Clear Sky Fraction

I live in a temperate maritime climate (i.e. the south coast of England) where clouds in various forms are frequently present in the sky.  Some simple experiments using a very small solar panel suggested that clouds have a significant effect on the performance of solar devices.  On a clear summer day, the global horizontal irradiance (GHI) at noon can be close to 1000 watts/ms, a few days later when the sky is overcast this can fall to less than 200 watts/m2.  In winter, the higher frequency of occurrence of clouds further increases the overall attenuating effect.  Also, the nature of the irradiance changes, under a clear sky the diffuse fraction might be around 15% with, under an overcast cloud sky, the diffuse fraction rises to 100% and there is no direct beam irradiance.  I wanted to quantify the attenuating effect of clouds one possibility is a statistic called the clear sky fraction (CSF).

This work has not been reviewed and should be treated with caution.

CSF is defined as the ratio of observed GHI under a cloud sky, to the estimated GHI under a clear sky (i.e.if the clouds were not present in the sky).

Unlike wind whose velocity can be more-or-less over several hours, solar irradiance is constantly changing, it is close to zero at sunrise and sunset and at a maximum around solar noon.  This makes it desirable to use a ratio which is independent of Sun-Earth geometry. The principal input for models of solar irradiance is air mass (AM) which is a ratio describing the amount of atmosphere the Sun's rays most pass before reaching the Earth's surface.  At solar noon close to the equator, the value of air mass is close to 1, whilst it is approximately 15 around sunrise and sunset in the temperate latitudes during the summer.  Based on observations in the south east of england, the author suggests that the "economic" range of air mass values is in the range 1 to 6.  At an air mass values of 6, the zenith angle is approx. 75 degrees (corresponding to an altitude of 15 degrees).  Depending on the terrain, when the sun is low in the sky, the shadow of hills, trees, buildings etc. effect the irradiance up a flat surface.  Experience suggests that within the range 1 to 6, CSF is more of less independent of air mass.

Horizontal irradiance was chosen because of the importance of diffuse irradiance under a cloud sky. Under a thick overcast sky there is no direct beam element to the irradiance which is all diffuse and is evenly distributed around the hemisphere of the sky.  Whilst it would be more convenient to consider a sloping surface (which is the normal way of mounding most solar devices), this would not account

 for all the diffuse irradiance.  Also, GHI is the most commonly collected form of solar irradiance data.

A problem in calculating CSF is the choice of method for estimating the clear sky irradiance.  There are two options.  The simplest is to use some form of model, many of these use atmospheric data such as water column and aerosol optical density and if this data is available, are capable of producing good estimates of direct and diffuse irradiance, the downside of these models is that detailed atmospheric data may not be available for the location where the observations are being made.  An alternative is to use observations of clear sky irradiance at the chosen location and the correlate thises with air mass.  Either approach has a degree of uncertainty associated with it.  not least of which is that whilst the reflection and absorption of clouds will be the dominant atmospheric effect, others such as moisture content will also have an impact.

I am currently messing with cloud sky models of irradiance which are based on CSF.