Clear Sky Chart

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Clear Sky Charts (called clocks until 2008-02-29) are web graphics which deliver weather forecasts designed specifically for astronomers. They forecast the cloud cover, transparency and astronomical seeing, parameters which are not forecast by civil or aviation forecasts.[1] They forecast hourly data, but are limited to forecasting at most 48 hours into the future. Each individual chart provides data for only a 9 mile radius, and so are essentially point forecasts. There are clear sky chart forecasts for over 5300 locations, though the coverage area is limited to Canada, the USA and parts of Mexico and the Caribbean. Locations are typically cities, professional and public observatories, colleges and science centers. However there are also clear sky charts for star parties and backyard observatories.

Sample image of a clear sky clock for one particular observatory.


In 2000, Allan Rahill, a meteorologist at the Canadian Meteorological Centre (CMC) and amateur astronomer, created a forecast processing step that took data from CMC's Global Element Multi-scale (GEM) forecast model and created a new forecast of cloud cover.[2] Rahill specially designed his cloud forecast to consider the formation of cirrus clouds. The cirrus cloud modeling distinguishes Rahill's model from other cloud forecast models, as sufficient cirrus clouds to make a night unusable for astronomers is still called "clear" by civil weather forecasts.

In later years, Rahill added a forecast of astronomical transparency, which is a measure of how much starlight traverses the Earth's atmosphere when otherwise free of clouds. Rahill also added a forecast of astronomical seeing which uses forecast data of turbulence and temperature gradients in the atmosphere to forecast its optical steadiness.

In 2001, Attilla Danko, computer programmer and amateur astronomer, began to summarize Rahill's hundreds of forecast maps by displaying only one pixel, from each map, laid out in rows.[3] The resulting meteogram, called a clear sky chart, showed all of Rahill's forecast data, but for only one location. Danko writes "It shows at a glance when, in the next 48 hours, we might expect clear and dark skies for one specific observing site". Danko accepts requests from observatories and private individuals to create new CSCs for locations not currently covered.[4] However, since CMC's GEM model only covers North America, CSCs are limited to North America.

Name change[edit]

On February 29, 2008 Danko changed the name of the CSCs to "clear sky charts" to avoid any possibility of legal action on the part of Skyclock company of Michigan who owns a USA registered trademark on the name "SKYCLOCK".[5] Danko's attorney opined that he was not infringing Skyclock Company's trademark, but also advised that changing the name was the "least painful" and least "expensive" solution.[6] However, there continue to be references to the old name on web sites not controlled by the domain

Alternative Services[edit]

In January 2017, Astrospheric was created to summarize Rahill's astronomy forecast and make better use of the increased data output from the CMC's Regional Deterministic Prediction System (RDPS). While both Clear Sky Charts and Astrospheric display similar data, Astrospheric uses the underlying binary data to provide wind direction, dew point temperatures, and improved Transparency forecasts,[7] all using bi-linear interpolation of model data.[8] Astrospheric also generates forecasts dynamically which allows for a visual forecast to be built for any location within the RDPS window instead of using pre-defined locations.


Rahill and Danko have received awards from meteorological and astronomical organizations:

Since the clear sky charts terms of use permit non-commercial web sites to display clear sky chart images, they are most commonly recognized by private and club astronomy websites in North America.


  1. ^ A. Danko., "'A Weather Clock for Observers'", Sky and Telescope Apr 2003; 62; [1]
  2. ^ *Ivan Semeniuk., "STARSTRUCK: Clear Sky Clock", Daily Planet, Discovery Channel. April 15, 2005
  3. ^ Lydia Lousteaux., "'Interview with the creators of the Clear Sky Clock'"; [2]
  4. ^ "The Clear Sky Clock" Sky News magazine. #54 | Volume IX, Issue 6 | March/April, 2004
  5. ^ "'Clear Sky Chart FAQ: Why the name change?'";;
  6. ^ "'Clear Sky Chart News'"; Feb 29, 2008;;
  7. ^ [3] "'Improved data availability'"
  8. ^ [4] "'Bilinear interpolated map data'"
  9. ^ "Astronomical Society of the Pacific : GET INVOLVED : Membership : Become a Member".
  10. ^ CMOS Prizewinners 2004 / Lauréats des prix de la SCMO, 2004
  11. ^ "Tim Cole., Astronotes August 2005". Archived from the original on 2006-10-03. Retrieved 2008-01-02.
  12. ^ TSP History Archived 2006-02-07 at the Wayback Machine
  • "A Weather Clock for Observers". A.Danko. Sky & Telescope Magazine; Apr 2003; 62;
  • "The Clear Sky Clock" Sky News magazine. #54 | Volume IX, Issue 6 | March/April, 2004
  • "Interview with the creators of the Clear Sky Clock" Lydia Lousteaux.
  • "STARSTRUCK: Clear Sky Clock", Ivan Semeniuk'. Daily Planet, Discovery Channel. April 15, 2005
  • "Weather Resources on the Internet", Observer's Handbook 2008, pp78–82, Royal Astronomical Society of Canada
  • "How We Really Predict Clouds" Danko, A., Journal of Irreproducible Results, 2006, VOL 50; NUMB 1

External links[edit]