Why is the sky blue?

In the story The Outcasts of Poker Flat, the sky is described as both clouded and overcast.... never blue.

He looked at the gloomy walls that rose a thousand feet sheer above the circling pines around him, at the sky ominously clouded...

But when he returned to the party, he found them seated by a fire—for the air had grown strangely chill and the sky overcast—in apparently amicable conversation.