But each year , the dogs are still sent out into the snow whenever a traveller is in difficulty.

这个句子中的whenever引导的并不是让步状语从句，而是时间状语从句（兼有条件意味）。whenever改为if，句子意义基本不变。

But each year , the dogs are still sent out into the snow if a traveller is in difficulty.

主句和从句都不是表示单个事件，而是表示习惯意义。