A Question of Temperature

I've been reading more and more about the operations of airships recently and can't decide whether I'm missing something or there is some contradictory information out there.

Some info speaks of the rise in outside (air) temperature and how it degrades the performance of an airship, whilst other reports tell of how sun warming the envelope (and I assume heating the helium inside) provides increased lift.

Can anyone clear this up for me? What effect does temperature have on the capability of an airship?