Well, there's a reason it took a while to find a hitter
the science /aerodynamics/ operational/legal economic/weather and performance issues that must be stated to answer your query requires a long and complex explaination to be thorough as all pilots are:
check out the practical test standards for dispatchers and you'll see you've asked a question that requires a long-winded answer:
I'll try to ponder a reasonable [ and short] explanation, but ---I hope someone beats me to it
I hope I can at least give your post a new light---one little reply may just do it