How do I read the Sky Clock

How do I see the full map?

Hey, this is cool. How do I make sure I keep getting it?

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Introduction: What is a Clear Sky Weather Clock

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 the area around the University Of Maryland Observatory in Adelphi, Maryland will have good weather for astronomical observing. The Observatory is quite near to Company Seven in Laurel and so we employ this clock as a public service for our readers.

The forecast data for the above image comes from those very cool guys at the Canadian Meteorological Center. They run three computer weather simulations every twelve hours. The output looks just like satellite pictures, but the dates are from the future. To see them, click on the 'cloud', 'tran' or 'seeing' blocks blocks on the chart below.

CMC's numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 763 forecast images. It can be a chore to find the one you want. So, Attilla Danko wrote a script to generate the images like the one above which summarizes CMC's forecast images. This one is just for the UM Observatory in Adelphi and the surroundings out to about 10 miles.

There are also clocks for 2409 other locations.

Image Control Explain color and details when you mouse-over: Click on a block to show full forecast map Last updated 2005-06-24 01:50:19. No Image below? Read this. Other Clocks All Maryland Within 60 mi Within 120 mi

How do I read the Sky Clock? 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. 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. 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. 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. 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 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. 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 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 be different. Wind Storm Strong Moderate Light to Moderate Light Calm 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 <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% 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. 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. Also, 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. Temperature < -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 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. 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 clock 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. Hey, this is cool. How do I make sure I keep getting it? Just keep using it. The author intends to keep updating this image for 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 clock, or CMC maps linked by the colored blocks, useful please send Allan Rahill of the CMC an E-Mail and feel free to send a copy to the author Mr. Danko. Allan needs to show his boss that his astronomy forecasts are actually being used. You can also help keep clear sky clocks free for everyone by being a sponsor. Please feel free to tell sponsors that they're cool.