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high mountain barrier may be much less than that for a similar layer extending down to sea ]evel. . Cutting off about 7,000 feet from the bottom of a column of pseudoadiabatic satnrated atmosphere reduces W p by about one-half. 2,2.6 Ascending air may also carry a consid- erab]e amount of Vquid water in addition to water vapor. Until recently observations of liquid water content of clouds consisted of drops in samples swept out by inst,ruments in airplane probing flights. Using this method, Weickma.nn and aufm Kampe [5] and Draginis [6] showed that there is a great deal of scatter between the measurements of liquid water content and the amount computed on the basis of moist-adiabatic ascent. The maximum concentration of liquid water they observed was 10 gm./m.' in a cumu]o- nimbus cloud. To]efson [7] reported a measure- ment of 9.25 gm./m.' in a cumulonimbus cloud. Probably the highest liquid water content that could be inferred from the samples would be 3 to 4 inches. In more violent stonns than can be sampled by ai,rp]a.ne probes, where violent up- drafts are able to keep large hailstones in suspen- sion, the liquid water content may be higher, but data are lacking. 2.2.7 The liquid water content of clouds can also be measured by radar with varying degrees of precision, giving a three-dimensional integra- tion through time of the water content. Ligda [8] gives an excellent description of the manner in which weather radar operates. Briefly, radio pulses are transmitted and their echoes received and portrayed on a scope. The strength of echo is a function of the mass of liquid water which inter- cepts a.nd reflects the transmitted beam. Consid- eration is given to drop-size distribution, strength of beam, distance, attenuation, and other influ- ences. Recent and continuing work with rada,r may provide much-needed infonnation on the mechanism of condensation and growth of rain- drops. Profiles of reflectivity given by Dona]d- son [9] and Chme]a [10], when converted to amounts of liquid water, indicate that the 3 to 4 inches referred to previously is not an overesti- mate. Their observations represent a few sam- pIes of stonns in northeastern United States. If more samples were available from other parts of the country, so as to include more storms and more violent stonns, larger concentrations of liquid water aloft might be found. 2,2,8 Dona]dson and Chme]a both show that 6 3.00 2.50 '" :I: '" 2.00 , ~ z 0 ;::: '" 1.50 0- iL U "' " a. "- o. 1.00 "' ~ " .50 o ~ ~ ~ ~ ~ ro ~ W 7PF 1000 MB. TEMP. FIGURE 2-1.-Rates of precipitation from pseudoadiabat. ically ascending saturated air extending from sea level to 9 km., assuming a linear decrease of convergence with height to zero at 4.5 km. (about 15,000 ft.). the maximum reflectivity occurs at approximately 20,000 feet. In the Handbook of Geophysics for Air Force Desig1WT8 [11], Donaldson states that the maximum concentration is usually at an a]ti- tude corresponding to 1h to % of the cloud height. It is thus suggested that while the maximum con- densation may ocour at lower elevations, rising currents of ..ir in the clouds carry the drops higher and tend to hold them in suspension. The maxi- mum amount (and concentration) of liquid w..ter th..t can be held ..]oft and the mechanism (and rate) of its release from the supporting updraft ..re still ]argely matters of conjecture, pa.rticu]arly for short durations ..nd small ..reas. 2.3 Rate of conversion of moisture into pre- cipitation 2.3,1 The precipitation process w..s described in chapter 1. 'With sufficient moisture availab]e, the precipitation mte then depends on how rap- idly the moisture c..n be converted into precipit..- tion. Since high precipit..tion rotes depend chiefly on rapid cooling of adi..batically rising moist air necessarily associated with convergence and/or orographic lifting, precipitation rates