The Clear Sky Chart helps predict the best viewing conditions at Observatory Park in Great Falls, VA.

The darker the colored blocks, the better are the conditions for viewing the night sky.

Above the chart, times are given in local Eastern Standard Time (EST), using a 24 hour clock.  For example 18 is 6pm.

  1. Cloud cover: the darker the block the clearer the sky
  2. Transparency: the darker the block the less humidity and the clearer the sky
  3. Seeing: the amount of star twinkling.  The darker the block the steadier the air and seeing
  4. Darkness: black is night, white is day


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Clear Sky Charts are created for observing sites all across North America.  In the map below, each black dot shows the approximate location of an observing site using the Clear Sky Charts with weather data developed by the Canadian Meterological Center.  It is a Crown copyright, product of Environment Canada.