benefits of green Japan Rail Pass?

I am about to buy 7 day rail pass for visit to Japan in September. Can you tell me the benefits of buying a green rail pass instad of an ordinary one?
We are visiting Tokyo, Kyoto,Hiroshima,Osaka and Mount Fuji. Can you tell me what the weather will be like? Will I need warm clothing for the evenings! Many thanks Maggie

Can you tell me the benefits of buying a green rail pass instad of an ordinary one?

Green cars come with more space and service and are usually less crowded.

Can you tell me what the weather will be like?

Here you go:
http://www.japan-guide.com/e/e2273.html

Ordinary JR Passes are fine..save the extra for meals etc...you really do not need a Green Car Pass..most trains don't have them anyway.
Layers are always the go with clothes when travelling, and I doubt you will need any heavy clothing in September

you really do not need a Green Car Pass..most trains don't have them anyway.

Virtually all long-distance trains have them. One prominent exception: the Hikari Rail Star trains between Osaka and Hakata.