Bentsen-Rio Grande Valley State Park
Clear Sky Chart
legend page

What is it?

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 Bentsen-Rio Grande Valley State Park, TX, 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 Bentsen-Rio Grande Valley State Park and the surroundings out to about 10 miles.

There are charts for 6145 locations.

How do I read it?

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 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 Bentsen-Rio Grande Valley State Park is -6.0 hours from GMT.) Single blocks mean separate forecast data for each hour. Connected blocks mean one forecast calculation for several hours.

Transparency

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. 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 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.

Seeing

Too cloudy to forecast

Bad 1/5

Poor 2/5

Average 3/5

Good 4/5

Excellent 5/5

The line, labeled Seeing, forecasts astronomical seeing. "Seeing" 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 Seeing Forecast for astronomical observations 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.

Darkness

-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

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 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.

Smoke

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

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.

How do I see the full maps?

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.

May I put the Chart on my web page?

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 don't copy other html or text from this page.

Hey, this is cool. How do I make sure I keep getting it?

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.