December 2010

Weather forecasts for astronomers



A new i-Phone application is now available for
amateur star-gazers.

Weather forecasts have an enormous range of uses on land, sea... and in the sky. By applying the power of supercomputers to numerical weather prediction ten years ago, Canadian meteorologist Allan Rahill developed a way to help amateur astronomers find the clearest night skies for their observations.

His weather predictions cover 4076 locations in North America, and have become standard use for major professional and personal observatories. The Astronomical Society of the Pacific awarded its 2010 Amateur Achievement Award to Mr Rahill, on behalf of the Clear Sky Chart team, which offers highly accurate forecasts for professional and amateur astronomers.

A space platform may be the best place to do carry out astronomical observations, but it is costly and technically challenging, notes Mr Rahill. Despite enormous technical advances in astronomy, such as the Hubble space telescope and the next generation James Webb space telescope scheduled for launch in 2014, most modern astronomy is still done from the earth's surface.

Changing weather, with cloud cover and turbulence, poses challenges to astronomers. While powerful telescopes of major observatories reduce the effect of turbulent weather, the trade-off is a very small field of view. More amateurs are also using the growing number of powerful, affordable telescopes available.

The biggest challenge for amateurs and professionals alike is to find a place for observations. Ideally, these tend to be located in remote areas where the sky is dark and the weather is clear and dry. Observatories are usually in remote deserts or on mountaintops.

Yet visibility is never perfect, and varies from night to night. Astronomers require a way to forecast seeing (turbulence) and sky transparency. Observatory operations are scheduled with the expected night sky quality. If poor viewing conditions are forecast, they will do photometry or spectroscopy work. If the forecast is good, they will do interferometry or high-resolution planetary imageries. They need to know in advance, as it can take half a day or a full day of work to install and calibrate telescope instruments.

Astronomical observatory schedules are based on projects submitted 12-15 months in advance. The administration gives an approximate window on when the data will be acquired by the technicians. Ideally, it is done within a window of a week or two, depending on weather conditions that match the type of observation needed for a project.

Most amateurs do basic sky observations. Due to big city light pollution where only a few stars are visible, they drive up to hundreds of kilometers to dark sites. It helps to know in advance if clouds will affect their sky observations, or if the sky quality is poor and not transparent.

On exceptional nights, one sees details on Jupiter, Saturn's rings and polar caps on Mars. Stars won't twinkle and will appear sharper, and deep sky objects such as planetary nebula display more detail.

This context led Mr Rahill to develop a weather Web site for astronomers with a cloud, seeing and transparency forecast,, ten years ago. It soon became a favourite of amateur astronomers across Canada, followed by the United States. Within a year or two it became the standard meteorological source for observatories and amateurs.

Using this weather information, another amateur astronomer, Attila Danko, created the Clear Sky Chart, which displays astro-weather forecasts in a simple, accessible format at

There are 4076 clear sky images for North America online. By clicking any point on the map, a list of images for locations within 30 km around the selected point will appear in a new window. See

Another interesting feature is the alarm, which provides an email alert when chosen conditions are met.


The i-Phone version is:

For an interview with the ClearSkyChart team, see:




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