When to not use SI symbols for units?

Question

In the book What If?, the author usually writes the units out as kilometer, atmosphere, megawatt instead of km, atm, MW. However, he does use the symbols in the drawings:

In one line, both mm and millimeter are even used together:

The core of a lightning bolt is a few centimeters in diameter. A bullet fired from an AK-47 is about 26 mm long and moves at about 700 millimeters every millisecond.

This makes me confused because I can't find a reason to not use the SI symbols. I guess the reason is similar to writing numbers in words when doing so doesn't look silly for a better reading-flow-feeling. But in the above example, I can't explain why the first number should use mm while the second one should use millimeter.

Is there a general rule for writing out the unit instead of its SI symbol?

Answer 1

The most important thing (at least, in almost all circumstances) is consistency. Most of the time, as long as you're consistent, you'll be forgiven for whatever choice you make.

If you're writing in a particular context, and other people writing in that context write the units in a certain way, do that. If you know of some specific reason why a certain choice is wiser or more appropriate than another, do that. Otherwise, just take your pick and be consistent.

"What if?" is essentially a comedy book (or at least, due to its association with a well known humorous web-comic, comes with some expectation of comedic content), so it's very possible the writer is being deliberately inconsistent for comedic effect.

Also possible is that the writer and editor both simply missed it. It happens.

Answer 2

Know your audience

is an important concept to keep in mind when writing a book.

This book was published by Penguin House, one of the big 5 publishers, with excellent editors. While anyone can make a mistake, it stands to reason that this pattern of mixed notation is deliberate and not the result of sloppiness.

I reason that since this book was written for a general audience, and not just the author's devoted XKCD following, they decided that writing for maximum clarity of the average person was of more importance than following notational standards of scientific publications.

Since the book, and its sequel ('What If 2'), are excellent examples of reasoning and the application of the scientific method it stands to reason that this is very deliberate. I imagine that when the author needed to make a choice between studious adherence SI notation and readability and understandability, for non-technical audiences, he chose to side with readability and understandability. The reasoning seems sound since an individual versed in SI units will understand what he is saying and find it entertaining. Someone not versed in SI units might find the notation itself daunting and put the book down. As the author, it is in Randall Munroe financial best interest when people read all or most of his book and then tell everyone how great it was and that they should also buy a copy and read it.

Answer 3

If you want to know the standard way {1}, see: http://physics.nist.gov/cuu/pdf/sp811.pdf gives the rules, including capitalization and abbreviations. {1} NIST was formerly the National Bureau of Standards