Cloud Forecasts for Astronomers

Edit Oct 2009: Since I originally wrote this article, SkippySky has become available. It uses the same US GFS weather model data so the discussion about accuracy is still relevant, but SkippySky is a much more convenient way to access the cloud forecast data, with great maps for all of Australia among other places.

Astronomers want good weather forecasts. In particular, they need to know about cloud. When you're deciding whether to travel two hours to a dark sky site for the weekend, an accurate forecast is invaluable.

Saturday 13th September 2008
1 Day Ahead Forecast

The 'Clear Sky Clock' is available via the web to amateur astronomers across North America, and includes not only cloud forecasts but also estimates for transparency and seeing. For those of us in the rest of the world, there are several other options:



APanel from 7Timer: The Clear Sky Clock for the rest of the World

Mid-2008 saw the trial launch of 'APanel' (Astroweather Panel) from 7Timer - an alternative to the Clear Sky Clock for the rest of the world. Here is a live APanel forecast for Heathcote, Victoria:




Explanation Key:



APanel is also based on data from the GFS model (as are most weather websites, since the raw data is made available for free). For short range forecasts, the GFS model is the best global model available (others may claim that prize for medium and long term forecasts). As computing power has increased, so has the accuracy of these global computer models. In 2005, the resolution of the GFS model was substantially increased from one degree (~55km) to around half a degree (~35km).

Although the global GFS model is not quite as good in terms of resolution as the Canadian model behind the Clear Sky Clock which is specific to North America, 7Timer's Astroweather Panel does provide forecasts of cloud cover, transparency and seeing specifically designed for astronomers anywhere in the world. However, the seeing appears to almost always forecast a worst case and the real variables governing transparency are not well captured by weather models. None the less, the Clear Sky Clock for the rest of the world has arrived!

Start here to choose a location in Australia from the 7Timer Astroweather Panel website.



Astroforecast.org

Jeff Bullard has a simpler forecast which shows just night time cloud forecasts for a given location, but is based on the same GFS data. Here is the live Astroforecast for Heathcote, Victoria:





Cloud Forecasts Maps from the US Global Forecast System

Data from the United States Global Forecast System (GFS numerical weather model) is made available for free, albeit in not very user friendly formats. These instructions look long winded, but there's really just a sequence of screens to click through which you can do quite quickly once you get used to it:




Step 1: At this starting page, Select "Cycles - All" and "Forecast Hours - All" then click "Build Order".




Step 2: Click "Plot Data".




Step 3: Depending on the Time of Day, there may be one or more options below the heading "Select a Dataset". Choose the bottom one in the list which will be the most recent.
Step 4: Select "Display Single Variable" and "Animation", then Click "Next Page".




After those tedious pages, now you are at the page where all the magic happens.

Step 5: For Variable 1, Select "Cloud Cover, Total, Atmosphere Column".
Step 6: For Variable 1 Representation, Select "Greyscale".
Step 7:

  • For Map Region, Select "Custom LatLong".

  • Enter Longitude Coordinates of 110 degrees and +50 degrees (ie. start map at longitude 110 degrees and make it 50 degrees wide).
  • Enter Latitude Coordinates of -45 degrees and +35 degrees (ie start map at latitude -45 and make it 35 degrees high)
  • Change the Water Boundary Color to Orange (which will give the outline of Australia).

    Step 8: Click "Create Plot".




    Step 9: At first, you won't see much, but click "Next" and the first image in the sequence should be shown (as in the figure below).




    Alternatively, rather than a cloud map, you can produce a graph of Cloud Cover vs Time for your location:

    Step 4b: Select "Overlay Two Variables" and "Time Series", then Click "Next Page".

    Here you select a combination of two forecast elements and locations to compare. There are endless opportunities here, but what I am generally choosing is to produce a chart which plots both Total Cloud Cover and Low Cloud Cover for my chosen location.

    Step 5b: From the first drop down box Select "Cloud Cover, Total, Atmosphere Column" and from the second Select "Cloud Cover, Total, Low Cloud Level".
    Step 6b: Enter the co-ordinates of a box one degree wide (minimum) in both columns. For the Astronomical Society of Victoria's Dark Sky Site near Heathcote, the box would be latitude -37 to -36 and longitude 144 to 145.
    Step 7b: You can adjust the Date Range if you wish, otherwise just Click "Create Chart".




    Since I made the location the same in both columns, the Total Cloud Cover (black line) and Low Cloud Cover (green line) are for the same place over the time period on the bottom (the time here is UT ie add 10 hours for AEST!). If the black line is high while the green line is low (as in the first few days of this example) that means that it is high/medium cloud rather than low cloud that is present. That kind of detail is very important when trying to intepret the forecast against satellite images and other weather charts.




    Below is an example of how well the system works. This forecast was from Thursday 28th August for north of Albury, where the Border Stargaze was being held.

    The cloud levels start very low as they were on the Thursday night and most of Friday. The forecast then indicates high cloud coming through (where just the black line heads up) around 08Z on the Friday (6pm AEST). That's pretty close to what happened, although luckily for the hundreds of locals who came on Friday night, the cloud was a few hours later than that (and high cloud can be quite 'thin' even at 100% cover).

    For Saturday 30th August, the forecast indicates comprehensive cloud cover which is also exactly what we got, from Saturday until we left on Sunday. Not bad for a forecast made on Thursday.




    Air Force MM5 Model

    The US Air Force also has a cloud forecast for the Australian region available on their website. It shows cloud top temperatures (higher cloud = colder temperatures) and appears to only plot the temperature where cloud cover is above a certain level so it tends to under-estimate cloud cover. However, it is useful for comparison with the GFS model, is much easier to access and appear to be updated more rapidly.

    AEST is 10 hours ahead of Time shown at top right of image. Click the image to load the interactive animation loop and choose an image for a different forecast time.

    Colours indicate cloud top height (thousands of feet). No colour equals clear-ish sky.



    Satellite Images

    You can access satellite images at WeatherZone, and if you sign up to a Silver Subscription you can get hourly satellite updates and 'loops' up to 24 hours long. Hourly updated images can be very useful for the final go/no-go decision and also good for working out if the cloud that arrived in the middle of the night is there to stay or not (assuming you have remote internet access).

    It also seems possible to get the latest hourly satellite image from AviationWeather.gov, which is the source of the image below. This is a raw image from the satellite and is not cropped to show just Australia but hey, it's free:



    And Finally..

    Also see Cloud Forecast Accuracy for comparisons of the forecast charts with actual satellite images.