Early that day the weather turned and the snow was melting into dirty water. Streaks of it ran down from the little shoulder-high window that faced the backyard. Cars slushed by on the street outside, where it was getting dark. But it was getting dark on the inside too.

What this means.please explain.

Without seeing the complete passage, I can only guess that the weather was getting warmer so that the accumulation of snow was melting. The last sentence could mean that it was getting darker inside the house and within the person.