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fell at night, between 2100 and 0300 MST, reaching a minimum of 8.3 percent during the period from midnight to 0300 MST. When these curves were compared to a similar analysis done by Super et ale (1986), some notable differences were revealed. In Colorado, the frequency of precipitation occurrence at any hour was much higher than in Arizona, ranging from a low of 22.4 percent at 2300 MST to a high of 33.0 percent at 0700 MST. This would indicate that winter precipitation occurs much more often on the Grand Mesa than it does in the vicinity of the Mogollon Rim in Arizona. This result was not a surprise for one would expect that the Grand Mesa, lying some 700 kilometers north of the Mogollon Rim, should be within the path of a larger number of winter storm systems moving across the United States. The amount of storminess is also likely influenced by topography. With a top near 3260 meters, the Grand Mesa rises abruptly some 1900 meters above the sur- rounding terrain affording substantial lifting of the air passing over it. The lower elevation Mogollon Rim, at about 2450 meters, does not tower above its' immediate surroundings but rises more gently both to the east and the west offering less lift to the air passing overhead. In Colorado, the frequency of occurrence curve peaked near sunrise (0700 MST) with generally higher frequencies during the night-time hours (between 2300 and 0700 MST). In Arizona, this period was shifted by approx- imately 12 hours with the most frequent precipitation occurring during the daylight hours. This fact may be another indicator that localized convection may playa more significant role in the Arizona precipitation regime than in Colorado. Like the frequency curve, the percentage of total precipitation plot in the Colorado data showed the hours contributing the most precipitation came at night, peaking around 0300 MST. 3-39 I i I I I I I t t I i I I I I I 1 I I