|
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 Rockford is -5.0
hours from GMT.)
|
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.
| 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.
| 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.
| -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, dust, clouds, snow cover or the observer's visual acuity.
So your actual limiting magnitude will often be different.
| >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.
| <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 the cloud
forecasts 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.
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.
| < -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.
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 |