tive drops are carried upward and ond in order to build up the away from the larger positive drops charge at the rate which actually by the air-currents in the cloud; thus, occurs. the upper part and the trailing part A theory proposed some years ago of the cloud should on that theory con- by C. T. R. Wilson, in which electrifi- tain most of the negative charge. The cation by the "breaking-drop" process average height of the negative part is not invoked, would require that the may be expected to be as great as or positive charge be located in the upper greater than that of the positive. How- part of the cloud. The small drops ever, measurements of the direction of are positively and the large are nega- the electric field-changes under thun- tively charged. The large drops fall der-, which have become more faster than the small ones and the re- abundant in recent years, indicate that sult is a positive charge in the upper the upper part of the cloud is gener- part and a negative in the lower part ally positive. Nine-tenths of the meas- of the cloud. Laboratory experiments urements made in South Africa, most- indicate that this process is probably ly during "dry storms," require the capable of separating charge in the positive charge to be about two kilo- cloud at the required rate. A mechan- meters higher than the negative. Fur- ism by which larger drops become pre- thermore, the positive charge appears dominantly negative has been sug- to be at an altitude of four to six gested by Wilson and recent labora- kilometers, where the breaking-drop tory tests support that suggestion.— process may scarcely be expected to O. H. G. take place. Registrations show that Discussion—Professor John Zeleny the charge of a thunder-cloud is gen- mentioned a case of a -stroke erated at the rate of about twenty photographed during a volcanic erup- coulombs in five seconds. Calculations tion in the Philippines which was ap- based on the size of drops and quan- parently 100 feet wide. He also stated tity of water in the active part of the that laboratory experiments which he cloud, together with the value found has been making indicate that the spe- in the laboratory for the quantity of cific charge which is developed on charge developed on each drop which breaking drops depends quite appre- is broken would require that each ciably upon the speed of the air blast available drop be broken up every sec- which is used for disrupting the drops.

AN INTRODUCTION TO THE STUDY OF AIR MASS ANALYSIS By JEROME NAMIAS II. CONSERVATIVE PROPERTIES OF AIR MASSES.1 An air mass is defined as an exten- main fairly constant at any given sive body of air which approximates level. These large scale currents of horizontal homogeneity. The proper- air have their origin at a source re- ties of the air mass which are con- gion—a large area characterized by sidered in this homogeneity are mainly sameness of surface conditions and and moisture. Thus over evenly distributed insolation. Thus the the earth's surface one may observe northern part of Canada in winter large currents of air within which the may be considered as a source region temperature and moisture content re- in that it is practically -covered, and the amount of insolation received 1 This is the second article of the series be- gun in the Aug.-Sept. BULLETIN, pp. 184-190. is almost evenly distributed over the

Unauthenticated | Downloaded 09/30/21 10:08 PM UTC entire area. A large body of air sentative of these properties. The which remains over a source region a most representative observations are sufficient length of time assumes cer- those made by means of upper air tain definite properties in the vertical, soundings; at the surface the most particularly with respect to tempera- representative observations are those ture and moisture. Once these prop- made at elevated and exposed sta- erties have been attained, equilibrium tions. Examples of non-representa- with respect to the source region is tive observations are those of valley reached, and any further stagnation stations, or those greatly affected by or movement of the body of air over the proximity of a lake or perhaps a the source region will not appreciably mountain barrier. In the latter case affect the balanced distribution. It is there might be an appreciable foehn clear, however, that any movement of effect. the air mass away from its source As an air mass progresses numer- region will result in a modification of ous changes take place, brought about its properties. For example, if a body by radiation, mixing (turbulent ex- of air from northern Canada moves change), adiabatic expansion or com- southeastward into the United States, pression, and or evapor- there is bound to be some warming ation. Some meteorological elements and moistening. Such modifications will remain more constant than others tend to destroy the homogeneity orig- as the air mass moves from point to inally established at the source re- point. Furthermore, quantities indi- gion. It is obvious that the modifica- rectly obtained by calculation from tion will take place essentially within the observations will vary in con- the lowest layers of the air mass, the stancy. The relative degree of con- upper layers being modified only grad- stancy of a meteorological quantity ually by means of the indirect pro- within a moving air mass is defined cesses of mixing with the modified as its conservatism. low layers and by radiation chiefly We are now in a position to test from the surface of the new region the meteorological elements and the over which the air mass is traveling. indirectly calculated quantities with The theory of air masses as entities respect to their degree of constancy is based upon the fact that the varia- (conservatism) as the air mass moves. tions of any property in the horizon- tal in an air mass are small compared A. TEMPERATURE. with the rapid change of properties The temperature of any given par- observed at the boundary between two ticle of air within a moving air mass air masses which come from different is influenced by the following factors: source regions. This boundary zone 1. Conduction and mixing. of rapid transition is a front. 2. and condensation. From the definition of an air mass 3. Expansion and compression it is clear that in order to identify (adiabatic changes). sections of a current as belonging to 4. Insolation and radiation. one and the same air mass we must Temperature, particularly at the not only know the properties at the surface, is so much changed by these source region and the modifications in- factors that it cannot be regarded as troduced, but we must also deal with a very representative element by observations which are most repre- which to identify an air mass after

Unauthenticated | Downloaded 09/30/21 10:08 PM UTC it has moved away from the source This was pointed out in the first art- region. icle of this series. During ascent, The effect of evaporation and con- therefore, the densation may be eliminated by the of the saturated particle will increase use of a quantity called equivalent by virtue of the of con- temperature. This is defined as the densation. temperature a particle of air would The most conservative thermal have if it were made to rise adiabati- quantity is the equivalent potential cally to the top of the atmosphere in temperature. This is the tempera- such a manner that all the heat of ture the chosen air particle would condensation of the were have if it were brought adiabatically added to the air and the sample of to the top of the atmosphere so that dry air were then brought back to its along its route all the moisture were original . Numerically this condensed (and precipitated), the la- is not much different from the tem- tent heat of condensation being given perature the mass of air would have to the air, and then the remaining dry if all its moisture were made to con- sample of air compressed to a pres- dense and the heat given off by con- sure of 1000 mb. The equivalent po- densation were added to the remaining tential temperature may also be de- dry air. Any change in the moisture fined as the potential temperature of content of the air mass by evapora- the ; that is, it tion or condensation will not affect the can be determined by finding the equivalent temperature of a particle, equivalent temperature, then reduc- since the quantity of moisture added ing this adiabatically to a pressure of by evaporation or subtracted by con- 1000 mb. The equivalent potential densation involves a certain loss or temperature combines the processes gain of heat which is implied in the involved in the definition of the po- definition of equivalent temperature. tential and the equivalent tempera- Changes in the temperature of a ture; hence it is independent of any particle by means of expansion or effects due to expansion or compres- compression (adiabatic changes) may sion as well as condensation and evap- be eliminated by the use of the poten- oration. If we deal with the equiva- tial temperature. Potential tempera- lent potential temperature of a par- ture is that temperature a parcel of ticle then the only processes which air would have if it were brought change its value are (1) conduction adiabatically to a pressure of 1000 mb. and mixing, and (4) insolation and If a dry particle were vertically dis- radiation. (1) and (4) obviously placed it would warm or cool at the have their maximum effect in the sur- dry adiabatic rate, and therefore its face layers, and are probably much actual temperature would differ from less important at higher levels. There- its potential temperature by about fore the thermal properties of an air 1 C deg. per 100 meters from the mass are far more conservative at level where the pressure is 1000 mb. upper levels than in the low layers. This holds only in the event that the Consequently it is best to use upper air particle remains unsaturated, for air data rather than surface observa- as soon as it becomes saturated, latent tions as criteria for the determination heat of condensation is realized and and identification of air masses. the particle no longer follows the dry B. . adiabat, but the saturated adiabat. The lapse rate associated with an

Unauthenticated | Downloaded 09/30/21 10:08 PM UTC air mass is frequently a good index D. CONDENSATION FORMS. for identification purposes. As in the The type of cloud formation is case of thermal quantities, the lapse largely a result of the vertical dis- rate is much more conservative at tribution of temperature (the lapse upper levels. In the surface layers rate) and moisture. Since both these the lapse rate will be found to vary quantities are fairly conservative, it appreciably from day to day, and follows that certain condensation from nighttime to daytime. These forms are more or less characteristic effects are mainly the result of radia- of each type of air mass. Care must tion and turbulence. For example, in be exercised in differentiating be- the early morning hours there is apt tween clouds formed within an air to be a ground inversion, while dur- mass and those formed by the inter- ing the afternoon an adiabatic lapse action at the front between two dif- rate may extend up to about 800 ferent air masses. In addition, local meters. At higher levels surface ef- condensation forms, such as ground fects are comparatively small, but , must be eliminated. there are times when the lapse rate E. . aloft changes because of extensive The visibility in the lower layers of rising or sinking movements. In spite the atmosphere is generally an indi- of variations in the lapse rate in the cation of the lapse rate therein. If surface layers, the lapse rate is often the lapse rate is stable, then indicative of the trajectory of the air and tend to remain close to the current, for when moving over a cold surface; if the lapse rate is steep surface the low layers of the current then vertical motion is easily possible tend to become more stable, whereas and the foreign matter diluted by be- when constantly moving over a warm- ing mixed throughout a layer of con- er surface the lapse rate becomes siderable thickness, thereby increas- steeper. Characteristic types of ing the visibility in the surface layers. clouds are commonly associated with In cold masses which are moving certain lapse rates. over a much warmer surface a steep C. . lapse rate is soon established and vis- In an article entitled Specific Hu- ibilities become good. On the other midity as a Conservative Element, in hand, when a warm current moves the January, 1934, BULLETIN, the over a much colder surface the marked writer pointed out that of the four stability keeps the foreign matter quantities, , relative concentrated in the lower layers mak- humidity, absolute humidity, and spe- ing the visibility poor. As an index cific humidity, the last named is by of the air mass present, visibility far the most conservative. If the must be used with considerable care, reader is not familiar with this ar- since there are numerous factors other ticle, it is suggested that he read it than turbulence affecting visibility. at this point. Specific humidity, be- F. DIRECTION AND VELOCITY. ing, by definition, the mass of water and velocity are in vapor per unit mass of air, remains themselves not very conservative ele- constant with vertical displacement ments. Polar air masses are fre- providing no condensation, evapora- quently observed with having a tion, or turbulent exchange of mois- southerly component, and tropical air ture takes place. masses not infrequently have sections

Unauthenticated | Downloaded 09/30/21 10:08 PM UTC in which the wind may blow from the following and identifying air masses. northwest. This is true particularly Of these, the equivalent potential tem- in upper levels. In the placement of perature is the most conservative, fronts, however, winds are very im- combining the conservative qualities portant. Some of the fundamental of both the potential temperature and concepts of this phase of the problem the specific humidity. The above dis- will be taken up in a later article in this series. cussion, necessarily brief and sketchy, Of the six elements named above, will enable us to take up the Rossby- the thermal and hygrometric quanti- diagram in the next article of this ties are the best indices to be used in

Further Suggested Reading in English on Air Mass Analysis J. BJERKNES and M. A. GIBLETT, Retrograde Depression in the Eastern United States, Monthly Review, Vol. 52, 1924, pp. 521-527. C. K. M. DOUGLAS, Further Researches Into the European Upper Air Data, Quart. Journ. Roy. Met. Soc., Vol. 50, 1924, pp. 339-363. , The Physical Process of Cloud Formation, ibid., Vol. 60, 1934, pp. 333-334. The Problem of Rainfall, ibid., Vol., 60, 1934, pp. 143-152. H. R. BYERS,The Air Masses of the North Pacific, Scripps Inst, of Oceanography Bulletin, Technical Series, (in press). W. C. KAYE and C. S. DURST, Some Examples of Development of Depressions Which Affect the Atlanticy Quart. Journ. Roy. Mfet. Soc., Vol. 58, 1932, pp. 151-164. S. PETERSSON, Kinematical and Dynamical Properties of the Field of Pressure With Applications to Forecasting, Geofysiske Publikasjoner, Vol. 10, No. 2, Kr. 9.00. I. P. KRICK, Foehn Winds of Southern California, Gerlands Beitrage z. Geo- physik, Vol. 39, 1933, pp. 399-407. H. C. WILLETT, Synoptic Studies in , M. I. T. Met. Papers, Vol. 1, No. 1, 1932, 50 cents. J. NAMIAS, Subsidence Within the Atmosphere, Harvard Meteorological Studies, No. 2, 1934, Harvard Univ. Press, Cambridge, Mass., 85 cents. K. EARL and T. A. TURNER, A Graphical Means of Identifying Air Masses, M. I. T. Miet. Course, Prof. Notes, No. 4, 1930, out of print. H. B. HUTCHINSON, A Fog Situation in the United States During the Winter of 1928-29, M. I. T. Met. Course, Prof. Notes, No. 3, 1930, out of print.

PAPERS AND ABSTRACTS OF THE BERKELEY MEETING, JUNE, 1934 The following papers presented at California. R. C. COUNTS, JR. (Un- the Berkeley Meeting, June, 1934, assigned). A study of the reduction of baro- have already been published or ac- metric pressure over the plateau to cepted for early publication in the the 5,000-foot plane. D. M. LITTLE and Monthly Weather Review (1934) : E. M. VERNON (May). Local winter air mass displace- Long period fluctuations of some ments in California. A. W. COOK meteorological elements in relation to (February). California forest-fire problems. L. G. The Los Angeles area excessive GRAY (July). and flood. L. H. DAINGERFIELD Further conclusions from additional (March). observations in the free air over San averages for the state Diego. D. BLAKE (June). of Washington as affected by habita- The diurnal variation of ceiling bility. L. C. FISHER (July). height beneath Stratus clouds. E. M. Winter fogs in the Great Valley of VERNON (Unassigned).

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