Brighton Webs Ltd.
Statistics for Energy and the Environment
The attenuation of solar irradiance is influenced by clouds. A cloud is is a mass of water droplets in suspension. For a cloud to form two conditions must be met, the first is that some water vapour is present and the second is that atmospheric conditions must allow water vapour to condense. As the relavative humidity increases, so too does the probability that water vapour will condense and form a cloud.
It appears the the attenuation of solar irradience is related to the amount of condensed water encountered by the Sun's rays before reaching the Earth's surface.
The graph below shows the distribution of pricipitate water on an almost cloudless summer day over southern England.
A high proportion of the water vapour is found within 6,000 feet of the Earth's surface. Whilst the above graph relates to a specific location, similar distributions can be found in other climates. In this example, only a very small amount of the water vapour has condensed into cloud.
The distribution of water vapour correlates with the attenuation of solar irradiance. Only occassionally do high level clouds (base greater than 18,000 feet) have a significant effect on solar irradiance, typically, they consist of a thin layer of ice crystals, the modal value of CSF is around 0.75 Broken and and overcast layers of medium level cloud (base in the range of 6,000 to 18,000 feet) have greater impact with typical values of CSF around 0.30. It is low level cloud (base less than 6,000 feet) which has the greatest effect on solar devices. Typically, this interval contains more than 70% of the water vapour, the state of this closely linked to the ground temperature. In hot desert areas, surface heating ensures that the water content is remains as vapour. In temperate areas, the vapour, even in summer condenses into clouds with CSF values around 03. In winter, the dense, overcast cloud can form, often with the base around 1,000 feet. In these conditions, the CSF can frequently be in the range 0.10 to 0.20.