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<br />a low level inversi~n. The neutral category IS used when it is uncertain <br />as to whether the lower level of the sounding should be classified as <br />unstable or stable. These situations tend to occur in the morning hours <br />after sunrise. In two cases a very stable sounding in the vicinity of <br />700 mb was classified as stable even though there was no inversion at the <br />surface. These cases occurred during daytime. <br /> <br />Examination of Table 2 indicates that inversions were found in only <br />one-third of the soundings. However, it is evident that low level 1nver- <br />sions occur more often than as indicated by this simple statistic. Based <br />upon the data of Table 2 it is evident that inversions are likely to <br />occur after 1900 MST even in cloudy conditions. Apparently, these <br />inversions endure throughout the night and are broken only after sunrise. <br />The reasons are that the surface wind is typically weak at night and the <br />radiational factors remain virtually the same. The earliest soundings <br />(0600-0700) indicate a pn~ponderance of inversions; subsequently neutral <br />and unstable soundings are typical. <br /> <br />It is noted that these observations are made toward the end of the <br />winter season. During December and January the period of inversions <br />might be expected to be 1 or 2 hours longer due to the longer night and <br />lower elevation angle of the sun during daytime. In any case the period <br />of inversions during February and March 1981 is typically 12 hours even <br />when cloud cover is present. During the daytime hours 0900-1700 MST, <br />about one 1n S1X soundings exhibited inversions. <br /> <br />Thus, while only one--third of the soundings show low level inver- <br />sions 1n terms of time, inversions may typically exist as much as <br />two-thirds of the tUne over the full 24 hour day. As a means of assessing <br />the impact of inversions during the period of study, the dates and times <br />when seeding was in progress in the Tushar area are indicated in Fig. 4. <br />During the nighttime some form of seeding was in progress a total of 137 <br />hours. No attempt is made to assess whether the low level or. two high <br />--- -""- <br /> <br />elevation seeders were used. Our purpose is directed to evaluate the <br />~___- _~_=-_~_~~---=o~'="__~~._~'=~-_-? <br /> <br />impact of inversions when it is deemed appropriate to seed. If one-sixth <br />of the daytime seeding and all of the nighttime seeding occurred during <br />inversions (as indicated by the soundings), then over one-half of the <br /> <br />16 <br />