This, of course, was summarized most eloquently at the zenith of human evoloution: the 1982 hit single by Toto clearly stating, "I bless the rains down in Africa."

You think rain is your ally?

You merely adopted the damp. We Brits were born in it, molded by it. I didn't see dry sand until I was already a man...

Was that area a desert 250,000 years ago?

I'm still missing something here. For it to be useful, I'd imagine that it would need to inform decisions, and do so where existing senses would fail.

At least in my environment, if I can smell rain, I could also just as easily use my eyes to see the cumulonimbus clouds and say "rain, due east".

In the savanna are there scenarios where the only awareness of rain would be smelling it? Can you derive directionality at 5 parts per trillion? Does it matter?

You'd think more African animals (especially predators) would have that ability, then