Graduate Thesis Or Dissertation
 

Cloudiness in the Willamette Valley of Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj430720m

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  • Temporal and spatial patterns of daytime cloudiness in the Willamette Valley of Oregon were studied by utilizing monthly and daily sky cover data published by the U. S. Weather Bureau for the Portland, Salem, and Eugene stations. The 15-year period, 1949 through 1963, was selected as the maximum reliable period length common to all three stations for accurate comparisons, for in the latter 1940's hourly observations of the number of tenths of the sky field of view obscurred by cloud cover for the time between sunrise and sunset were initiated. The measures of cloudiness used in this study, each of which is based on sky cover in tenths, include: 1. Percent sky cover. 2. Clear and cloudy days (0-5 and 6-10 tenths). 3. Cloudless and overcast days (0 and 10 tenths). 4. Clear, partly cloudy, and cloudy days (0-3, 4-7, and 8-10 tenths, respectively). The average annual and monthly sky cover in tenths and the number of clear, partly cloudy, and cloudy days can be ascertained directly from Weather Bureau data sources. The principal objectives of this study, however, include the comparison of cloudiness among stations and data presentation which affords a more revealing image of the character of cloudiness. To achieve the latter, variation, persistence, and a range of temporal frameworks were employed as well as the liberal use of graphs and tables. The Willamette Valley averages nearly 70 percent sky cover annually, and only July, August and September average less than 60 percent sky cover. During most months the three stations compare closely in the amount of cloudiness. In July and August, however, Portland averages ten percent more cloudiness than Eugene, and Salem has intermediate cloudiness characteristics. Although overall annual cloudiness in the Willamette Valley may fluctuate as much as 15 percent between extremes, the three stations tend to be affected by the same cloudiness patterns for a given year. The winter half of the year, which is characteristically quite cloudy, shows the least variation in cloudiness, but the late spring and summer show monthly ranges between extremes approaching 50 percent sky cover. The duration of periods of specified amounts of cloudiness indicates frequent changes in the amount of cloudiness, although exceptionally long periods of cloudy weather in winter have occurred. The three stations experience the same periods of cloudiness throughout the year. Contrasts in the persistence of cloudiness among stations, however, are primarily reflected in the summer when Eugene has somewhat longer lasting periods of clear weather than Portland. Partial cloudiness tends to be a transitional rather than persistent cloudiness characteristic, for, despite the overall considerable cloudiness in the Willamette Valley, most clear (0-5 tenths) days that do occur tend to be cloudless or have only scattered cloudiness.
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