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The Bulletin American Meteorological Society THE BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY Entered as second class matter September 24, 1945, at the Post Office at Lancaster, Pennsylvania, under Act of Aug. 24, 1912. Acceptance for mailing at special rate of postage provided for in the Act of Feb. 28, 1925, embodied in paragraph 4, section 538, P. L. and R., authorized Sept. 24, 1945. Published monthly except July and August. PUBLICATION OFFICE. American Meteorological Society, Prince & Lemon Sts., Lancaster, Pa. EDITOR. Robert G. Stone, P.O. Box 2061, South Station, Arlington, Va. SECRETARY. Charles F. Brooks, American Meteorological Society, Milton 86, Mass. TREASURER. Ralph W. Burhoe, Milton 86, Mass. Annual subscription, $3.50; single copies of this issue, 35^. Vol. 27 January, 1946 No. 1 Comparison of Lower-Cloud Bases with Condensation Levels" MAJOR R. C. BUNDGAARD, A.C. Hq. AAF Weather Service, Langley Field, Va. N IMPORTANT aeronautical requirement shore winds. Col. Floyd B. Wood 3 showed A is information concerning the existing that for stratus clouds formed by convec- height of the lower-cloud base. Since tion current within a turbulent layer the ob- direct measurement^ have been infrequent, served cloud base is higher than the lifting and visual estimates are inaccurate, the condensation level of the ground surface air meteorologist has been obliged to evaluate and is lower than the mixing condensation the heights of clouds as the condensation level4 for a completely stirred layer be- level for tlie layer of air near the ground neath the turbulence inversion. Binder,5 surface, using the easily observed ground- comparing the observed heights of lowest surface temperature and humidity.1 stratocumulus clouds with heights of the Numerous investigators have considered lifting condensation level in the interior to what extent there is agreement between United States, found that the ratio of com- the observed cloud base and the several puted to measured heights averaged .74, a computed condensation levels. McAdie,2 value distinctly higher than the .60 value comparing calculated and measured values for Britain given by Dines.6 Recently, for 80 observations made at Valencia, Ire- Jaeckisch 7 prepared a comparative investi- land, found that the heights of the lifting gation, pertaining to the methods of com- condensation level were generally lower puting the condensation level. Using 18,808 than the heights of the observed lower-cloud comparisons between the observed lower- limit of the stratified clouds formed in on- cloud limits and the computed condensations * Published with permission of the War Department. 3 Wood, F. A.: The Formation and Dissipation of 1 For every degree of dewpoint spread, 400 feet of Stratus Clouds Beneath Turbulence Inversions, Bul- lifting is required for the air parcel at an isobaric sur- letin of the American Meteorological Society, Vol. 19, face to reach its condensation level, provided that no No. 3, March 1938, page 98. mixing occurs between the parcel and the air through 4 Actually, the only condensation level heights which which it is being lifted. From 1000 mbs to 500 mbs can be computed from ground surface observations are the correction to this rule is: for the lifting condensation level. 5 Binder, R. C.: Estimating Cloud Heights from Dewpoint (°C) — 50 —40 — 30 — 20 — 10 Humidity Observations, Bulletin American Meterologi- Correction % — 2.4 — 1.6 — 0.6 + 0.6 + 1.6 cal Society, Vol. 22, No. 6, June 1941, page 256. 6 Dines, L. H. G.: Humidity Observations as an Dewpoint (°C) 0 10 20 30 Aid in Estimating Cloud Heights, Meteorological Correction % +3.0 +4.5 +6.4 +8.8 Magazine, Vol. 56, 1921, page 226. 7 Jaeckisch, Hans: Vergleichende Untersuchungen tiber 2 McAdie, A.: A Quick Method of Measuring Cloud Die Methoden Zur Berechnung Des Kondensations- Heights and Velocities, Annals of the Astronomical Ob- niveaus, Annalen der Hydrographie und Maritimen servatory, Harvard College, Vol. 83, 1919, Part IV. Meteorologie, Vol. 72, 1944. Unauthenticated | Downloaded 09/28/21 09:27 AM UTC levels based on different methods, he at- temperature T0 and dewpoint r0) and the tempted to determine whether one method pressure level P2 at which convection con- deserves preference and, if so, under what densation should occur for the particle of conditions. The comparisons were selected surface air which first has been heated or classified according to all possible com- (Ta — T0) in order that it may always be binations of the following conditions: warmer than its environment while ascend- geographical location—coastal or inland; ing adiabatically through the layer from the lower cloud forms—stratus, stratocumulus, surface to the convection condensation level fractostratus, fractocumulus, and cumulus; P2. It is clear from FIGURE 2 that clouds weather situations—anticyclonic weather, may occur at two generally differing con- intermediate high-pressure situations, zonal- densation levels, the determination of which weather cyclones, meridional-weather cy- rests on two fundamentally varying con- clones, other cyclones; air masses—arctic, cepts as to the process leading to the for- temperate actic, temperate tropical, tropical, mation of the clouds. and other air masses; and type of conden- An hypothesis for the process of cloud sation level. The results from his pains- formation at the LCL is that there is Aus- taking exact investigation contribute to the tausch motion of air particles moving up- clarification of the practice for weather ward along an isentropic surface. The par- forecasting and the theory of cloud develop- ticles must have begun their ascent from the ment. The results of his findings now will ground not at the point of observation but be presented. at a distant point where the initial tempera- The assumptions that the lower cloud base ture was Tb. At this distant point the dew- coincides with Lifting Condensation Level point must have been higher by TB — T0, (LCL) or the Convection Condensation Level than the dewpoint at the point of observa- (CCL) 8 are based on different concepts of tion. If the cloud-base at LCL were com- cloud-formation. For comparison, FIGURE 1 posed of air particles that had been lifted shows pressure level Pt at which lifting from the ground surface at the point of observation it necessarily would be colder by Tb — T0 than the Observed temperature for the surrounding air—an inconceivable condition. Clouds can develop at CCL only if a dry- adiabatic lapse rate exists between the sur- face and CCL. An illustration of this proc- ess is afforded by heat cumulus clouds forming in the morning as a consequence of increased heating of the air layers near the ground surface. However, such a condition does not exist in FIGURE 1; therefore, no clouds would develop there at the time of observation from cloud particles originating at the observation point. Nevertheless, it is possible that clouds such as stratocumulus may exist at the CCL at the time of obser- FIG. 1. Computation of Condensation Levels for vation. Such clouds could have formed Lifting (LCL) and Convection (CCL). earlier as fair-weather cumulus at another condensation should occur for a particle of locality where the lapse-rate of the air mass air rising adiabatically from the surface was dry-adiabatic. When the air mass (with surface values of pressure P0, actual reaches the place of observation it may have become stable again. 8 In order to compute the CCL it is necessary to The most conspicuous features reflecting take into account the vertical distribution of tempera- the influence of the weather situation, air ture. Unauthenticated | Downloaded 09/28/21 09:27 AM UTC FIG. 2. Comparison between computed and observed cloud heights for specified con- ditions and methods of computation. The locations of the dots and crosses in the diagrams were determined as follows:—The computed heights were divided into hectometers (328 ft.). Successive intervals were selected so that they overlapped one another by two-thirds. For example, the first interval was 0-2 hectometers; the second, 1-3 hectometers; the third, 2-4 hectometers, etc. For each such interval the most frequent observed value was deter- mined. The abscissa of this most frequent value was then used as the abscissa of a dot, the ordinate of the dot being the midpoint of the interval height. Where the frequency dis- tribution was bi-modal, a cross was entered for the second highest frequency value. Unauthenticated | Downloaded 09/28/21 09:27 AM UTC TABLE I. Comparison Between Computed and Observed Cloud Heights for Specified Conditions and Methods of Computation (Figures in body of table indicate number of cases, in percent, for which there was ten percent or less difference between computed and observed heights) St+Fs All St Fs Cu Sc All +Cu Clouds Observs. LCL CCL LCL CCL LCL CCL LCL CCL LCL CCL LCL CCL LCL CCL Anticycl. weather 38 25 40 23 50 28 19 30 43 26 35 27 20 15 Intermed. high- 36 14 45 31 54 42 18 31 46 30 36 31 32 27 press. situations Zonal-weather 30 23 37 30 46 43 21 38 36 30 32 32 28 29 cyclones Meridional 57 27 55 19 50 50 13 22 55 28 44 26 40 20 weather cyclones Other cyclones 42 32 46 15 41 53 15 25 43 29 36 28 33 26 Other weather 36 22 43 33 50 46 22 30 43 33 37 32 30 25 situations All weather 37 22 42 29 50 43 20 31 43 31 36 31 29 25 situations Arctic air masses 44 23 52 33 44 49 23 30 47 37 37 35 33 30 Modified arctic 37 33 46 29 55 47 22 30 46 36 39 36 34 30 air masses Modified trop. 37 19 41 35 46 44 18 39 40 31 33 49 28 28 air masses Trop.
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