Foothill Observatory
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 Foothill Observatory, CA, will have good weather for astronomical observing. (Not my observatory.)

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 Foothill Observatory and the surroundings out to about 10 miles.

There are charts for 6142 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 Foothill Observatory is -8.0 hours from GMT.) Single blocks mean separate forecast data for each hour. Connected blocks mean one forecast calculation for several hours.

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.

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'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.

ECMWF Cloud A Sponsored Feature

Overcast

Cloud 75%

Cloud 50%

Cloud 25%

Clear Sky

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 13:00 and 01:00 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 furthest out forecasts may be for 6-hour blocks. Those are shown on the charts with groups of 6 blockes linked together to remind that the time resolution is much lower.

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 known as MET Norway.

The ECMWF full forecast for near Foothill Observatory is here.

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.

Wind

>45 mph

29 to 45 mph

17 to 28 mph

12 to 16 mph

6 to 11 mph

0 to 5 mph

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 vertical position.

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%

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.

Temperature

< -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

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.

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.