But clouds got in my way

I'm old enough to remember when software came with manuals bound up in ring binders which allowed pages to be replaced as errors were found, bugs removed and features inserted.  Not a few of these volumes had chapters prefaced with a quote from "Alice in Wonderland" often without any context.  Having contributed to software documentation I will confess seeking out displacement activities such as the hourly mug of coffee, routine admin chores and listening to the radio, maybe I should have delved into poetry and and attempted to engage my reader (assuming that there ever was one).  Another useless fact, all modems in the 1970's were called Gandalf.

Over the past few months, I've been up working my way through a diverse collection of routines and attempting to package them up into commented Python modules.   The comments include a description of the functionality and a reference which points to source of an algorithm.  This usually takes longer than writing the code, but less time than rewriting it when the re-use event fires.  I'm also contemplating adding some poetic quotes to the comments.

I was revisiting the code which produces these cloud height and extent diagrams (which is written in VB.Net) when I heard the radio playing Joni Mitchell's "Both Sides Now" and thinking these graphics were a poor substitute for "Rows and flows of angel hair, And ice cream castles in the air, And feather canyons everywhere...."

Clouds have a significant effect on the output of solar devices.  At the time of writing during the afternoon of an overcast July day, I guess the solar irradiance is around 250 watts/m2, two weeks ago when the sky was almost clear of cloud, it was closer to 850 watts/m2 or in human terms frowns and smiles.   The ideal place to put a solar device, but possibly not to live if you like sea breezes, is a hot dry desert, the clear sky irradiance is high and the number of clouds few.  This is illustrated in the diagram below:

Each circle is a pie chart showing the extent of the cloud cover over a given month for a given height interval.  The dark slices denote an overcast sky and the light ones that there are just a few clouds.  Visually this means the darker the pie, the more clouds in the sky.  The yellow pies show the proportion of clear sky.  This example is for a desert location with lots of clear sky, when clouds do appear most of them are high in the sky and in general, high level cloud causes less attenuation of solar irradiance than low level ones.

Compare this with a plot for a temperate maritime climate, the most striking difference is that most of the cloud is low level, in the south of England this frequently overcast stratus in winter and few and scattered cumulus in summer (except to day which is just dull).  The intervals of clear sky are much shorter than in the desert and there just a lot more clouds.

These diagrams do not take into account the seasonal variations in solar irradiance due to Sun-Earth geometry which cause the irradiance to be significantly lower in winter than summer.

The source data for these plots is Metar reports which are designed to facilitate the safe operation of aircraft, but can also be applied to solar energy.  The plot is based on the highest reported layer of cloud.

At the risk of going off an a tangent, most of this blog was written using a Raspberry Pi.  This was acquired for an energy management project, but is proving to be an effective alternative to my laptop.  I have not done any serious energy analysis, but it is quiet (no cooling fan) and nothing is warm to the touch, so there is little heat dissipation.  About twenty years ago when I first started writing software in this room, the four computers where the heating system, now I have to light a fire in winter to keep warm.