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H TRAJECTORIES;; - .,.. ~ SNOWFLAKES ,\NO RAINDROPS' 'S6 " e " \ , --~ FIGURE 2-3.-8chematic illustration of spill-over. liquid or solid. In addition, as snow formed at hig-h altitudes, melted, and turned into rain, there would be a zone in which the trajectories would have a rather sharp change in shape. FaIling rates of raindrops are fairly well known, and while little is known about those of snowflakes, they are undoubtedly much less. The actual wind flow over the usual orographic !:mrrier would not be anywhere near as smooth or uniform as that de- picted in figure 2-3. An actual profile across the Sierra Nevada (fig. 2--4) gives a good indication of the degree of generalization inherent in com- putations of orographic precipitation based ou simplified wind-flow patterns. 2.3.10 The extreme distance for spill-over of heavy rainfall in the Sierra Nevada from 01'0- g-raphic effects alone is estimated to be roughly 10 miles. The storm precipitation distribution across a barrier has never been measured accurately, but it probably varies widely from storm to storm, particularly for short durations and small areas. 2.3,ll In an attempt to determine the oro- graphic effects on precipitation rates, the maxi- mum observed clock-hour and 24-hr. precipitation for stations on the western slopes of the Sierra Nevada in California were plotted against the station elevation (figs. 2-5 and it-6, respectively). The data. are from recording-gage stations having at least 8 years of record between 1940 and 1951. Fig-ure 2-5 shows that, within the range of ob- served data, maximum clock-hour precipitation is very poorly related to elevation. In other words, ..000 , , ~ 8000 -1 , ~ 6000 ,~ 0, 60 00 100 DISTA'<C' F"OM SAW J04<lU,N RIV'R IMIUSl ,~ FIGURE 2-4.-Topographic profile across Sierra Nevada from 36049' X., 120022' W. to 38022' N., 118015' W. the plot suggests that it can rain as hard for one hour at a low elevation as it can at a high eleva- tion. Figure 2-6, on the other hand, shows a slight tendency for maximum observed 24-hr. pre- eipitation to be higher at the higher elevations, although the eorrelation is admittedly poor. A similar plot (fig. 2-7) of maximum observed ob- servational-day precipitation for Colorado sta- tions west. of the Continental Divide also shows a slight tendeney for higher values at higher eleva- tions. Here again, however, the correlation is poor. 2.3.12 Comparison of figures 2-6 and 2-7, which are for regions of eomparable orography, reveals that the latter shows mueh lower preeipita- tion values level-for-Ievel than does the former. Obviously, other factors besides elevation and slope affect preeipitation rates. The various fae- tors governing availability of moisture were dis- eussed in section 2.2. Distance from a moisture source was one of the factors mentioned. How- ever, reduetion of atmospheric water vapor with distance from the moisture source, as observed in the Plains Region, for example, is much too gradual to account for more than a small part of the difference between California and Colorado storm preeipitation indicated by figures 2-6 and 2-7. Neither eould the difference be eXplained on the basis of latitudinal or seasonal variations in atmospherie water-vapor eontent. Current kuowledge of storm meteorology is admittedly limited, but what little is known suggests no great differenee in the preeipitation-producing e/lieieney of storms in these two regions. 2.3.13 It would appear from the preeeding paragraph that there is no known explanation for 9