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Start Times of Storms Knowing the time of day when stor.ms occur is important since traffic compounds problems of snow removal and enployees sometimes work overtime when snow falls in the early evening or overnight. Observations are not taken frequently enough at most cooperative observer sites to determine time of fall. Researchers have studied time of fall at several sites in Colorado, however, (Grant, ed., 1969; Crow, 1969) and their findings, discussed below, are used to assign start times of each storm. Figures 1-4 show variations in times of recorded snowfall at Hot Sulphur Springs in central Colorado and at Ouray, Durango and Silverton in the southwest. A marked peak in frequency during the early morning followed by a dip around noon is typical at Hot Sulphur Springs and at other stations in the north and central parts of the state (Grant, ed., 1969). The pattern at Ouray is somewhat similar. But in Durango, snow is more likely to fall in the morning and less likely to fall in the late afternoon and evening, and in Silverton snow is most likely to fall in the late afternoon. Grant, ed., (1969) attributes these differences to exposure--that is, to the location of observation stations relative to surrounding mountains. We used the probabilities of occurrence of snow depicted in Figures 1-4 to assign a start time to each stor.m randomly. The Hot Sulphur Springs variation was used for counties :in north and central Colorado; Ouray, Silverton and Durango variations were used for Ouray, San Juan, and La Plata counties, respectively. We assumed that the probability of storms starting at a given hour is, in the long run, identical to the probability of snow falling during the same hour. If, for exarrple, snow at a given station is twice as likely in the early morning as in the early afternoon, the model assigns start times that reflect this pattern. -11-