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 1:00 pm) this refers to it's local time in 24 hour format. Local time for University of Maryland Observatory is -4.0 hours from GMT Standard Time.
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.
CMC's text page explaining this forecast is
| Overcast || 90% covered || 80% covered || 70% covered || 60% covered || 50% covered || 40% covered || 30% covered || 20% covered || 10% covered || Clear |
| 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. 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
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.
Look at the cloud forecast for the same time to see how much cloud there will be. CMC's text page explaining this forecast is
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
| Bad 1/5 || Poor 2/5 || Average 3/5 || Good 4/5 || Excellent 5/5 |
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
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
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 Sugarman'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
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
| Storm || Strong || Moderate || Light to Moderate || Light || Calm |
This forecasts ground-level relative humidity. Humidity variations won't determine whether or not you can observe,
but it might affect observer comfort and can indicate the likelihood of dewing.
| <29% || 30% to 35% || 35% to 40% || 40% to 45% || 45% to 50% || 50% to 55% || 55% to 60% || 60% to 70% || 65% to 70% || 70% to 75% || 75% to 80% || 80% to 85% || 85% to 90% || 90% to 95% || 95% to 100% |
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 and valleys.
a forecast of 95% humidity under an opaque sky is a pretty good clue that it's 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.
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
| < -35C || -35C to -30C || -30C to -25C || -25C to -20C || -20C to -15C || -15C to -10C || -10C to -5C || -5C to 0C || 0C to 5C || 5C to 10C || 10C to 15C || 15C to 20C || 20C to 25C || 25C to 30C || 30C to 35C || 35C to 40C || > 40C |