In the Harry Potter series book 5 (The Order of the Phoenix), when Hagrid comes back, the three (Harry, Ron, Hermonie) go to his cabin "through the thickening snow". How could, a considerable time later, Umbridge discern their tracks and follow them? That should not have been possible, as the thickening snow would have hidden their tracks in a matter of minutes. Did Umbridge use magic, or what?
It's just "thickening", which doesn't say anything about the speed. Footsteps could last a while and you don't need every track to see where it leads.
If she's working from an assumption the path was already heading cabin-wards, then just a few footsteps would do.
Thanks for the comments to form this answer.
Normally, the answer to a question like this might simply be that the author forgot that she had described the snow as thickening, or that she didn't fully think through the implications that would have for track-following.
However, in this case, it seems from a later passage that the author had both the thickening snow and the tracks in mind while writing this. At the end of the very chapter in question, the trio return to the castle with the following description:
"I dunno if you got through to him," said Ron a short while later when, having checked that the coast was clear, they walked back up to the castle through the thickening snow, leaving no trace behind them due to the Obliteration Charm Hermione was performing as they went.
It thus seems clear that, at least in the author's opinion, thickening snow alone would not be enough to conceal tracks.