What is the basis for the proverb 'Flowers smell best just before a rain'?

That moist air is more amenable to transmitting scents is the basis for the proverb "Flowers smell best just before a rain."

Specifically, moist air is a harbinger of coming rainfall. Atmospheric and terrestrial moisture enhances the strength of an odor. That is why flowers release fragrances just before rain, and earthwormy smells pervade grounds and sidewalks.