system whose chemical properties, dynamic motions, 

 and physical processes constitute the subject mat- 

 ter- of meteorology. 



The earth's atmosphere extends from the solid 

 or liquid surface of the earth to an Indefinite 

 height, its density asymptotically approaching 

 that of Interplanetary space. At heights of the 

 order of 80 km (50 ml) the atmosphere Is barely 

 dense enough to scatter sunlight to a visible 

 degree. At heights of the order of 600 km (370 mi) 

 the atmosphere's density becomes so low that the 

 properties typical of a gas cease to exist and the 

 free molecular paths are long enough that one must 

 consider them as portions of elliptical orbits in 

 the earth's gravitational field. At 1000 km (600 

 mi) the density of the atmosphere is still suffi- 

 cient to yield readily observable auroral effects. 

 At about 30,000 km (18,600 ml) above the earth's 

 surface, a molecule moving as if in rigid rotation 

 with the earth could not be held to such an orbit 

 by the earth's gravitational attraction, so this 

 height might be taken as an extreme upper limit of 

 the possible atmosphere. 



The atmosphere may be subdivided vertically 

 into a number of ATMOSPHERIC SHELLS, but the most 

 common basic subdivision Is that which recognizes 

 a troposphere from the surface to about ten kilo- 

 meters, a stratosphere from about ten kilometers to 

 about eighty kilometers, and an ionosphere above 

 eighty kilometers; and each of these is often fur- 

 ther subdivided. 



Because the troposphere contains the bulk 

 (about three- fourths) of the atmospheric mass and 

 because It contains virtually all of the atmospheric 

 water vapor, ordinary weather events are most in- 

 timately concerned with the tropospheric phenomena. 

 (24) 



ATMOSPHERIC PRESSURE . The pressure exerted by the 

 atmosphere as a consequence of gravitational attrac- 

 tion exerted upon the "column" of air lying direct- 

 ly above the point in question. As with any gas, 

 the pressure exerted by the atmosphere is ultimate- 

 ly explainable in terms of bombardment by gas mole- 

 cules ; it is independent of the orientation of the 

 surface on which it acts. 



Atmospheric pressure is one of the basic 

 meteorological elements . It is measured by many 

 varieties of barometer, and is expressed in several 

 unit systems. The most common unit used is the 

 millibar (1 millibar equals 1000 dynes per cm ). 

 Unique to the science of meteorology is the use of 

 inches (or millimeters) of mercury; that is, the 

 height of a column of mercury that exactly balances 

 the weight of the column of atmosphere whose base 

 coincides with that of the mercury column. Also 

 employed are units of weight per area and units of 

 force per area. A standard atmosphere has been 

 defined in terms of equivalence to each of the 

 above unit systems, and it is used as a unit itself. 

 (24) 



ATMOSPHERIC SHELL . Any one of a number of strata 

 or "layers" of the earth's atmosphere. 



Temperature distribution is the most common 

 criterion used for denoting the various shells. 

 The TROPOSPHERE (the "region of change") is the 

 lowest 10 or 20 km of the atmosphere, characterized 

 by decreasing temperature with height. The term 

 STRATOSPHERE is used to denote both (a) the rela- 

 tively Isothermal region immediately above the 

 tropopause, and (b) the shell extending upwards 

 from the tropopause to the minimum temperature 

 level at 70 to 80 km; the MESOSPHERE is the shell 

 between about 20 and 70 or 80 km which has a broad 

 maxlititMi temperature at about 40 or 50 km; and the 

 THERMOSPHERE is the shell above the mesosphere with 

 a more or less steadily increasing temperature with 

 height . 



The distribution of various physico-chemical 

 processes is another criterion. The OZONOSPHERE, 

 lying roughly between 10 and 50 km is the general 

 region of the upper atmosphere in which there is an 

 appreciable ozone concentration and in which ozone 



plays an important part In the radiative balance of 

 the atmosphere; the IONOSPHERE, starting at about 

 70 or 80 km, is the region in which ionization of 

 one or more of the atmospheric constituents Is sig- 

 nificant; the NEUTROSPHERE Is the shell below this 

 which is, by contrast, relatively un- ionized; and 

 the CHEMOSPHERE, with no very definite height 

 limits, is the region in which photochemical re- 

 actions take place. 



Dynamic and kinetic processes are a third 

 criterion. The EXOSPHERE is the region at the 

 "top" of the atmosphere, above the critical level 

 of escape, in which atmospheric particles can move 

 in free orbits, subject only to the earth's gravi- 

 tation. 



Composition is a fourth criterion. The 

 HOMOSPHERE is the shell In which there is so little 

 photo-dissociation or gravitational separation that 

 the mean molecular weight of the atmosphere is sen- 

 sibly constant; the HETEROSPHERE Is the region above 

 this, where the atmospheric composition and mean 

 molecular weight is not constant. The boundary 

 between the two is probably at the level at which 

 molecular oxygen begins to be dissociated, and 

 this occurs in the vicinity of 80 or 90 km. 



The term mesosphere has been given another 

 definition which does not fit into any logical set 

 of criteria, i.e., the shell between the exosphere 

 and the ionosphere. This use of the word mesosphere 

 has not been widely accepted. (24) 



ATOLL . A ring-like "coral" island or Islands en- 

 circling or nearly encircling a lagoon. It should 

 be noted that the term "coral" island for most of 

 these tropical Islands is incorrect, as calcareous 

 algae often forms much more than 50% of them. (27) 



ATOLL REEF . A ring-shaped, coral reef, often 

 carrying low sand Islands, enclosing a body of 

 water. (10) 



ATOM. The smallest particle of an elementary sub- 

 stance. It consists of one or more positive pro- 

 tons and neutral neutrons surrounded by such a 

 number of electrons as will balance the positive 

 charge. (36) 



ATOMIC ^rtJMBER . The number of positive charges or 

 protons inside the nucleus of an atom; it is also 

 the number of electrons possessed by an uncharged 

 atom. The atomic number distinguishes each one of 

 the chemical elements and starts with hydrogen with 

 the atomic number of 1 and goes through uranium 

 with 92, the last of the natural occurring elements. 

 (39) 



ATOMIC WEIGHT . The determination of the weight of 

 an atom as measured arbitrarily with oxygen having 

 the weight of 16.0000. It is found that the weight 

 of atoms can be expressed very nearly as whole num- 

 bers. These whole numbers represent the rough 

 atomic weight which also express the sum of the 

 particles in the nucleus of an atom. Most uranium 

 has 92 protons and 136 neutrons. The sum of these, 

 or 238, is the rough atomic weight of the most 

 abundant kind of uranium. (39) 



ATS. Air Transportable Sonar. 



ATTENUATION ANOMALY . That part of the propagation 

 anomaly which may be identified with that portion 

 of the total loss which appears as a constant 

 fractional change per unit length of path. (4) 



AUDIO FREQUENCY . An audio frequency is any 

 frequency corresponding to a normally audible 

 sound wave. (Ranges roughly from 15,000 to 20,000 

 cycles per second.) (1) 



AUGUSTE FICCARD . A manned submersible vehicle 

 (Swiss). 



