by Todd Carlson

Is there anything more disappointing to stargazers than watching perfectly clear daytime skies disappear behind a thickening blanket of clouds shortly after dusk? Although satellite photos and weather forecasts are available on the Internet, there’s nothing specifically designed for astronomers to check nighttime cloud cover—until now.

The Clear Sky Clock (CSC), developed by Ottawa software guru and astronomy enthusiast Attilla Danko and Environment Canada meteorologist and amateur astronomer Alan Rahill, is a free observing tool that night-sky enthusiasts will find invaluable. In effect, it provides a private, astronomy-oriented, overnight weather forecast for any site in North America.

The CSC is a small chart designed for specific locations (the one shown here is for my town). It displays cloud, transparency, seeing and darkness on a scale rated by coloured blocks, with dark blue being the best and white the worst. Daylight and moonlight are also factored into the darkness scale. Updated twice daily, the CSC presents an exceptionally accurate forecast for the coming night.

Many times at my observing site, the CSC has forecast clearing skies later in the evening, even after a completely cloudy day. Often, the sky clears within 30 minutes of the predicted time. The CSC chart above shows that the skies in my area should be clear tonight (Wednesday), but Thursday night looks like a write-off. This information allows me to plan my observing.

Danko has also incorporated other handy features. From any CSC, you can be linked to satellite predictions as well as road, topographical and light-pollution maps for that particular location. To access the CSC, simply connect to the website www.cleardarksky.com and select the existing clock site nearest you. The CSC is accurate for a 25-kilometre radius. More than a thousand CSCs already exist, so there is likely one near you.

You can also submit your observing site’s geographical coordinates, elevation and time zone, and Danko will design your own CSC accessible from the website. Although Danko provides the CSCs for free, users can become sponsors of any particular CSC to help defray the cost of running the website.

FAIR-WEATHER FRIEND

Updated twice daily, the Clear Sky Clock is the only weather-forecasting tool specifically designed with backyard astronomers in mind. Forecasts are provided for more than 1,600 locations.