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The Impact of Clouds on Dark Sky

Clouds make the night darker right?

Not always. On nights when there is no visible moon and the sky is dark, cloud cover will, in fact, reflect back gound light. You will observe evidence of this over any major city. In an area such as Lough Gur where there is minimal ground lighting, on cloudy nights light can be bounced back from near-by towns and Limerick city.

But on a bright night …

However, on nights where the sky has a light source such as the moon, cloud cover can then block out some of that light making it darker than it was before, even allowing for the reflected ground light.

Observation from April 27th to April 28th

The infographic to the right show data gathered on the night of April 27th, 2016. On this night a member of our project team stayed up to make visual observations of the night sky to explain the data. Here are some key points.

  • The Sun set at 20:55hrs, at which point there was twilight until around 23:00.
  • The sky was clear for most of the night.
  • At 23:00 it was truly night with the light meter recording SQM values in excess of 21.5
  • Shortly after midnight (00:15) we observed a slight “brightening” in our data, dropping to 21.29. Visual observation identified that cloud cover coming in over the lake was reflecting ground light back down.
  • Sky darkness peaked at 21.65 prior to moonrise.
  • At 01:45 the moon started to rise, at this time we start to see a steady decline in the sky darkness as the moon continues to rise.
  • At 02:45, 03:10 and again at 03:45 our light meter registers a slight darkening in the sky. Once again visual observations identified cloud cover was blocking out the moon.

In this case observations of the sky by members of the team allowed us to interpret the SQM data. Going forward with our SQM data, Lunar & Weather data we can explain what is happening in our sky based on the data itself.

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