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I am trying to clarify data on weather from the Royal Netherlands Meteorological Institute (KNMI); I have incorporated the Solar Position Algorithm (SPA), such that I can calculate sunset and sunrise (by means of the zenith angle, i.e. setting the zenith to 90 degrees). What I am trying to do is get a model of the behaviour of the irradiance (joules per square centimeters J/cm2). That is, by means of the irradiance measurements per hour provided by the KNMI. The thing that struck me is that the overall irradiance is quite variable.

In the underlying figure the sinusoid is a model which should resemble the measurements on a clear sky day. The spiky curve reflects the actual measurements, and the somewhat horizontal line is cloudcover, i.e. 8 is totally cover and 0 is clear sky.

figure

So, what strikes me is for example that the irradiance measurements of two days later (day 59) has a maximum irradiance of 46 J/cm2 with full cloud cover (measure 8).

In other words; full cloud cover on day 57 at 12:00 results in $\approx$ 65 J/cm2, whereas on day 59 with the same cloud cover and same timestamp results in $\approx$ 27 J/cm2.

It seems as if there is another relevant factor which has a significat impact on irradiance. What could it be?

METADATA (Source: KNMI):

  • Cloud cover (in octants), at the time of observation (9=sky invisible)
  • Global radiation (in J/cm2) during the hourly division
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  • $\begingroup$ The dot around 8 am corresponds to the sunrise. Note that positive irradiance is measured before sunrise. $\endgroup$
    – onimoni
    Commented Jun 10, 2013 at 15:05

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The reason for this is that cloud cover doesn't tell you much about the optical properties of the clouds. There are lots of different types of clouds and they differ greatly in terms of droplet sizes, density and thickness. All of this affect their scattering and absorption properties. Cloud cover is just a measure of what fraction of the sky is covered by clouds and doesn't take into account what type of clouds it is.

Even if you have full cloud cover that doesn't transmit any light directly, some light will be transmitted through volume scattering in the clouds. The amount of light transmitted this way will depend mostly on the thickness of the cloud layer. Sometimes you even have total overcast, but the clouds are so thin that you still see shadows on the ground and the countours of the sun through the clouds. From what I understand this is still classified as full cloud cover, even though direct sun light is clearly transmitted.

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  • $\begingroup$ So, the actual irradiance (assuming clear sky) changes gradually over time. Or in other words, the irradiance is in some sense deterministic? $\endgroup$
    – onimoni
    Commented Jun 11, 2013 at 10:43
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    $\begingroup$ @user61001 Yes, assuming clear sky, the irradiance on a horizontal surface will change very predictably based on the solar angle. This is the green curve in your plot. Clouds will generally cause the irradiance to be lower than this, but cloud cover alone is not enough to say how much lower it should be. $\endgroup$
    – jkej
    Commented Jun 11, 2013 at 15:37

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