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<br />, <br />" <br /> <br />7 <br /> <br />T ABLE II <br /> <br />Distribution of convective summer rainfall in west Texas by radar echo type, 1978-80 <br /> <br />Type of echo <br /> <br />Frequency of occurrence <br />(%) <br /> <br />Rainfall contribution <br />(%) <br /> <br />Cell <br />Small cluster <br />Large cluster <br />Nested cluster <br />Unclassified <br /> <br />3 <br />34 <br />41 <br />19 <br />3 <br /> <br />0.1 <br />4.3 <br />27.5 <br />67.9 <br />0.2 <br /> <br />Total <br /> <br />100 <br /> <br />100.0 <br /> <br />After Jurica et al. (1981). <br /> <br />T ABLE III <br /> <br />Average lifetimes of cumulus clouds <br /> <br />First pass liquid <br />water concentration <br />(g m-') <br /> <br />Lifetime to <br />O.lgm-' <br />(min) <br /> <br />Number of <br />clouds <br /> <br />Yellowknife, N.W.T. <br />1975 0.50 >20 8 <br />1976 0.40 12 35 <br />Thunder Bay, Onto <br />1977 0.88 8 15 <br />1978 1.44 6 23 <br />Miles City, MT <br />1979 0.64 12 11 <br />1980 0.34 3 10 <br />After Schemenauer et al. (1981). <br /> <br />20 HIPLEX-l clouds. However, the Canadian results on cloud lifetimes at <br />Miles City are supported by detailed studies of the evolution of HIPLEX-l <br />clouds by Cooper (1981). <br />Table III shows that some clouds with significant liquid water concentra- <br />tions on a first aircraft pass have less than 0.1 g m-3 of supercooled cloud <br />water left 5 min later. As the time required for the Bergeron Process to pro- <br />duce precipitation from ice crystals is 10 to 15 min, it is not surprising that <br />many isolated cumulus clouds fail to produce precipitation. Table IiI ..uso <br />shows interesting variations in cloud behavior with location and from one <br />summer to the next. <br />