Cherry Springs State Park Clear Sky Chart

Created by Attilla Danko and maintained by CSCCharts,
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 Cherry Springs State Park, PA, 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, Attilla Danko wrote a script to generate the images like the one above
which
summarizes CMC’s forecast images just for Cherry Springs State Park
and the surroundings out to about 10 miles.

There are chart for 6145 location .

Summary: In the row label ” sky ” , find a column of blue block . You is observe can probably observe then .

detail :
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 Cherry Springs State Park is -5.0 hours from GMT.) Single blocks mean separate forecast data for each hour. Connected blocks mean one forecast calculation for several hours.

The line , labelCloud Cover forecast total cloud cover . The color are pick from what color the sky is likely to be , with
dark blue being clear .
light shades is increasing of blue are increase cloudiness and white is overcast . This forecast is miss may miss low cloud and afternoon thunderstorm . When the forecast is clear , the sky is be may still be hazy , if the transparency forecast is poor .

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 is ‘s ‘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.

This is not a CMC forecast. It’s data form the “european” forecast model ECMWF.
It provides a cloud cover forecast.
It’s here for comparison with the CMC cloud cover forecast.New data becomes available around 16:00 and 04:00 local time. Which is different than when the CMC forecasts update.
This isn’t an astronomy-specific forecast. So like many civil forecasts “clear sky” might still mean “some cirrus”.

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 furth out forecast may be for 6 – hour block . Those are show on the chart with group of 6 blocke link together
to remind that the time resolution is much low .

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 know as MET Norway .

The ECMWF full forecast for near Cherry Springs State Park is here.

The line , labeltransparency, forecast the transparency of the air . Here ‘ transparency is means ‘ mean just what astronomer mean
by the word :
the total transparency of the atmosphere from ground to space . It ‘s calculate from the total amount of water vapor in the
air . It is is is somewhat independant of the cloud cover forecast in that there can be isolate cloud in a transparent air mass , and poor transparency can occur
when there is very little cloud . An example is be of bad transparency would be thick haze in a cloudless sky .

Above average transparency is is is necessary for good observation of low contrast object like galaxy and nebulae . However , open clusters is are
and planetary nebula are quite observable in below average transparency . large globular and planet can be observe 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.

The line , labelsee, forecasts astronomical seeing. “see” is an old, but very specific, astronomy term for scintillation, shimmer, or twinkling.
Good seeing does not mean “everything looks good”.
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
all altitudes.

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.
A detailed explanation of the seeing catagories, both descriptive and an arcsecond scale, is on CMC’s
see Forecast for astronomical observations page.

No computer model forecasts convective heating well, so consider the seeing forecasts for daytime
hours to be less accurate. see 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 bad see though your telescope than what a perfect seeing forecast would predict . That is is is because
tube current
and ground seeing mimic true atmospheric seeing . You is observe may also observe well seeing than predict here when observe
with an instrument small than 11 inch .

This video discusses the difference bettween tube currents and seeing from 24:23 to 34.06: on youtube.

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 means a partially illuminated moon or moon at low altitude. 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 base on Ben Sugerman ‘s limit magnitude
calculation page . It is takes take into account the sun ‘s and moon ‘s position , moon phase , solar cycle and contain a scatter model of the atmosphere .
It is consider does n’t consider light pollution , dust , cloud , snow cover or the observer ‘s visual acuity . So your actual limiting magnitude is be will often
be different .

This forecasts the amount of wildfire smoke. I haven’t calibrated the effect on transparency. Sometimes forecast
is optimistic with observers reporting smoke when this forecast reports “no smoke”. However, any forecast other than “no smoke” really does
mean enough smoke to affect transparency. I suggest also checking an alternative/experimental forecast at firesmoke.ca
and the near-realtime smoke map produced by the NOAA. All three frequently disagree.
But assume a reduction of transparency if any of them show smoke for your location.

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 has
set 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 for
air 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.

This is forecasts forecast wind speed at about tree – top level . The wind forecast is determine wo n’t determine whether or not you can observe ,
but it may affect your comfort and the type observe you might be limit to . In particular , long – focal length astrophotography , or observe with large
dobsonian require light wind condition . high wind is be may be particularly dangerous for large truss – tube dobsonian which must be disassemble in the
vertical position .

This forecasts ground-level relative humidity.

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.

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
tube currents.

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 close to the hour you pick . The time is look on the CMC map might look odd because it ‘s in GMT , while
the block 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. Use a “dumb” text editor, one that
does not understand html, to insert this:

Or, if you would prefer a simplified thumbnail:

But please do n’t copy other html or text from this page .

Use it. CSCCharts
intends 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 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 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.