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hours when there is precipitation at points in the entire San Juan area is considerably greater than when there is precipitation at any single station. It does rot seem unreasonable, therefore, to estimate that there are at least 700 to 800 hours of precipita- tion in the San Juan Mountains during an average winter. Investigations are in progress to refine the above estimates and extend them in order to determine the number of hours that can be expected for various meteorological conditions with varying degrees of suitability for weather modification operations. b. Daily precipitation The average wintertime precipitation (November - April) between November 1953 and April 1960 at Climax was 14.14 inches. Since the average number of days with precipitation during this interval was 85, the mean daily precipitation on days with precipitation was . 17 inch. The average of 85 days, with snowfall per winter season amounts to 47%, or almost half, of the 181 days during the November - April period. 35% of the days with precipitation had. 06 inch or less, 500/0 had. 11 inch of less, 800/0 had. 26 inch or less, and the extreme value recorded was. 95 inch. 90% of all the daily precipitation events during this interval were in the range from. 03 to .50 inch. While Climax is representative of much of the area that accumulates snowpack in the northern Colorado Rockies, many higher elevations and areas exposed to stronger orographic influences receive total winter snowfall of nearly double the Climax amount. The additional snowfall at thes e locations results both from additional days of snowfall and from somewhat greater daily amounts. The general pattern of light snowfall with precipitation occurring on about half of all winter days is representative, however, of all important snow-producing areas of the northern and central Colorado Rockies. This can be seen from Table XI. This shows the monthly, seasonal, and 3-year average precipitation and the number of precipitation days for Berthoud Pass, Fraser, Grand Lake, and Hot Sulphur Springs. All of these stations are in the Upper Basin of the Colorado River. Berthoud Pass is in one of the higher precipitation areas. Most daily snowfalls, even in the heavier snow areas, are less than. 20 inch per day with daily totals greater than 1 inch very infrequent. The three season (1964-67) record of days with greater than. 40 inch per day for Berthoud Pass is shown in Table XII. It can be noted that there :were four days in three years with daily precipitation greater than 1 inch, the greatest being 1. 27 inches. It can be noted further that there were only nine days in the three years (average of 3 per year) with daily precipitation as great as .75 inch per day. In contrast to the northern part of the Colorado River Basin, the San Juan Mountains on occasion receive heavy ds..ily precipito..tion amounts which result from storms moving ~n from the south- I west. Many mountain stations in tpat area can expect daily amounts of precipitation in excess of 1 inch several times each winter. In the !higher precipitation areas, precipitation amounts excefding 1 inch can be expected on the average from 3 to !6 times each winter season. Daily amount s in excess bf 2 inches can be expected at some of these stations:on an average of to 3 days each winter and occasiodally daily amounts in excess of 3 inches are reported~ The relative importance of the few days of hea.Jy precipitation is reflected in Figure 35 where it car!). be noted that slightly over 30% of the total seas~nal snowfall occurs on only 10% of the heavier snowfal~ days. Despite the fact that a few heavy precipitationjdays occur each year, there are a relatively large ;number of days with light precipitation. This can also Ibe noted in Figure 35 by observing that 50% of the precipitation days contribute less than 20% of the totfl snowfall. To summarize this i section on daily snowfall, approximately 50% of the days each winter have snowfall at mountain elevations above 9,000 to 10, 000 ft at anyone location. Daily precipitation amounts in general are light, with daily averages being under. 50 inch on some 80% to 90% of all days. Precipitation amounts in excess of 1 inch can be expected on a few days each winter in the San Juan Mountains and occasionally at the ihigher precipitation areas in the northern portion of the Basin. c. Storm duration The duration of individual storms affects both the planning and oper~tions in a weather modification program. An arbitrary definition has been used to identifv storm days, . and on this basis, ," . I Table Xl[ has been prepared to show the frequency of storms of different duration. The storm day has been defined as a day on which precipitation of at least. 01 inch of precipitation was measured at three or more of the four hourly recording stations in the San Juan area. Using this definition, there were ap- proximately 80 storm periods and 162 storm days in a set of three years' data from November 1964 through May 1967. Since the 7-month, November-May period contains 636 days for the 3-year period; 162 storm days constitute approximately one-fourth of the total 636 elapsed days. Most storm periods are 3 days or less. These include 72 of the 80 !3torm periods shown in Table XIII. Less important storm days not included Uc-1Table ALII indicate there were ,47 additional days which had precipitation recorded at only 2 of the 4 stations during the 3 seasons. There were an addi- tional 64 days when precipitation ,occurred at only one station. d. The areal extent of prec,ipitation during an episode , General storms pr,oducing many hours of precipitation in an area frequently do not have coincident precipitation at stations separated by only a short distance. The hours dur~ng which coincident precipitation occurs at 3 or more of the recording stations in the San Juan area constitute less than half of the hours of total precipitation. Ouray, Telluride, and Silverton are less than 20 miles apart in the higher mountainous area and Durango is approximately 40 miles from the other three and to the south of the barrier. Ouray has an upslope , 46