Brighton Webs Ltd
statistics for energy and the environment
Brighton Webs is an independent research company located in the south east of England
interested in the economics, design and performance of sustainable energy systems.
We are currently developing software to link weather data to the output of wind
and solar devices.
The solar irradiance of the Earth's surface has two components. The direct
bean which is usually called sunshine is the most signifiant and radiation
scattered by the molecules of the air and particles suspended in it becomes the
diffuse component. On a clear day when the sun is high in the sky around
noon, the diffuse component accounts for approximately 8 - 15% of the total.
Around sunrise and sunset, the diffuse component can account for 50% or more of the
The graph below shows part of the raw output of a simple shaded radiometer, this
device measures the total horizontal and diffuse horizontal irradiance,
the output is in mV which have to be converted to irradiance. Completely clear days
are rare in England and whilst the part of the sky around the Sun was clear when
the observations were made, there was usually some haze or cloud present.
These comments are based on notes made whilst making observations:
This was presnet on several summer days, the effect was to increase the diffuse
irradiance to over 200 watts/km2.
If cumulus is forming or present in the sky, the diffuse iradiance will rise
above the clear sky level. The prescense of low level cloud implies that the
relative humidity in the lower part of the atmosphere is close to 100%.
High Level Cloud
Except when the high level cloud is between the Sun and the photocell, high level
cloud does not have the effect of increasing the diffuse irradiance.
Occaisionally, the total irradiance would dip as an airliner came between the Sun
and the photocell. One morning, a larger than average jet trail appeared,
this was probably created by an Airbus A380 bound for one of the Paris airports.
It is common for there to be significant differences in the irradiance before and
after solar noon. In the south of England, the sky is often clear during the first half of the morning after which clouds start to form, as
a result, the irradiance is higher in the morning than the afternoon. In
other climates, irradiance can be at a maximum in the early afternoon.
Two factors which influence solar irradiance are the water content of the
atmosphere and aerosols and these vary according to the season and climate.
In southern England the water content is higher in late summer and early autumn
than in the winter months.
The equipment seems to attract good natured dogs and it has had to be repaired on
at least three occasions following a canine encounter
This stuff is work in progress and should be treated with caution, a more detailed description
can be found at:
At the time of writing data has been collected for the period Apr-13 to Dec-13, the plan is
to continue until a full year's data is available for analysis.
New posts usually appear on Friday mornings.