Randall Garrett & Robert Silverberg 
Shrouded Planet 

Shrouded – perpetually shrouded – by a blanket of clouds, the planet of Nidor made its slow way through space. A year on Nidor was equal to 3000 Earth-years. To the Nidorians change was unthinkable. Until the Earthmen arrived. And then a change cataclysmic, and the first for many a millennium, hit Nidor. A change that was to split the planet against itself.