It's the astronomer's forecast. At a glance, it shows when it will be cloudy or clear for up to the
next two days. It's a prediction of when Rancho Cordova, CA, will have good weather for astronomical observing.
The data comes from a forecast model developed by Allan Rahill of the
Canadian Meteorological Center. CMC's numerical weather forecasts are unique because they are specifically designed for astronomers.
But they have 763 forecast maps. It can be a chore to find the one you want.
So, I (Attilla Danko) wrote a script to generate the images like the one above
summarizes CMC's forecast images just for Rancho Cordova
and the surroundings out to about 10 miles.
There are charts for 6137 locations.
Summary: In the rows labeled "sky conditions", find a column of blue blocks. You can probably observe then.
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 Rancho Cordova is -7.0 hours from GMT.)
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.
| Overcast || 90% covered || 80% covered || 70% covered || 60% covered || 50% covered || 40% covered || 30% covered || 20% covered || 10% covered || Clear |
CMC's text page explaining this forecast is
| 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 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
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.
CMC's text page explaining this forecast is
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
| 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 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.
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
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.
The line labeled darkness is not a weather forecast. It shows when the sky will be dark,
assuming no light pollution and a clear sky.
Black is a dark sky. Deep blue shows interference from moonlight. Light blue is full moon. Turquoise is twilight. Yellow is dusk and white
is daylight. For those who prefer numbers, the scale is also calibrated. The numbers are the visual limiting magnitude at the zenith.
(The brightness of the faintest star a standard observer can see straight up.) Mouse over a darkness block for details.
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
This 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. In particular, long-focal length astrophotography, or observing with large
dobsonians require light wind conditions. High wind may be particularly dangerous for larger truss-tube dobsonians which must be disassembled in the
| >45 mph || 29 to 45 mph || 17 to 28 mph || 12 to 16 mph || 6 to 11 mph || 0 to 5 mph |
This forecasts ground-level relative humidity.
| <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 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
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.
This forecasts temperatures near the ground. While temperature variations won't determine if you
can observe, the forecast can be handy choosing clothing for cold observing conditions. (In general, dress as if it were
20 degrees F or 10 degrees C colder than the forecast.) Observers with thick primary mirrors should take note of
falling temperature conditions because their mirrors may require additional cooling to reach equilibrium and so prevent
| < -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.
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.
If your website does not make money from ads, does not charge admission and does not sell anything, then yes. Please use this html:
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.