Solar Envelope probelm

Dear Boris, Chris, Mostapha,

I have practiced the Solar Envelope example file and it was extremely cool!!

I have several questions as I took a close look at the definition. Could you kindly explain to me?

1. The definition only takes into account the first and second day for each month. Why do we only have these two days for calculation?
2. The definition set a conditional statement for the dry bulb temperature smaller than 16℃. Why do we choose this bottom line to calculate the effect of solar access?
3. In terms of the practice I’ve done, I am a bit confused about the Solar Collection envelope. Most of the lowest heights from which a new built volume are under the ground. Is there any mistake involved? (see attachment)
4. 

Thanks a lot in advance!!!