Cumulus Support

I have been trying to work out the settings for (ie Sun threshold, Sun minimum and Trans Factor) for the calculation of sunshine hours. I have read the thread back in 2011 but found it did not help.
Am I trying to to make the sunshine trace match the max. solar radation, is it the threshold that changes this?
If I want to record the sunshine at sunrise and sunset do I set Sun Min to 0%?

CGL

CGL wrote:Am I trying to to make the sunshine trace match the max. solar radation, is it the threshold that changes this?

Have you looked at the help for the station settings screen? The threshold is the percentage of theoretical max above which the current actual reading is regarded as indicating that the sun is shining. The trans factor affects the the theoretical max calculation, so if you are trying to match the two curves when the sky is clear, it's the trans factor that you adjust. Note that due to the nature of the sensor and the variability of the atmosphere, it's not easy to get a perfect fit.

If I want to record the sunshine at sunrise and sunset do I set Sun Min to 0%?

If you do that it is likely that your sun hours figure will be spuriously higher due (amongst other things) to the low numbers involved at those times. Note that it is not a percentage, it's the absolute solar radiation reading.

DOH! Steve beat me to it, I'll cross post anyway!

Transmission factor tries to model how transparent the air is, so making this lower will lower the calculated value. Adjust this to match (or read just above) your sensor reading on a clear sunny day.

Threshold is what percentage of the calculated value will trigger the sunshine flag to be set. So if you set 70%, your sensor must read >= 70% of the calculated value to set the sunshine flag.

Minimum value is used to exclude recording sunshine when the sun is low is the sky, at these low elevations the sensor can depart from the calculated value considerably due to lots of factors (the light traverses a lot of atmosphere, shading, reflections from nearby buildings and clouds etc.). Also the numbers involved are relatively small, so the relative percentage error goes up. So the lower this value to closer to sun rise/set you will record sunshine hours, but the more prone to error those figures will be.

Thank you for the replies that helps a lot, will the sun shine again!

CGL

Spent a lot of time sorting this out myself!

Mark's comments are on the mark (pun intended!)

Also, nothing you do with those three settings will change the amount of W/m2 recorded or displayed on the graph.
Trans Factor changes the Theoretical Max curve up or down.
It takes days (of plenty of sun) to get the Trans factor right. Bear in mind that the W/m2 can spike and even remain for a time above the theoretical maximum. We have recently had clear sky plus a lot of small bright white clouds passing. If the clear patch is big enough then the curves may match, if the clouds are any where near then they reflect sun back from a different angle adding to the total W/m2.

My current settings are 75%, 50 and 0.87 and will stay that way as we are currently in mid summer so the best time to configure these settings.