THE ATMOSPHERE

Taranaki Daily News, 5 November 1932, Page 1 (Supplement)

THE ATMOSPHERE

ITS . HEIGHT AND WEIGHT

(‘By

Rev. B. Dudley, F.R.A.S.)

The pretty idea that 'appealed to us in early youth that just as animals of various kinds'live at the bottom of the sea. so we live at the bottom of an ocean of <air, is more than a poetic fancy; it is sober truth. .One great difference between’ the two things is that ’the air-ocean is very much deeper than . the water-ocean. . Science has shown that the former is at least 100 miles; deep/ /' There are several ways of determining the. question, It can be arrived at •approximately, for one thing, by observing .meteors,, or so-called shooting stars. These , ar-e. dark masses of matter Which enter, bur< atmosphere from outer space and 'become visible only because -they- strike the air with such terrific .force as to' make them glow 'with. heat,-. The height at which the air • is,. sufficiently dense to render a meteor, incandescent .is learned- by . two or more -astronomers working conjointly and observing the meteor’s direction from their respective stations on the earth’s.surface at the moment when it first becomes .visible. It,is,-found .by a multiplicity of tests that ’ meteors become visible generally ■at a height of from .60 to 90 miles. At this altitude, therefore, the atmos Sphere must exist in sufficient qu'anti•ties to. retard meteors to such an extent ’as to make them incandescent It •is likewise found that meteors cease to glow ■ when they have descended t-o within 30 to 4's' miles, of the earth’s ■surface.

The height, -too, to which an ..auroral streamer extends can be determined by (observations m'ade In like manner, it has been found that auroral phenomena appear, at heights greater than those at ■which meteors become visible. Therefore it is- easily contended that the atmosphere must extend beyond 90 miles.. It probably exceeds 100 miles. The duration of ” twilight yields further .evidence on. this point. Hawn and twilight.are due. to those portions of' th.e .•illuminated atmosphere which -are visible, to us,, although the sun is ■below the -horizon. \ The greater the •height' of the atmosphere the earlier would dawn- appear, and the longer would twilight •linger!. This, however, does not afford ‘additional information, since twilight. does? not ■ show above'” 50 mjles, the'density; in still higher regions •than .tjiat being insufficient to reflect sunlight. '

■A ■ • difference between the Kjce'an wound-? us and'the air above is that, whereas; the ■ former is almost uniformly .dense? the, latter is exceedingly elastic and: compressible. The lower ■atmospheric strata are .enormously cop'pressed;’ by the- excessive weight of those abovei yhjlp : the. upper layers are •ifiqre'.'-tequeius,, the, •tenuity ■ increasing 'with distance from' the surface -of the :e'irtlv^lt;bh;.s>h^!en i ?a^ce r tain?.d thatthe ■atmospheric density at the height of si- miles 'is half that-at the earth's 'surface.' ‘' Although Vyarying consider* ably in' the; on after ;■ of 'density, half - of •its- entire ’ihasp .fe within the area Ppmed; that is to.yay, inside.the area ?whd'Se'b6iiiidhrie's:*aTA3i miles from the •earth iniail.'.directipPß- ’ It takes the ’whole. of the atmosphere above that 'height to ’ equal: th'e amount of air contained'',beneath '’’it. ■'■ ' : A' •, • \ At 'sea-level-, the average weight of tfie atmosphere is about 15 factually 14.7) pounds-per " square inch; and the latest .calculations give the mass (or total amount) of ■ the ' atmosphere as rather less thap one-millionth that of the entire earth. From the known area bi the earth's siirfa.ee' the weight of the atmosphere is calculated to be approximately. ' 6,000,000,000,000,000,000,000, 000 tons. At sea-level the density of-the air.’ is abqut one' eigait-hundredth that ofwater.' The greatest altitude so far attained ‘by artillery projectiles isjneariy ~'2s'miles, Here the air density is only. ©he 'two-hundred-and -fiftieth part of. its density fat sea-level. In' composition the'atmosphere vtaries somewhat.'with ’altitude. Very little •wjater-vapour reaches levels - beyond ttbree ’or four miles. The proportion of iga&es is four parts .(nitrogen) to one i(oxygen), water-vapour being generally la'b out one per cent. Other gases are present in very small quantities, chief ’among these, being carbon dioxide. These igaees are quite independent of eaen fother-;' they are mixed only after the (mahaier. in whibh "sand and sugar are Bqid) to' mix.' They can be easily separJated/aghin. . Nitrogen has the effect of toning down' the oxygen.; Without nitrogen, in an atmosphere of oxygen, as iia sometimes pointed out, “not only •would our fires bum, but also our fireplaces- and- everything' else, and we our* istelves would ’ have' the whole activity land excitement of our entire live® packed into'a few glorious hours.” 1 Water v'apour 'and . carbon dioxide play tan'important part in determining the temperature : of'the earth’s surface, since' they are relatively transparent- to •sunrays and ' relatively opaque to the •outgoing heat rays from the earth, the final result being that the earth e temperature at the surface, is probably '.somewhat higher on the average than di> "would be: were these compounds ab•sent. Indeed, theory has it that glacial periods 'have come'about from time to •time when the earth’s atmosphere has ibee.n poor" in carbon dioxide ■ The blueness peculiar to the atmosphere is in. all probability due to the ■fact that the rays of'sunlight entering it are broken up into their primary colours by "the' tiny dust particles, suspended in ft, just as a'prism breaks up 'apd .bends the light that falls upon it. The short blue rays are the only ones .'that reach’our vision, the ruddier tints 'being diverted away. , _ 'The earth’s atmosphere refracts the IMit of the sun and other celestial objects bv amounts which increase from nil at the zenith, or point exactly over the observer’s head, to about 35 minutes of arc at'his horizon, certain modifications taking place as the result .of changes in temperature, humidity and barometric pressure. ‘Since the density ■of the atmosphere increases its outer 'limits, down'to the surface of the earth, refraction gradually' increases as light penetrates to lower and lower levels. It is this'influence which is responsible for.the false altitude of heavenly bodies. 'Until, they attain a height near the zenith ' these ‘bodies appear higher than, they; really are,, so t'h'at we see the sun and moon just above the horitepn when they.'are : as a matter of fact Ijust below it. Herein, too, lies the explanation of' the flattened appearance lof the solar and lunar discs when setting or rising. The lower. edge of the •rising sun has to pass, through a denser n'ayer of . atmosphere than the upper •edge, thus bringing about the well•known distortion of' image.