Created by Attilla Danko,it's the astronomer's forecast. At a glance, it shows when it will be cloudy or clear for thenext few days. It's a prediction of when Indianapolis, IN, will have good weather for astronomical observing. (Not my observatory.)
The data comes from a forecast model developed by Allan Rahill of theCanadian Meteorological Centre. CMC's numerical weather forecasts are unique because they are specifically designed for astronomers.But they have 1180 forecast maps. It can be a chore to find the one map that tells you if you can observe tonight.
So, Attilla Danko wrote a script to generate the images like the one abovewhich summarizes CMC's forecast images just for Indianapolis and the surroundings out to about 10 miles.
There are charts for 6126 locations.
Summary: In the rows labeled "Sky", find a column of blue blocks. You can probably observe then.Details: Read the image from left to right. Each column represents adifferent hour. The colors of the blocks are the colors from CMC's forecast maps for that hour. The two numbers at the top of a column is the time. A digit 1 on top of a 3 means 13:00 or 1pm. It's local time, in 24hr format.(Local time for Indianapolis is -4.0 hours from GMT.) Single blocks mean separate forecast data for each hour. Connected blocks mean one forecast calculation for several hours.
| Overcast | 90% covered | 80% covered | 70% covered | 60% covered | 50% covered | 40% covered | 30% covered | 20% covered | 10% covered | Clear |
Accuracy averaged over North America for a 30 day period: when the forecast is predicting less than 12 hours into the future,mostly-clear forecasts (cloud<25%) have been right 80% of the time. Mostly-cloudy forecasts (cloud>75%) have beenright 91% of the time. When the forecast is predicting 36 to 48 hours into the future, the mostly-clear accuracyis 76% and the mostly-cloudy accuracy is 89%. Accuracy beyond 48 hours in unknown.
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.
CMC's text page explaining this forecast ishere.
ECMWF Cloud
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| Overcast | Cloud 75% | Cloud 50% | Cloud 25% | Clear Sky |
Accuracy Summary: The ECMWF forecast is more accurate than the CMC cloud forecast when predicting mostly clear, but less accurate whenpredicting mostly cloudy. The far future ECMWF cloud forecasts are still usable.
But note that the furthest out forecasts may be for 6-hour blocks. Those are shown on the charts with groups of 6 blockes linked togetherto remind that the time resolution is much lower.
Accuracy details: Averaged over North America for a 30 day period: when the forecast is predicting less than 12 hours into the future,mostly-clear forecasts (cloud<25%) have been right 85% of the time. Mostly-cloudy forecasts (cloud>75%) have beenright 75% of the time. When the forecast is predicting from 60 to 64 hours into the future, the mostly-clear accuracyis 81% and the mostly-cloudy is to 71%. Beyond 64 hours accuracy is unknown and time resolution changes from data for each hour to data for every 6 hours.
Data from the Norwegian Meteorological Institute, also known as MET Norway.
The ECMWF full forecast for near Indianapolis is here.
| Too cloudy to forecast | Poor | Below Average | Average | Above average | Transparent |
The line, labeled Transparency, forecasts the transparency of the air. Here 'transparency' means just what astronomers meanby the word:the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in theair. 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 occurwhen there is very little cloud. An example of bad transparency would be thick haze in a cloudless sky.
Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clustersand planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poortransparency.
A forecast color of white formally means that CMC didn't compute the transparency forecast because the cloud cover was over 30%. So it maybe possible to observe during a white transparency forecast, but the real transparency is usually yucky.This forecast does not consider smoke. So see the separate smoke forecast line on this chart.
CMC's text page explaining the transparency forecast is here.
| Too cloudy to forecast | Bad 1/5 | Poor 2/5 | Average 3/5 | Good 4/5 | Excellent 5/5 |
Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to thewater vapor content of the air.
The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range.A detailed explanation of the seeing catagories, both descriptive and an arcsecond scale, is on CMC'sSeeing Forecast for astronomical observations page.
No computer model forecasts convective heating well, so consider the seeing forecasts for daytimehours to be less accurate. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. Those triplet boxes for the same forecast map are showen linked as a reminder.A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it.
Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currentsand ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observingwith an instrument smaller than 11 inches.
This video discusses the difference bettween tube currents and seeing from 24:23 to 34.06: on youtube.
| -4 | -3 | -2 | -1 | 0 | 1.0 | 2.0 | 3.0 | 3.5 | 4.0 | 4.5 | 5.0 | 5.5 | 6.0 | 6.5 |
It is based on Ben Sugerman's Limiting Magnitudecalculations page. It takes into account the sun's and moon's position, moon phase, solar cycle and contains a scattering model of the atmosphere.It doesn't consider light pollution, dust, clouds, snow cover or the observer's visual acuity. So your actual limiting magnitude will oftenbe different.
| No Smoke | 2ug/m^3 | 5ug/m^3 | 10ug/m^3 | 20ug/m^3 | 40ug/m^3 | 60ug/m^3 | 80ug/m^3 | 100ug/m^3 | 200ug/m^3 | 500ug/m^3 |
A note about CMC's smoke forecast colors:The chart shows different colors than the corresponding CMC maps because the maps use white to mean "no smoke" but the cloud and transparency forecasts use white to mean "opaque sky".I've chosen colors for the lowest levels to suggest sky colors. But the EPA hasset a limit for 24hr exposure of 35 micrograms per cubic meter. So I have colored levels with more smoke than that with red colors and the worst with brown. People should look forair quality warnings from their local health authorities if the smoke line shows red blocks. If the smoke line shows brown blocks, I'd stay indoors.
The smoke data on the chart comes from Environment Canada's Canada's Wildfire Smoke Prediction System.
| >45 mph | 29 to 45 mph | 17 to 28 mph | 12 to 16 mph | 6 to 11 mph | 0 to 5 mph |
| <25% | 25% to 30% | 30% to 35% | 35% to 40% | 40% to 45% | 45% to 50% | 50% to 55% | 55% to 60% | 60% to 65% | 65% to 70% | 70% to 75% | 75% to 80% | 80% to 85% | 85% to 90% | 90% to 95% | 95% to 100% |
Humidity variations can indicate the likelihood of optics and eyepieces dewing.
But dewing is not simply correlated to relative humidity. Dewing tends to happen when the sky is clear, the temperature is dropping and thereisn't much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, thehumidity forecast does not have the spatial resolution to know about small hills, valleys, or observatory walls. All of which can reduce dewing.
A sudden spike in the humidity forecast, an hour or so after the cloud forecast predicts a sudden transition from cloudy to clear, when there is no wind, means that ground fogwill form.
Also, when the cloud forecast is opaque and the humidity forecast is 95%, rain is likely: a good time to cover the telescopes.
Since there are many different levels in this forecast, with similar looking colors, it's best to activate the"explain colors when you mouse over" to interpret the colors.
| < -40F | -40F to -31F | -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 | 104F to 113F | >113F |
Cold temperatures also mean reduced battery capacity, stiffer lubricants, stiffer electrical cables and slower LCD displays.Camera sensors will have reduced noise. But, in general, electronics have a lowest temperature at which they will work.
They are the same Cloud and ECMWF-cloud forecasts as for the first few days, but extended far into thefuture. Each block represents 3 hours. ECMWF data this far into the future is available only every 6 hours. That is shown with two linked blocksfor each forecast point.
This far into the future, time resolution is lower and accuracy is unknown. But one should assume the accuracy is low. Such forecasts may offer hope for planning future observingif the Cloud Cover and ECMWF forecasts agree. Otherwise they are offered as entertainment.
To see CMC's full map for a particular hour, click on a colored block. The CMC map your browser will loadwill be the map closest to the hour you picked. The time on the CMC map might look odd because it's in GMT, whilethe blocks on the chart are in local time.
It's worth checking a few of the full maps beforecommitting to a long drive out to an observing site.
If your website does not make money from ads, does not charge admission and does not sell anything, then yes. Use a "dumb" text editor, one thatdoes not understand html, to insert this:Or, if you would prefer a simplified thumbnail:
But please don't copy other html or text from this page.
Use it. Attilla Dankointends to keep updating this image for as long people use it and as long as CMC is willing to generate the underlying maps. But there are waysthat you can help:
If you find this clear sky chart, or CMC maps linked by the colored blocks,useful please send Allan Rahill of the CMC an email (and feel free to copy me). Allan created these forecasts and would appreciate hearing that they are used.
You can also help keep clear sky charts free for everyone by being a sponsor. Please feel free to tell sponsors that they're cool.
