I wanted to run a point in time illuminance simulation at 9:00 using a perez sky model and I came across a small problem.
I noticed that when the DIVA simulation starts it states the direct normal irradiation used. In my case 590. However, when I checked the direct normal irradiation in the weather file the irradiation at 9:00 is 720, and it is 590 at 8:00.
The first hour with sun at that day is at 6:00.So, I run a few tests and I found out that when I simulate at 6:00 the direct irradiation selected for the simulation is 55, which is the same value as in the weather file at 6:00. However, when I simulate at 7:00 the direct normal irradiation is again 55 instead of 243 as in the weather file. So then at 8:00 the irradiation used is 243, which is the irradiation at 7:00 in the weather file and so on. I though that it could have something to do with the solar time vs local time but it doesn’t make sense to shift for only the first morning hour.
Anyone know why this is happening or am I missing something obvious?
I attached a couple of screenshots with the simulation and the hourly direct irradiation on that day.