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.

Diffuse Irradiance

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 total.

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.

Diffuse irradiance - Haze, cumulus and cirrus and clear sky

These comments are based on notes made whilst making observations:

Haze

This was presnet on several summer days, the effect was to increase the diffuse irradiance to over 200 watts/km2.

Cumulus Cloud

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.

Jet Trails

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.

Morning/Afternoon Variations

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.

Seasonal Variations

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.

Dogs

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.

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