Go to the Clear Sky Clock site for a complete explanation of this clock.
The line, labeled Cloud Cover forecasts total cloud cover. The colors are picked from what color the sky is likely to be, with Dark blue being clear. Lighter shades of blue are increasing cloudiness and white is overcast. This forecast may miss low cloud and afternoon thunderstorms. When the forecast is clear, the sky may still be hazy, if the transparency forecast is poor.
This forecasts wind speed at about tree-top level. The wind forecast won't determine whether or not you can observe, but it may affect your comfort and the type observing you might be limited to.
The line, labeled Transparency, forecasts the transparency of the air. Here 'transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. It is somewhat independant of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.
Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.
The line, labeled Seeing, forecasts astronomical seeing. (It's an experimental forecast.) Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.
Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to the water vapor content of the air.