I'm discussing with a couple of friends, and we can't settle.
Do dementors fly in the books?
We recall them flying in the movies, but we're not sure about the books. Its there a specific quote stating whether they fly or not?
Dementors are described in the books several times as gliding. For example, from the attack on Dudley and Harry in The Order of the Phoenix:
A towering hooded figure was gliding smoothly towards him, hovering over the ground, no feet or face visible beneath its robes, sucking on the night as it came
Of course, gliding does not quite equate to flying, but it certainly means they weren't walking in the books.
In addition, during Harry's trial in OotP, Amelia Bones explicitly states to Mrs. Figg:
"Dementors don't run, they glide."
Unfortunately, I do not recall a single quote that would directly prove or falsify Dementor's ability to fly.
As usual, it's hard to prove a negative. However, the time I remember seeing dementors flying in the films was during the Quidditch match in Prisoner of Azkaban. If I look up the passage in the book, the dementors are on the ground and not flying.
Before he'd had time to think, Harry had taken his eyes off the Snitch and looked down.
At least a hundred dementors, their hidden faces pointing up at him, were standing beneath him.
Harry Potter and the Prisoner of Azkaban Chapter 9: Grim Defeat
This is the moment when we see the flying dementor in the film.
There is no quote directly stating it one way or another but there is some evidence both for and against.
Others have given many quotes from the novels which describe the Dementors gliding (and we get a sense that this is just above the ground).
I have been studying Prisoner of Azkaban and in Chapter 12 'The Patronus' as Harry is learning about them with Lupin, it says,
The Dementor stepped from the box. (Page 238 Scholastic paperback edition).
As Harry faces them in defence of Sirius in Chapter 20 'The Dementor's Kiss' on page 384, it says,
It couldn't walk through the cloud of silver mist.
I provide these as evidence that they most definitely glide very low to the ground, surely in order to add to their fear-factor, but they remain very human-like and certainly don't fly as in the films.
We can't emphatically rule it out but there is no direct example of Dementors flying in the books. The closest we get to it is here:
"And as if that all wasn't bad enough," said Fudge, barely listening to the Prime Minister, "we've got Dementors swarming all over the place, attacking people left right and centre..."
The Prime Minister sank, weak-kneed, into the nearest chair. The idea of invisible creatures swooping through the towns and countryside, spreading despair and hopelessness in his voters, made him feel quite quaint.
(Half-Blood Prince, Chapter 1, The Other Minister).
Now swarming is a verb that is often used to describe flying animals (insects, bats, birds and the like) but can also encompass non-flying animals and people as well. The Prime Minister pictures the Dementors swooping around, which may imply flight. It's possible that he's imagining Dementors swooping out of the sky, bearing down on people from a height. But they could also be swooping at ground height. The passage arguably implies that they fly, but this hardly a clear-cut example.
We also have this excerpt, which suggests that Dementors don't touch the ground when they move.
A towering, hooded figure was gliding smoothly towards him, hovering over the ground, no feet or face visible beneath its robes, sucking on the night as it came.
(Order of the Phoenix, Chapter 1, Dudley Demented).
Here they are hovering over the ground, not flying.
I think that there is strong evidence that Dementors don't fly in the books, however - courtesy of The Prisoner of Azkaban.
And then Harry saw them. Dementors, at least a hundred of them, gliding in a black mass around the lake towards them. He spun around, the familiar, icy cold penetrating his insides, fog starting to obscure his vision; more were appearing out of the darkness on every side; they were encircling them...
(Prisoner of Azkaban, Chapter 20, The Dementor's Kiss).
If Dementors could fly, why would they bother gliding all the way around the outside of the lake when they could just breeze over the top of it? Bear in mind that the Dementors were crazed with the prospect of fresh souls and were eager to devour their prey. They would have taken the most direct route towards Sirius, Harry and Hermione. The reason they didn't fly over the lake (as they did in the films) was because they couldn't.
So if the books don't have floating Dementors, why do the films have them?
This behind-the-scenes production documentary from the Prisoner of Azkaban DVD gives an insight into the thought process of some of the filmmakers. Stuart Craig, the film's Production Designer, betrays something of an assumption about Dementor movement (4:23)
Dementors don't walk, they float. And so if they float, I guess it was a fairly natural step to say, "OK, well, what would happen to the robes? How would they move most effectively?"
His starting point is a belief that Dementors "float" (perhaps based on the excerpt from Order of the Phoenix quoted above). Floating is not flying. But it's credible that during the creation process an assumption that Dementors float evolved into the idea that they also fly. They seem to be flying in all the concept art that's playing in the background.
Staying close to the books doesn't seem to have been a priority for these guys. Alfonso Cuarón (the Director) stated that "we wanted to do something that was abstract and metaphysical" with the Dementors. Steve Rawlins (Lead Animator): "it was really more a matter of trial and error and imagination".
Remember that Harry Potter and the Prisoner of Azkaban was released in May 2004, less than a year after the conclusion of Peter Jackson's hugely popular Lord of the Rings adaptation. The Nazgûl had a big role in those films and their look, feel and characterisation had already been well-established in the minds of audiences in the early 2000s. Dementors share a lot in common visually with the Nazgûl - possibly because they are based on them - and so part of the job of the filmmakers was to give the Dementors a distinctive look. Allowing them to fly certainly accomplished this.
I suspect that the main reason that the Dementors fly in the films is because of creative decisions taken by the filmmakers and because of the need to distinguish them from the Nazgûl.
There appears to be a clear example of dementors flying. In the beginning of Order of the Phoenix when Harry stops the dementors we have the following description (my emphasis):
The dementor’s eyeless face was barely an inch from Dudley’s when the silver antlers caught it; the thing was thrown up into the air and, like its fellow, it soared away and was absorbed into the darkness. The stag cantered to the end of the alleyway and dissolved into silver mist.
The dementor was launched into the air by the patronus, but it did not return to the ground. From its airborne position it soared away, which ostensibly means that it flew.