Poseidonho

I had read an article about a hot air balloon, when coming into balloon shape design, I had question on it.
For best shape of the air balloon was the tear drop shape, but I do not know what is the physics behind it. From the article I read is about the pressure vector, and he is using an example of the water pressure and the depth. But I cant found any mathematical proving on it or the detail.

So any suggestion where I can find more detail information about the shape of hot air balloon?