The Clear Sky Chart shows an hourly forecast for observing conditions, with information on each of four different sky variables. Times are given in 24-hour format, so 0 is midnight local time, 12 is noon. In general, the darker the squares the better for viewing. First look at the fourth row, Darkness, to see which hours will be dark. For a moonless sky, these squares are black, while for a full moon they might only be light blue. Next look at the first row, Cloud Cover. Dark blue squares forecast clear skies, while white is completely cloudy. Now look at the second row, Transparency. Dark blue squares again indicate best conditions. Lighter shades indicate haze that might be present even in the absence of clouds.
More detail on reading Clear Sky Charts can be found by clicking on the chart to bring up the chart in its native cleardarksky.com setting. From there one can also click on individual squares to see a map of that quantity for that hour. These maps are useful in judging the reliability of the forecast. If the observatory is close to the boundary between good and poor conditions, then only a small error in the forecast position of the boundary could change the forecast.