Cloudiness Over the Southwestern United States and its Relation to Astronomical Observing

Abstract

A number of types of cloudiness statistics for Weather Bureau stations in the southwestern United States are analyzed in terms of their implications for astronomical observatory site-selection. In all but one of the analyses, Yuma proves to be distinctly superior to other stations with respect to clearness of skies. Lack of nearby mountain peaks extending above haze and dust layers plus poor seeing due to inevitably high thermal instability throughout much of the year render the immediate vicinity of Yuma astronomically unattractive, however. Hence the difficult task of comparing the relative cloudiness of the region roughly concentric with Yuma is the practical problem confronting the astronomer seeking new observing sites. Inherent limitations in available types of meteorological observations are disctissed; but these limitations are not precisely defined, since they are not quantitatively known to the meteorologist at present. In view of these uncertain limitations in each individual type of data, the safest procedure becomes that of assembling all possible types of independently observed data and assessing the site problem in terms of the overall implications of all of these data. ~e present report consists chiefly in such an assembly and assessment of meteorological data. In addition, a review of past studies of the site-selection problem is given. It is concluded that during the winter half-year (more significant to astronomical observing than the sunnner half-year for night-duration reasons), the area extending out about 200 miles northeast and east of Yuma 1s the best portion of Arizona for observing sites. Sites in this area will have clearer winter skies than those over coastal southern California, and will: be somewhat superior to those north of the Mogollon Rim where more frequent migratory cyclonic storms increase the mean winter cloudiness to values higher than those found to the south. The southeastern corner of Arizona is unattractive in the summer rainy season (July-August) due to high-frequency of thunderstorms, and equally undesirable in summer is the whole Mogollon Rim whose thunderstorm frequencies are nearly as high, on the average, as those of extreme southeastern Arizona. A rapid westward diminution of summer thunderstorm activity across southern Arizona (due to upper-level flow conditions governing moisture distribution over the Southwest) makes summer conditions increasingly more favorable from Tucson westward to Yuma; and in the winter the entire border area west of Tucson to and beyond Yuma is quite favorable. Haze and dust tops average about 5000 to 6000 ft. msl. in winter and probably 8000 to 10,000 ft. msl. in summer in southern Arizona. Areas of agricultural cultivation, as the Salt River Valley area around Phoenix, have a locally severe transparency problem. The general altitude of the haze and dust layers plus other seeing difficulties leave only a few peaks in southwestern .Arizona as feasible sites. A peak at 5672 ft. in the Harquahalas and Kitt Peak (6875 ft.) in the Quinlans seems to offer the the meteorologically best possibilities in .Arizona. Several California peaks considered by Irwin may be almost as favorable as those in southwestern .Arizona, but no first-order Weather Bureau station data are available for those areas. However, the only peaks above 6000 ft. msl. in the area. west of Yuma are those just south of San Jacinto Peak and these ranges undoubtedly have about as high a winter cloudiness as San Diego, which proves to be distinctly higher than either Tucson or Yuma.