Scrooge has no reaction to snow or any other external forces.

At the beginning of A Christmas Carol, Scrooge lives a lonely existence and is very much in his own world. He is not affected by the people — or weather — around him.

External heat and cold had little influence on Scrooge. No warmth could warm, no wintry weather chill him. No wind that blew was bitterer than he, no falling snow was more intent upon its purpose, no pelting rain less open to entreaty. Foul weather didn’t know where to have him (Stave 1).

The description of Scrooge’s reaction to snow, or his lack of reaction to snow, is really a metaphor for how Scrooge has withdrawn from the world. Scrooge created an insulation of misery. He ignores people and the weather. He doesn’t notice summer or winter.

One of the ways Dickens subtly indicates Scrooge has changed is that, when he wakes up Christmas morning after being visited by the three ghosts, Scrooge notices the air is cold.

Running to the window, he opened it, and put out his head. No fog, no mist; clear, bright, jovial, stirring, cold; cold, piping for the blood to dance to; Golden sunlight; Heavenly sky; sweet fresh air; merry bells. Oh, glorious! Glorious (Stave 5)!

Before being visited by the Spirits, Scrooge was miserable and he didn’t notice the cold. He wasn’t annoyed with it or made miserable by it, but he also didn't enjoy it. Now, Scrooge is a happy man ready to find joy in the simple things in life. He finds pleasure in the cold air; it makes him feel alive. When Scrooge opens the window with the snow-influenced air, now crisp and clean but still cold on Christmas morning, he is happy. It is refreshing, and he is ready to start a new life.