preposterous - Contrary to reason or common sense; utterly absurd or ridiculous.
When Sandy's stock broker called and suggested that he sell half his stocks because he felt the market was overvalued, Sandy thought the idea was preposterous - but it turned out that his stock broker was right.
unforgiving - Not making an allowance for error or weakness in others; not willing to forgive others for things they might have done wrong.

Because they knew that Alaska in the winter is an unforgiving environment, they had to plan their expedition very carefully.