The snow had become impacted

Let’s look at the next sentence.

The snow here had become impacted: it was hard and slippery where people had trodden on it all day.

Here Rowling is describing the snow for specific effect.

The snow had become impacted: it’s not nice and soft, it’s impacted, meaning…
It was hard and slippery: not pleasant
Where people had trodden on it all day: a motif in this passage is people around but just out of sight or out of reach.

Can you find the same pattern in these examples?

The windows in this alley had been shuttered: they were closed tight as people had retreated inside their homes for the night.

The smell in this part of the chamber was overwhelming: the giant drains had become choked with rotting garbage washed down from the tenements above.

The wall here had been marked: it was scuffed and scratched where someone had once tried to climb out of the garden.

Imagine a thing, make an observation and then explain it in a slightly creepy way.

Gothic in the Deathly Hallows: Isolating the heroes