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How to read the Clear Sky Chart

Clear Sky Chart Example

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 Indianapolis is -5.0 hours from GMT.) In the rows labeled "sky conditions", find a column of blue blocks. You can probably observe then.

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.

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 independent 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.

Seeing:

Bad 1/5

Poor 2/5

Average 3/5

Good 4/5

Excellent 5/5

The line, labeled Seeing, forecasts astronomical seeing. 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.

Darknesss:

-4

-3

-2

-1

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.

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, is a good indication that ground fog will form. This is especially true if the transparency is good. Also, when the cloud forecast is opaque and the humidity forecast is 95%, rain is likely: a good time to cover the telescopes.

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.