The climate on the Grand Line makes no sense. There are islands where it’s permanently raining lightning (Raijin Island), islands where it’s always cold, hot, windy, rainy etc.
I believe that an Awakened Logia has the ability to change the weather of an area permanently. Over the centuries Awakened Logia users have created islands with crazy climate conditions. One of the most recent examples is Punk Hazard where Aokiji and Akainu fought each other.
They have applied a permanent weather effect to the island and made one side extremely cold and the other side extremely hot.
The Raijin Island might have gotten its weather conditions from a previous user of the Awakened Goro Goro no Mi.