It's the astronomer's forecast. At a glance, it shows when it will be cloudy or clear for the next few days. It's a prediction of when San Jose, CA, will have good weather for astronomical observing.
The data comes from a forecast model developed by Allan Rahill of the Canadian 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, I (Attilla Danko) wrote a script to generate the images like the one above which summarizes CMC's forecast images just for San Jose and the surroundings out to about 10 miles.
There are charts for 6199 locations.
Details: Read the image from left to right. Each column represents a different 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 San Jose is -8.0 hours from GMT.) Single blocks mean separate forecast data for each hour. Connected blocks mean one forecast calculation for several hours.
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 been right 91% of the time. When the forecast is predicting 36 to 48 hours into the future, the mostly-clear accuracy is 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 is here.
Accuracy Summary: The ECMWF forecast is more accurate than the CMC cloud forecast when predicting mostly clear, but less accurate when predicting 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 together to 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 been right 75% of the time. When the forecast is predicting from 60 to 64 hours into the future, the mostly-clear accuracy is 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 San Jose is here.
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.
A forecast color of white formally means that CMC didn't compute the transparency forecast because the cloud cover was over 30%. So it may be 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.
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.
The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC's seeing forecast page.
No computer model forecasts convective heating well, so consider the seeing forecasts for daytime hours 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 currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observing with an instrument smaller than 11 inches.
This video discusses the difference bettween tube currents and seeing from 24:23 to 34.06: on youtube.
It is based on Ben Sugerman's Limiting Magnitude calculations 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 often be different.
A note about CMC's smoke forecast colors: The chart shows different colors than the corresponding 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 smoke line on the chart that might better represent the color of a smokey sky -- except for the highest levels which, when forecast, means people should stay indoors.
The data comes from Environment Canada's Canada's Wildfire Smoke Prediction System.
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 there isn't much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, the humidity 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 fog will 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.
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 the future. Each block represents 3 hours. ECMWF data this far into the future is available only every 6 hours. That is shown with two linked blocks for 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 observing if 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 load will be the map closest to the hour you picked. The time on the CMC map might look odd because it's in GMT, while the blocks on the chart are in local time.
It's worth checking a few of the full maps before committing to a long drive out to an observing site.
Or, if you would prefer a simplified thumbnail:
But please don't copy other html or text from this page.
Just keep using it. I intend to keep updating this image for as long as CMC is willing to generate the underlying maps. But there are ways that 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 needs to show his boss that his astronomy forecasts are actually being 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.
Page updated by d800 at 2023-01-26 21:22:38 which is -8.0 hours from UTC.
Update rank: 225. Thanks to anyone thinking of sponsoring this page.
Copyright 2023 A. Danko. Page updated 2023-01-27 05:01:06UT on server3.