tive drops are carried upward and ond in order to build up the cloud 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-clouds, 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 lightning-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 temperature 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 snow-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 condensation 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. Evaporation 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 potential temperature densation may be eliminated by the of the saturated particle will increase use of a quantity called equivalent by virtue of the latent heat 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 water vapor 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 pressure. 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 equivalent temperature; 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.