(The brightness of the faintest star a standard observer can see straight up.) Mouse over a darkness block for details. The numbers are the visual limiting magnitude at the zenith. For those who prefer numbers, the scale is also calibrated. Deep blue means a partially illuminated moon or moon at low altitude. It shows when the sky will be dark,Īssuming no light pollution and a clear sky.īlack is a dark sky. The line labeled darkness is not a weather forecast.

This video discusses the difference bettween tube currents and seeing from 24:23 to 34.06: on youtube. With an instrument smaller than 11 inches. You may also observe better seeing than predicted here when observing That is becauseĪnd ground seeing mimic true atmospheric seeing. Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. Those triplet boxes for the same forecast map are showen linked as a reminder.Ī white block on the seeing line means that there was too much cloud (>80% cover) to calculate it. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. No computer model forecasts convective heating well, so consider the seeing forecasts for daytime The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. It's because seeing is not very related to the Often good seeing occurs during poor transparency. This forecast attempts to predict turbulence and temperature differences that affect seeing forīad seeing can occur during perfectly clear weather. Turbulence combined with temperature differences in the atmosphere. 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 The line, labeled Seeing, forecasts astronomical seeing. So see the separate smoke forecast line on this chart.ĬMC's text page explaining the transparency forecast is So it mayīe possible to observe during a white transparency forecast, but the real transparency is usually yucky. Large globulars and planets can be observed in poorĪ forecast color of white formally means that CMC didn't compute the transparency forecast because the cloud cover was over 30%.

However, open clustersĪnd planetary nebulae are quite observable in below average transparency. 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 occurĪbove average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. It's calculated from the total amount of water vapor in theĪir. The total transparency of the atmosphere from ground to space. Here 'transparency' means just what astronomers mean The line, labeled Transparency, forecasts the transparency of the air. It's always wise to click on a colored block for the hour you want to observe and see if your location is close to a cloud edge.ĬMC's text page explaining this forecast is Is 76% and the mostly-cloudy accuracy is 89%. When the forecast is predicting 36 to 48 hours into the future, the mostly-clear accuracy Mostly-clear forecasts (cloud75%) have been When the forecast is clear, the sky may still be hazy, if the transparency forecast is poor.Īccuracy averaged over North America for a 30 day period: when the forecast is predicting less than 12 hours into the future, This forecast may miss low cloud and afternoon thunderstorms. Lighter shades of blue are increasing cloudiness and white is overcast. The colors are picked from what color the sky is likely to be, with The line, labeled Cloud Cover forecasts total cloud cover. Connected blocks mean one forecast calculation for several hours. (Local time for Parrsboro is -3.0 hours from GMT.) Single blocks mean separate forecast data for each hour. A digit 1 on top of a 3 means 13:00 or 1pm. The two numbers at the top of a column is the time. The colors of the blocks are the colors from CMC's forecast maps for that hour. Summary: In the rows labeled 'Sky', find a column of blue blocks. Summarizes CMC's forecast images just for ParrsboroĪnd the surroundings out to about 15 Kms. So, Attilla Danko wrote a script to generate the images like the one above It can be a chore to find the one map that tells you if you can observe tonight. CMC's numerical weather forecasts are unique because they are specifically designed for astronomers.īut they have 1180 forecast maps. The data comes from a forecast model developed by Allan Rahill of theĬanadian Meteorological Centre. It's a prediction of when Parrsboro, NS, will have good weather for astronomical observing. At a glance, it shows when it will be cloudy or clear for the

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