Daylight visibility observations at the Sacramento Municipal Airport were used to investigate the possibility that visibility in the area has been declining due to rising levels of air pollution. Those visibility observations made with naturally occurring fog (defined as relative humidity greater than 90 percent) and/or precipitation were eliminated from the data. Observations for July, a warm, dry month with relatively slight climatic differences from year to year, and November, a cool month that often exhibits it considerable annual variation of climate, were tabulated for the three 4-year periods, 1935–38, 1945–48, and 1953–56. The results show that for July the percent frequency of poor visibilities steadily increased, and of good visibilities steadily decreased over the last 20-odd years. Such a trend is consistent with the Sacramento population increase. For November the trend was irregular. This is believed to be due to variations in the occurrence of meteorological factors, important to atmospheric dispersion. It is likely that such irregularities are characteristic of other months and areas where there are large annual climatic variations.
From concurrent surface wind observations, the preferred wind conditions for various visibility ranges were determined. These data indicate that poor visibilities at the airport, on the southern edge of the city, occurred most frequently with light winds from over the nearby Sacramento urban area. Good visibilities were favored by moderate winds from rural directions. Visibilities were reduced when the wind speed became strong enough for loose materials to be picked up from the ground.