This is the web supplement for the article of the same name in the February 2010 issue of Sky & Telescope. The primary purpose of the web supplement is to gather the URLs for the websites referenced in the magazine article into one place, so that you can click on the relevant hyperlinks instead of typing the URLs into your browser. See the main article to find out how to interpret the information on these websites and how to get your own stargazing forecast.
Astronomy Stargazing ForecastThe Clear Sky Charts are the first place that most North American stargazers look when they're trying to figure out whether observing conditions will be good for the next night or the one after. The underlying data for the Clear Sky Charts comes from the Canadian Meteorological Centre astronomy forecast. Each Clear Sky Chart gathers all the information for one location into a single view, while the Canadian Astronomy forecast has maps showing how conditions evolve over the entire continent.
Astroforecast and Skippy Sky have astronomy forecasts for Europe and other parts of the world, including North America. These are based on the Global Forecast System, which covers the entire planet and stretches more than a week into the future, but is less detailed and reliable than the model used by the Canadian Meteorological Centre.
General Weather Forecasting
It's always worth checking a general weather forecast as well, both as a cross-check for the astronomy-specific stargazing forecast and to include conditions such as wind speed that are important to astronomers but not included in the CMC astronomy forecasts.
Moreover, whereas the astronomy forecasts are produced blindly from computerized weather models, general weather forecasts also have input from experienced meteorologists who have seen these models go wrong in the past, and know how to interpret them. For US National Weather Service forecasts, click on the Forecast Discussion link in the lower-right corner of each local forecast page. These discussions can be a little cryptic, but they yield invaluable insight into how confident the meteorologists are in their own forecasts.
By all means consult the forecasts from your local news media. AccuWeather is one particularly handy source for private weather forecasts. But be aware that all other forecasters routinely consult the ones from the various national weather services, for instance:
To see what the weather is actually doing, check the US Geostationary Satellite Server.
Computer Weather Models
All modern weather forecasts are based on models running inside supercomputers at the various national weather centers. The two most important models for North America are the Global Forecast System (GFS) and the North American Mesoscale (NAM). The GFS covers the entire planet and runs two weeks in advance, but its model is fairly crude. The NAM is much more precise, but runs only four days in advance. Data from both models can be accessed at the US NWS Model Analyses and Forecasts website. Ground-level forecasts based on this data are shown at the Model Output Statistics website. Additional data is available from IPS MeteoStar, Inc. and
Penn State University's Electronic Map Wall.
What if you're planning a vacation, or looking for a retirement home with good astronomy conditions? Then you will want to check the long-term climatic conditions. The US National Climate Data center displays a huge amount of data both in tabular form and as maps. For data on nighttime cloud cover, which can be see only in infrared, consult the historical data from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instruments.