Clouds and Diffuse Irradiance

This page describe a quick-and-simple study to look at the distribution of diffuse irradiance under a cloudy sky.  The results are being published as they are collected in an attempt to illicit peer-review.

The Equipment

The equipment was constructed from materials to hand, they are not an optimum selection, rather a pragmatic one.  The  main component is a light dependent resistor mounted at the base of a 300 mm length of white waste pipe whose translucence was reduced with a wrapping of parcel tape.   The photo shows it in position.

Operation consists of setting the altitude to 15, 30, 45, 60 or 75 degrees, then rotating the instrument from 0 to 330 degrees in 30 degree increments and recording the resistance of the light dependent resistor.

The LDR resistor has a "typical" resistance of 20k at 100 lux.  A simple callibration gave the relationship between resistance an illuminance of:

The results should be treated as relative illuminance which consistent for the experiment, but are unlikely to represent accurate absolute values.

Overcast Sky - 26-Jul-2010 15:00

The irradiance increased with altitude, but was relatively constant with zenith.  The cloud was thick and low, there was a small break to the northeast, hence the higher illuminance in that direction

Broken Cloud - 27-Jul-2010 14:00

The early afternoon sky consisted of broken cloud, with the sun occasionally visible through the cloud. Compared to the overcast sky the day before, there was significantly greater illuminance in the part of the sky in which the Sun was located.

Few Clouds - 28-Jul-2010 11:30

As so often happens when there are a few clouds in the sky, there was some fluctuation in the readings.  The maximum illuminance was to the south east at an elevation of 45 to 60 degrees.

Clear Sky - 16-Aug-2010 14:00

The illuminance is clearly coming from the sun's disk, with high intensity, even though the readings were taken nearly three weeks those relating to the clear sky.