Tierra del Sol
Clear Sky Chart
legend page

What is it?

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 Tierra del Sol, CA, will have good weather for astronomical observing.

The data comes 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 which summarizes CMC's forecast images just for Tierra del Sol and the surroundings out to about 10 miles.

There are charts for 4314 locations.

How do I read it?

Summary: In the rows labeled "sky conditions", 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 Tierra del Sol is -7.0 hours from GMT.)

Cloud Cover

Overcast

90% covered

80% covered

70% covered

60% covered

50% covered

40% covered

30% covered

20% covered

10% covered

Clear

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.

CMC's text page explaining this forecast is here.

Transparency

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 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.

Seeing

Bad 1/5

Poor 2/5

Average 3/5

Good 4/5

Excellent 5/5

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 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. There are some more details in CMC's seeing forecast page.

There are gaps in the line of seeing blocks because CMC's seeing model does not consider daytime heating, so the forecast is only available for the night. 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.

You can help improve the seeing forecast by submitting observations to the Astronomical Seeing Observations program.

Darkness

-4

-3

-2

-1

0

1.0

2.0

3.0

3.5

4.0

4.5

5.0

5.2

5.4

5.6

5.8

6.0

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,