Abnormal sky radiation of Tregenza sky for low altitude sun positions

It seems when the sun altitude is low (early morning sun close to horizon), the Tregenza sky dome visualization of the sky radiations on the 145 patches is quite strange:

sun position for 1-Jan 8pm, using Singapore epw file:

Tregenza sky (145 patches) visualization:

The direct radiation from the sun is not calculated at all:

Using a higher resolution Reinhart sky subdivsion still produces the same result:

A reasonable visualization for sun position on higher altitude should look something like this, although the match between the positions of the sun and that of the sky patch with the largest radiation value is not always precise:

I worry that the calculation of the sky matrix for all the low altitude sun positions might be affected …

tregenza sky patch viz.gh (422 KB)

Hi Grasshope,

You can use a denser sky but I doubt that will change anything as based on the weather data there is no direct radiation at 8 am. That’s why you get a fully diffused sky. Here are the data from the weather file for 8 am and 9 am.

I forgot to answer the second part of your question. The position of the sun in sunpath and the sky is offset by half an hour. gendaymtx uses the wea file to generate the sky and wea file are considering the radiation values for the hour between time steps. 00:30, 1:30 and so on. Here is the adjusted sun position with a higher resolution sky.

Thank you, Mostapha.

Do you mean that, for a particular HOY, e.g. “10” or 1-Jan 10am, the skyDome component actually shows the radiation of the Tregenza sky (145 patches) when the sun is positioned at 9:30am?

If that is the case, does gendaymtx takes the average value between 9am and 10am for the direct normal, diffuse horizontal and global horizontal radiations from the weather file for its calculation?

May I ask the source of reference mentioning gendaymtx’s consideration of “radiation values for the hour between time steps” since it does not seem to be mentioned in the manual of gendaymtx?

It seems that the skyVisualizer produces a more reasonable Perez sky image for the hour with no direct radiation (e.g. 7:30am, 1-Jan) as compared to that by the skyDome component:

tregenza sky patch viz _v2.gh (422 KB)

Most of your assumptions are correct. gendaymtx uses a wea file to generate the cumulative sky, which is what ladybug is using. Wea file is generated using epw2wea which is set to generate the values which is offset by half an hour. Since the values are still hourly (and cumulative) they are not averaged. You can read more here why they pick the time between the hours in epw2wea (which was originally part of Daysim): https://groups.google.com/forum/#!topic/daysim/Fdajly9RRDA