The Solar Bucket

An Overcast Sky from Above

The picture was taken just before sunrise on an early morning flight from London to Glasgow in October 2011.

Weather reports suggest that this was a layer of stratus a few thousand feet thick.  From the ground this would have been a grey, overcast sky, possibly with some drizzle.

Stratus is a feature of the sky over England during the winter months, the diagram below shows how the nature of cloud changes during the year.

In winter, low, broken or overcast skies which are full of stratus are common, in summer these give way to scattered and broken cumulus.

The effect of a thick overcast sky on solar devices on the ground include:

• Significant attenuation of solar radiation caused by a combination of reflection and absorption, often this is less than 20% of the level that would be expected from a clear, dry sky for the same value of air mass.
• There is no direct sunlight (i.e. no shadows) and all the radiation is diffuse.
• Often the density of the radiation is equal from across the sky.  Some experiments suggest that the yield of PV panels might be higher under an overcast sky if mounted horizontally, rather than sloping to face the sun.

These effects are exaggerated because thick overcast skies occur in winter when the Sun is low in the sky (air mass around 4.0 at solar noon).