Some skyglow is surprisingly local. You can often see more stars 15 miles from a big city than a quarter mile from even one bad rural shopping center.

I've made extensive sky-brightness measurements of the zenith at two sites in Middletown, Connecticut: at the Van Vleck Observatory on the Wesleyan University campus, and at my home two miles away in wooded, semirural suburbia. The campus had, until recently, a night sky more than 20 times brighter than the natural sky background. The sky over my house is only four to five times brighter than the natural level. The change in two miles was remarkable — from a nearly invisible Milky Way to views of the Sagittarius and Scutum starclouds on good nights.

In 1994 the university agreed to replace most of its walkway lights within a block of the observatory with properly shielded fixtures. The sky brightness at the zenith dropped by almost half — a dramatic improvement of 0.6 or 0.7 magnitude.

Such observations point up the importance of dealing with local lights. You don't have to convert an entire city to see results. Hartford, Connecticut, a metropolitan area of a million people, is only 15 miles north of the Wesleyan campus. Its lights obtrude only marginally. Those of New York City about 90 miles southwest interfere not at all.

Another example appears on the light-pollution map of the Washington, D.C., region made by the Northern Virginia Astronomy Club and published in the June 1996 Sky & Telescope, page 82. The club members found surprising holes of relatively dark sky in a region around Washington that looks solid white in nighttime spacecraft photographs.

Watch the color of the daytime sky, especially near the horizon. The bluer the sky, the darker the night will probably be. Whiteness in a daytime sky is due to sunlight scattered by tiny particles. They do just as good a job of scattering artificial light at night. A deep blue sky in the afternoon should mean a transparent sky after dark.

Dry air is another good sign. Even if the upper atmosphere is fairly free of haze, high humidity is liable to bring haze (or fog) lower down. Watch for forecasts of low humidity.