The Signature of Clouds

Experience with a 4.5 watt solar panel in 2009 showed that clouds were a significant factor in the energy yield of the device.  Under a clear, summer sky the panel might generate a current of 300mA at noon, under an overcast sky in the same month, this would drop to less than 50mA.  My first attempt at understanding the attenuating effects of clouds was to sit in the backyard with a flat photodiode and a multimeter and watch the output change as clouds passed overhead.  Later I found some datasets which allowed some form of statistical analysis, whilst these have been instructive, there is a lot to be learnt from simply staring at the sky.  Whilst the objective was to suggest some form of model which related observed cloud cover to the attenuating effect of clouds, the variations in the meter readings suggested that cloud types have distinctive signatures.  This post is based on some hand sketched graphs based on staring at a digital multimeter.  I have recently acquired a Raspberry Pi and an interesting project would be to explore this concept further using the Pi to record images of the sky and the output of a photodiode.  One day...............

One way of describing the attenuating effect of clouds is the Clear Sky Fraction which is defined as:

Clear sky irradiance is constantly changing, it is close to zero at sunrise and sunset and peaks at solar noon, the attraction of this ratio is that it is independent of sun-earth geometry.  Experience suggests that it is more or less independent of air mass for values in the range 1 to 6.  Under a perfect clear sky, CSF is close to 1.0.

Samples of CSF for a given cloud base and extent (excluding an overcast sky) will form a bimodal distribution, but the nature of short time series may indicate the type of cloud. The graphics below are sketches based on manual observation, rather than a comprehensive analysis from the output of a data logger, thus they should be treated with caution.

Cumulus is a common feature of an English summer sky, typically the CSF varies between 0.4 and 1.1.  CSF values greater than 1.0 occur when the edge of a cloud pass passes across the sun's rays causing a short period of increased diffuse irradiance.  The ratio of the duration of high and low periods depends on the extent of the cloud.

In winter, the sky is typically overcast with a thick layer of stratus, then the CSF is generally in the range 0.2 to 0.4 with only small random fluctuations.

 High level cloud, such as cirrus,  has a much less attenuating effect than types such as cumulus,

These sketches are the result of observations from a sky with only a single layer of cloud.  The author's experience suggests that as the number, extent and complexity of the cloud layers increases, the values of CSF tend to behave like an overcast sky, thus a complex sky might not fit these somewhat idealized patterns.