Whither the Weather
Astronomers in the Midwest endure fickle weather that never seems to be clear at the times when we want to observe. As a result, we go through considerable effort to figure out what the sky conditions will be prior to committing to an hour (or more) drive to one of our dark sky sites. This is the reason why our plans to get together for observing are always a last-minute decision on that day.
Local weather forecasts offer some amount of help in the go/no-go decision. To see what conditions are like in adjacent states and to learn what sort of weather might be heading toward our site, there are the National Weather Radio stations KWO34 (Chicago) or WXJ74 (Rockford)*, the weather channel on cable TV, and weather maps on the Internet:
http://www.cnn.com/WEATHER/NAmerica
Sometimes it all boils down to your own sense of what the weather will be like for that night. Obviously, you wouldn't head out if a large bank of solid clouds was looming in the western sky. But what about broken clouds? Here some understanding of cloud formation helps. Light or broken clouds in the late afternoon or evening often disperse by nightfall due to cooling of the air. Clouds form when moisture (water vapor) in the air is cooled either to condensation or to "sublimation", which is the formation of ice crystals from the water vapor rather than raindrops. Daytime warming helps the process of cloud formation -- heat makes even moist air become lighter and more likely to rise to high elevations where it's cooler. As sunlight weakens and disappears in the evening, the means for heating the air and making it rise is no longer there, so clouds tend to disperse. Humidity may remain in the air closer to the surface where it's somewhat warmer, but if there's not enough of it to create clouds, then you at least can do some observing. If there's a lot of moisture, then as it settles close to the surface, ground fog may develop.
That said, let it also be noted that there have been times when we've headed out, banking on continued clear skies, only to see the clouds appear out of nowhere and thicken after nightfall. We've even been so desperate to observe as to ignore the present weather completely, hoping that it would clear. (It usually doesn't.) But there's an exception to every rule!
*The station from Rockford sometimes comes in more clearly, and the Rockford forecast often ends up being somewhat more accurate for our area than does the Chicago forecast.
