288 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Upper Atmospheric Nomenclature SYDNEY CHAPMAN * ABSTRACT It is proposed that stratosphere shall signify solely the nearly isothermal region above the troposphere; that the layer between the stratosphere and the deep temperature minimum some- what below 100 km be called the mesosphere; that the layer of rising temperature over this mini- mum be called the thermosphere. On the basis of composition, it is proposed to divide the atmosphere into the homo sphere (of substantially uniform composition from the ground up- wards) and the heterosphere (of different composition). On the basis of electron density, a correlative to ionosphere is proposed, the neutrosphere. Using pause to signify upper boundary, the stratopause, mesopause, homopause and neutropause are defined. Peak is suggested as the name for a level of maximum, e.g., mesopeak, ozone peak, E or F peak. Incline and decline are names suggested for the parts of a peaked layer below and above the peak, e.g., mesoincline (for the layer of rising temperature just above the stratosphere), mesodecline, E or F incline or decline. A "dip" in a peaked layer is called a syncline. HERE seems scope for a few additional dary, as introduced by Sir Napier Shaw in the terms connected with the upper atmos- term tropopause. Tphere. Taking stratosphere to denote the nearly iso- Various bases of characterization of different thermal region above the troposphere, its upper atmospheric regions and levels are in use, e.g., boundary, where the temperature first begins to the presence of ozone or of ionization makes it increase upwards more rapidly than is common in convenient to use the terms ozone layer (or ozono- the lower stratosphere, would be the stratopause, sphere) and ionosphere (due to Watson Watt) ; and the mesosphere would extend from this level similarly for the exosphere, the high region whence to the mesopause,2 at the level of the deep tem- there is escape of molecules from the atmosphere. perature minimum already mentioned. These terms are modeled on the names given by It is uncertain whether this usage can advan- Teisserenc de Bort, troposphere and stratosphere, tageously be applied to the ozone 3 or to the E and based on the thermal stratification first revealed F ionospheric layers, because of the indefiniteness by his sounding devices (kites, balloons). of their upper boundaries. As used by de Bort, stratosphere signified the For these layers and the mesosphere, the de- nearly isothermal region above the troposphere; fining characteristic (ozone density, electron den- in contrast to H. Flohn and R. Penndorf,1 I would sity, temperature) first increases upwards and prefer to restrict the term to this original mean- then decreases. I suggest that in each case the ing, despite its occasional use in recent years for level of maximum be called the peak, e.g., the E any level above the troposphere. and F peaks, and the mesopeak (this word, though Extending this thermal classification, I propose hybrid, seems more acceptable than the fully Greek the name mesosphere for the layer between the word mesoacme). It is a matter of speculation top of the stratosphere and the major minimum of whether or not the thermosphere has a thermo- temperature existing somewhat below 100 km (the 2 exact level is still uncertain) ; and the name ther- In reference (1) this is called the "upper tropopause," although the name "upper troposphere" is not given to mosphere for the layer of upward increasing tem- what is here (para. 9) called the mesodecline, and al- perature above that level; this layer should, I sug- though "stratosphere" is used for the layer extending up to this level. Instead of mesodecline ^ the name "upper gest, extend beyond the next temperature maxi- mixing layer" is used, based on considerations of com- mum on up to the next temperature minimum, if position, although the layer is defined on a thermal basis, such a maximum and minimum exist. and although present evidence indicates that mixing is effective throughout the whole region from the ground to Like Flohn and Penndorf, I would advocate a level above the mesopeak (para. 8) ; see K. F. Chackett, extended use (though in a manner different from F. A. Paneth, and E. J. Wilson, "The chemical composi- theirs) of the term pause to signify upper boun- tion of the stratosphere at 70 km height," Nature, 164: 128, 1949. (It may be remarked that the samples re- * Research Associate, California Institute of Technol- ferred to in this publication probably represent air typical ogy, under Signal Corps Project No. 24-172B; on leave of a level a little above 60 km rather than 70 km.) from Oxford University. 3 In reference (1) the mesopeak is called the "ozono- 1 H. Flohn and R. Penndorf, "The stratification of the pause," but it is doubtful whether the rather indefinite atmosphere," Bull. Amer. Met. Soc., 31: 71-77, 126-130, upper boundary of the ozone layer should be identified with the level of maximum temperature in the mesosphere. 1950. Unauthenticated | Downloaded 10/02/21 06:37 PM UTC VOL. 31, No. 8, OCTOBER, 1950 289 terially ; this level would be called the homopause. The name heterosphere is proposed for the over- lying region of different composition. The name homosphere may not need frequent mention, but for some years discussion is likely to remain ac- tive as to the level of the homopause, and as to the nature of heterospheric air (at higher levels), which means a very different gas from that at ground level. Similarly a term correlative to ionosphere is proposed, to provide a complete characterization of the atmosphere on the basis of electronic pres- ence or absence: the name neutrosphere is pro- FIG. a. FIG. b. posed for the region below the ionosphere, where the concentration of electrons is insignificant peak. The ozone layer can be considered under (apart from thunderstorms or meteor trails), at two aspects, both important, namely, absolute con- least from the standpoint of the radio physicist; centration or density, and relative concentration and where the air particles are almost all neutral, (the ratio ozone to air, by volume) ; these have far more completely so than in the ionosphere. different levels of maximum, which I suggest The transition level between the neutrosphere and should be called the absolute ozone peak and the the ionosphere is the neutropause, a word more relative ozone peak, respectively. likely than neutrosphere to be often needed, e.g., The parts of such "peaked" layers which lie "the neutropause is lowered during a solar flare." below and above the peak, where the defining The various layers or "spheres" are of course characteristic is respectively increasing and de- not exclusive, nor are they co-terminous; the ceasing upwards, may conveniently be called the ozone layer includes the troposphere, stratosphere incline and decline, e.g., mesoincline, mesodecline, and at least part of the mesosphere; the D layer ozone or E or F incline or decline; and the usage probably overlaps the mesodecline, the ionosphere may also be extended to thermoincline, although probably includes the whole of the thermosphere we do not know whether or where there is a and heterosphere (which probably have different thermopeak. lower borders, the mesopause perhaps being below If at some times and places a layer has two the homopause) ; in addition, the ionosphere prob- peaks (major and minor), the region between ably overlaps the mesosphere and homosphere. them, containing a minor minimum, may be called "Upper atmosphere" is a useful term, but its a syncline. Some rocket flights have suggested meaning depends on the context, and it is prob- the presence of an ozone syncline, and some rocket ably not convenient to limit its meaning too defi- data on upper-air temperatures have indicated a mesosyncline, though in both cases there is some doubt as to their reality. It may be useful also to classify atmospheric levels on the basis of composition. The main causes tending to non-uniformity of composition are diffusion (countered by turbulence) and photo- dissociation (countered by recombination). As long as they modify the composition only very slightly (e.g., in regard to rare constituents like ozone), the scale height of the atmosphere is sim- ply proportional to the absolute temperature (if the variation of gravity with height is neglected). The temperature-scale-height relation becomes more complicated where the composition changes materially with height. The name homosphere is suggested for the part of the atmosphere from the ground up to the level (probably about 100 km) where the composition first begins to change ma- FIG. C. Unauthenticated | Downloaded 10/02/21 06:37 PM UTC 290 BULLETIN AMERICAN METEOROLOGICAL SOCIETY nitely; the weather forecaster may use it to mean are very tentative; the scales of height h are not the stratosphere and perhaps part of the tropo- the same in the three diagrams, and all three scales sphere, whereas to the radio physicist it may sig- (h, T, ne) may be non-uniform. nify a region above the stratosphere as here In conclusion I should like to support C. T. defined. Elvey's proposal 4 of the name air glow (suggested Precision may be gained while retaining brev- by O. Struve) to signify the light emitted by the ity, in referring to different atmospheric levels, atmosphere, other than the aurora (and light- by using upper, middle, or lower in conjunction ning). Ordinarily airglow will signify the (non- with the specialized names of the layers or sub- auroral) light of the night sky, but for further layers ; e.g., one may say that only the middle part distinction one may call this the night airglow, in of the mesosphere has a temperature above 0°C, contrast to the twilight (sunset or dawn) airglow, or that the E decline may be in the lower hetero- and to the day airglow, which should now be ob- sphere.