[pct-l] when's it cold enough for pants?

[Moynihan]

When is it necessary to start carrying pants on the trail (vs shorts of
course, thinking ahead of someone replying with a ridiculous, but
still amusing remark)?
I don't plan on using pants to protect my legs from the sun. And I carry
tights so that would be extremely uncomfortable anyway.
Would it be wise to carry them the whole trip? Or perhaps send them to
myself up before the Sierras?
I run very warm and my legs can tough it out on an occassional cold night.

Thans
~Spirit