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How I Check the Weather Before Going Observing
By Michael Covington - 9/3/2005
Your local weather forecast won't necessarily tell you whether the sky is good for observing. To a TV meteorologist, "clear" only means there aren't any identifiable clouds. We astronomers know that the clarity and the steadiness of "clear" air vary quite a lot.
|Visible-light satellite image of Georgia and Florida. Courtesy NASA) |
When I'm planning an observing session, my first stop is the Clear Sky Clock (www.cleardarksky.com). This is a site of hour-by-hour forecasts extracted by amateur astronomer Attila Danko from Canadian government
weather data. There are hundreds of Clear Sky Clocks, and Mr. Danko will even create one for your location, on request, if it's not close to one of the existing locations.
Each Clear Sky Clock predicts transparency, steadiness, temperature, humidity, and wind. You can also see road maps, topographic maps, and light pollution charts for each location, to help you decide exactly where to go.
My next stop is the NASA GOES satellite image page at
http://weather.msfc.nasa.gov/GOES/. Each satellite has 3 kinds of images - visible, infrared, and water vapor - each of which can be magnified and animated.
The infrared images are the most useful since they show cloud cover and look the same by day or by night. However, visible clouds at low altitudes don't always show up, and that's what the visible cloud image is good for.
Conversely, a thin haze on the infrared image doesn't necessarily interfere with astronomy. The water vapor map usually shows haze everywhere, regardless of how the sky looks, but it will tell you whether the air above you is getting clearer.
Last, I check for the aurora borealis at http://www.sec.noaa.gov/pmap/pmapN.html.
Living in Georgia, I used to think the aurora was something I could ignore, but in the past few years we've had several fine displays, and this satellite map of the ionosphere helps me anticipate them. My experience is that when the red part of the oval gets within 1000 miles of me, there may be an aurora, and when it gets within 500 miles, there will surely be one.