Rehoboth, MA Observing Conditions
Cloud cover is an aggregate of three diferent layers.
Arc second refers to the minimal size of an object which is still visible using telescope.
Seeing is included in the arc second column as a color value and is computed based on turbulence in the air column.
Look for dark blue colors in the cloud cover for good visibility and green values in the arc second column for good seeing.
The forecast may miss very low clouds. The moon phase is shown to the right.
Clear Sky Chart Notes
Cloud Cover forecasts total cloud cover. The 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. Bad seeing can also occur during clear weather.
Transparency forecasts the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. Above average transparency is needed for low contrast objects like galaxies and nebulae. Open clusters and planetary nebulae are observable in below average transparency. Large globulars and planets can be observed in poor transparency.
Astronomical seeing is impacted by turbulence in the air combined with temperature differences. 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 but galaxies will be fine. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. Seeing is often good when transparency is poor. The seeing line is calibrated for cooled 11-14 inch scopes (so will be better for smaller equipment) and forecast in three-hour blocks.