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.
||29 to 45 mph
||17 to 28 mph
||12 to 16 mph
||6 to 11 mph
||0 to 5 mph
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.
||-30F to -21F
||-21F to -12F
||-12F to -3F
||-3F to 5F
||5F to 14F
||14F to 23F
||23F to 32F
||32F to 41F
||41F to 50F
||50F to 59F
||59F to 68F
||68F to 77F
||77F to 86F
||86F to 95F
||95F to 104F
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
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.