|Sponsor this Chart|
Bentsen-Rio Grande Valley State Park
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 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 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 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 6374 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 Bentsen-Rio Grande Valley State Park is -6.0 hours from GMT.)
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. CMC's text page explaining this 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. 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.
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.
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 2019-12-09 22:06:41 which is -6.0 hours from UTC.
Update rank: 3413. Thanks to anyone thinking of sponsoring this page.
Copyright 2019 A. Danko. Page updated 2019-12-10 04:06:41UT on server2.