and others). Wind pulsations, which represent the agitation of small and medium-size air vortices, are usually noticeable in all air flows. These pulsations are associated with the atmospheric turbulence that materially affects many atmospheric processes.
Climate and weather. The differences in the amount of solar radiation impinging on the earth’s surface at different latitudes and the complexity of the structure of the surface, including the arrangement of the oceans, continents, and large mountain systems, control the different kinds of climates on the earth.
The climate in the tropical latitudes is characterized by high air temperatures at the earth’s surface (on the average, 25°-30° C) that vary little during the year. In the equatorial belt there is usually a large amount of precipitation, which creates excessively moist conditions. In the tropics beyond the edges of the equatorial belt the amount of precipitation is reduced, and in a number of areas of the subtropical high pressure belt it becomes very small. The earth’s broad deserts are located in these areas.
In the subtropical and temperate latitudes, the air temperature is considerably lower and varies during the year; the difference between winter and summer temperatures is especially large in continental regions far from the oceans. For example, in certain areas of Eastern Siberia the temperature of the coldest month is 65° C lower than the temperature of the warmest. The moisture conditions at these latitudes are quite varied and are chiefly dependent on the state of the general atmospheric circulation.
In the polar latitudes, despite appreciable seasonal changes, the temperature remains low for the entire year and so favors widespread ice covering on land and oceans.
Against the background of a stable climate there occurs continuously changing weather, which is mainly controlled by the general atmospheric circulation. The weather is most stable in tropical countries and most variable in the circumpolar areas, especially in the North Atlantic and the Pacific oceans, where there are many cyclone tracks. Analysis of the causes of weather changes is the basis of weather forecasting methods, which depend on the plotting of daily synoptic charts that are analyzed by applying the general physical laws of atmospheric processes and various statistical procedures. Of increasing importance are the computational forecasting methods based on the solution of hydrodynamic and thermodynamic equations that describe the movement of the atmosphere.
Direct influences on atmospheric processes. A very important problem from both the scientific and practical viewpoints is that of actively influencing the atmospheric processes for the purpose of changing the weather and climate. Work in this direction, which was first started in the 1950’s in the Soviet Union, has already led to the creation of methods for acting on certain atmospheric processes. Specifically, scattering certain reagents in clouds alters the development of thunderclouds and prevents the precipitation of hail that causes large losses to agriculture. Methods have been developed for dispersing fog and protecting plants from frosts, and experimental work is being performed on clouds to increase the amount of precipitation. The majority of the methods now being used to influence atmospheric processes are based on the possibility of controlling unstable processes whose dynamics can be changed at the expense of comparatively small amounts of energy and reagents.
In addition to the active influences, notable changes in meteorological conditions are obtained by such reclamation measures as irrigation, protective forestation, and drainage of swamplands. These changes, however, are for the most part restricted to the lower (ground level) layer of air.
In addition to efforts to regulate the weather and climate, a number of aspects of human activity have an influence on climatic conditions. Thus, especially in recent years, pollution of the atmosphere has been increased substantially by the dust and various gases emitted from industrial enterprises. In view of this, efforts are being made in many countries to control air pollution and to limit the contaminating substances ejected into the atmosphere. The rapid increase of power use is contributing additional heat to the atmosphere, and though it is noticeable only in large industrial centers, in the comparatively near future it may lead to a change in climate over large territories. It is conceivable that soon man will to a considerable extent control atmospheric processes in order to change them in favorable directions and prevent effects that are harmful for agricultural activities.
Optical, acoustical, and electrical phenomena. The propagation of electromagnetic radiation in the atmosphere is connected with the occurrence of various phenomena that depend on the scattering and absorption of light and on refraction (bending the trajectory of a light ray). Such phenomena as the rainbow and the corona, which are caused by water drops scattering the sun’s light, are well known. The halo and coronas are seen when solar radiation is scattered by ice crystals. The scattering of light causes the apparent flattening of the firmament and the blue color of the sky, and the phenomenon of light refraction produces mirages. The optical instability of the atmosphere is an important factor in restricting opportunities for astronomical observations. The conditions for propagation of light in the atmosphere determine the visibility of objects. The transparency of the atmosphere at different wavelengths defines the propagation distance for radiation from lasers, which is of importance from the standpoint of using lasers for communication. The attenuation of infrared radiation by the atmosphere affects the functioning of various devices and instruments for infrared technology. The phenomenon of twilight is of great value in the investigations of optical heterogeneities in the stratosphere and mesosphere. For example, photographing the twilight from spacecraft makes it possible to locate aerosol layers. All these questions, as well as many others, are studied in atmospheric optics. It is the refraction and scattering of radio waves that govern radio reception possibilities.
The propagation of sound in the atmosphere, which depends on the spatial distribution of temperature and wind velocity, is studied in atmospheric acoustics; this is of interest in the development of indirect methods for sounding the upper layers of the atmosphere. As an example, observations of the audibility zone for the sound from a test explosion have made it possible for the first time to show the temperature increase with height in the stratosphere. The use of rocket acoustic methods has allowed a wealth of information to be obtained regarding the winds in the stratosphere and the mesosphere.
A fundamental problem in studies of atmospheric electricity is the presence of the earth’s negative charge and the resulting electrical field in the atmosphere. A vital part of this problem is the formation of clouds and thunderstorm electricity. The generation of lightning discharges involves the appearance of lightning strokes. The frequency of their occurrence has brought about the development of lightning protection methods for buildings, structures, and lines for electric power and communication. This phenomenon is particularly dangerous for aviation. Lightning discharges produce atmospheric radio interference, which is called atmospherics. During periods of sharp increase in the electrical field intensity, luminous discharges are observed from points and sharp corners of objects that project above the earth’s surface, on individual peaks in the mountains, and so on (St. Elmo’s fire). Because of the effects of various ionization processes, the atmosphere is always ionized and contains quantities of light and heavy ions which vary widely as a result of specific conditions and which govern the electrical conductivity of the atmosphere. The principal ionizing agents on the earth’s surface are the radiation from radioactive substances contained in the earth’s crust and in the atmosphere, as well as cosmic radiation. In the upper atmospheric layers the ionization is due to ultraviolet, corpuscular, and X-ray radiation from the sun. These are the factors that control for the most part the structure of the ionosphere, the conditions of which therefore are dependent on solar activity.
Study of the atmosphere. Although the study of the atmosphere began in ancient times, the science of meteorology was created only in the 19th century. Meteorology includes a number of disciplines which differ in the methods of investigation employed and the objects studied; among them are atmospheric physics, atmospheric chemistry, climatology, weather forecasting, and dynamic meteorology. The effect of atmospheric factors on biological processes is studied in biometeorology, which includes agricultural meteorology and human biometeorology. The classification of these disciplines has not yet been finally established and is in the development stage.
For observations of the atmosphere at the earth’s surface, an extensive network of meteorological stations and posts which are equipped with standard meteorological and aero-logical instruments has been created; in hard-to-reach regions, automatic meteorological stations have been set up. Radar is of great importance in the ground-level system of meteorological observations because it makes it possible to detect and investigate clouds, precipitation, turbulence, and convective atmospheric formations and also to measure the velocity and direction of the wind at high levels. Radio direction finding by means of atmospherics is also used extensively to locate thunderstorm cells. A major role in meteorological observations has been played by vertical soundings of the atmosphere made by means of radiosondes for the measurement of pressure, velocity and direction of the wind, temperature, and humidity in the free atmosphere.
In studying various characteristics of the atmosphere, aircraft and automatic balloons are used—for example, when investigating clouds and developing methods of actively influencing clouds, as well as for measurements in the fields of actinometry, atmospheric optics, and atmospheric electricity. During the period of the International Geophysical Year (1957–58) and in subsequent years, the utilization of meteorological rockets was begun for measuring temperature and atmospheric pressure in the upper stratosphere and the mesosphere. An important means of obtaining meteorological information, especially for the equatorial oceans and hard-to-reach territories, was opened up by meteorological satellites.
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M. I. BUDYKO and K. IA. KONDRAT’EV
ii. The unit of air pressure, based on the pressure exerted by the air on the surface of the earth (i.e., 1 atm (atmosphere) = 1.0333 kg/cm2/14.7 lb, or about 1,013,246 dyn/cm2.
atmosphereA unit of pressure. One atmosphere (one ATM) is equivalent to approximately 15 pounds per square inch. The following table shows ATM ratings for water resistance:
# Depth of RatingATMs (meters) Water Usage 1ATM 10 Splashes 3ATM 30 Brief immersion 5ATM 50 Shallow water swimming 10ATM 100 Swimming, snorkeling 15ATM 150 Scuba diving