Fig. 3. Annual frequency for which the transmission efficiency equals or exceeds 809? for the various U.S. regions. “better” sites, hole boring is significantly beneficial at later times, being able to penetrate thin, high clouds which often precede the passage of a warm front. We have observed cases where the use of hole boring extends the period of acceptable transmission efficiency by factors of 2 to 3. For sites having intermediate meteorological conditions, hole boring offers an improvement in the persistence frequency at slightly earlier persistence times, but increases in the period of acceptable transmission efficiency are not as dramatic. Only the southwestern sites give persistence frequencies exceeding 50% for 8-h persistence times. Even so, no site attains this level of performance for all seasons. As alluded earlier, the calculational results are particularly insensitive to the assumed values of cloud transmission efficiency, especially for models 1 and 2. Because the probability of a cflos, Q, is close to unity during periods of negligible or scattered cloud cover (j = 0 to 3), the (1 - CJtj term in Eq. 1 is small and less significant than the first term, Q. Since the sky-cover occurrence-frequency curve is usually U-shaped, as illustrated by the solid histogram in Fig. 5, differences in the numerical results of various cloud-transmissivity models are small if the Tj curve decreases rapidly with increasing sky cover (e.g., models 1 and 2). If partial transmission is assumed during periods of overcast conditions (j = 8 to 10), the (1 - C^Tj term in Eq. 1 dominates. The calculated Tj is then statistically significant because of the magnitude of Ks_i0. Hence, model 3, which assumes a slower decrease in Tj with increasing sky cover, shows markedly different behavior when compared with the first two models and is subject to greater possible error. Because (1 — CJtj « Cj in
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