The Lake County Astronomical Society

Clearly, Not "Fair" Weather

You've probably heard weather forecasters say we'll be enjoying "fair" weather, which led you to assume that it would be a good night to observe. Then you see a hazy sky or a large amount of clouds. What the media often refer to as "fair" weather or a "sunny" day is not what astronomers consider sufficiently clear. This does, in fact, reflect how the Weather Bureau interprets the words. According to Chicago meteorologist Tom Skilling: "Another cloud term, 'fair' is most often used at night, usually indicating high cirrus clouds. Often, the moon and some bright stars are visible with fair skies." That's certainly not the kind of "fair" weather we like!

Perhaps the Weather Bureau is putting the best possible spin on our weather. In the Chicago area, we average just 51 days a year that are totally cloudless (only 14% of the total). That doesn't provide many opportunities for observing, on top of which, the cloudy days all seem to occur on the weekends! (They really don't.) With some luck, the broken clouds will dissipate after dark when the atmosphere cools. This may mean that the humidity that formed the clouds remains in the atmosphere, thus reducing the transparency of the sky. But observing is still possible. If the clouds hang around, then just get used to taking advantage of sucker holes!