Changes I would make.

[Carol]

No: HPFGUIDX 177655

<snip previous dispute over "spirals" vs. "helixes," which I'm sure
everyone has read>

Margaret: 
> That's right.  Technically "steam" is water in gaseous form, and
while it is in that form and at that temperature (over 100 degrees
Celsius) it is invisible.  You can only see it once it cools down and
condenses enough to be actually water vapor.
>
Carol responds:

But JKR is no more a scientist than she is a mathematician or a
historian. The average person speaks of "clouds of steam" coming from
a teakettle or steam engine as if steam were visible, and of "spirals"
rather than "helixes" ("spiral staircases" having already been listed
as a case in point). Hermione would have sounded pedantic if she had
referred to "vapor rising in characteristic helixes." Or how about
"helices," if we really want her to sound, erm, impressive and
technically correct? Ugh. I'd rather that she sounded poetic than
pedantic here, especially since the image created in the minds of most
readers would be correct. Note that "spiral" as a verb is commonly
used with "downward" or "upward": 

inflected Form(s): -raled or -ralled; -ral·ing or -ral·ling
intransitive verb : to go and especially to rise or fall in a spiral
course <costs spiraled upward> (Merriam-Webster online)

I do think that JKR made some stylistic errors that should have been
corrected by the copyeditor(s), as well as factual errors (Venus
visible at midnight) and all sorts of inconsistencies. She has DD
sepak of Tom Riddle as being in his sixteenth year when he's really
already sixteen and therefore in his seventeenth year. She gives
James's age as fifteen when it should be sixteen. She gives NHN an
anachronistic Elizabethan ruff and, even worse, a Jacobean plumed hat,
neither of which existed in 1492, when Henry VII, Elizabeth I's
grandfather (and my least favorite English monarch) was king of England.

If I were her copyeditor, I would have pointed out these problems and
others (including "fug," which, word or no word, I would suggest
changing to "fog"), but as purely stylistic choices, suitable for the
average reader and especially for children, I would have left "steam
rising in characteristic spirals," technically incorrect though it is,
simply because it creates the right picture in the mind of most
readers and sounds graceful and natural.

Carol, noting that an author has the choice of accepting or rejecting
a copyeditor's corrections and suspecting that JKR would have rejected
the ones Eggplant is suggesting (and most of mine, as well)