gemination orthography


In this sentence

> ミサイルは高【たか】さ100kmより低【ひく】い所【ところ】を東【ひがし】に750km飛【と】びました  
>  _The missile flew 750 kilometers east at a height lower than 100
> kilometers._

appearing here
<https://www3.nhk.or.jp/news/easy/k10013261541000/k10013261541000.html> the
recording sounds like there is gemination between the end of ひゃく and the
beginning of キロメートル.

What is the orthographically correct way of writing "100km"?

> ひゃっキロメートル

or possibly

> ひゃッキロメートル




I agree with @Simon in the comments: since the gemination comes from the く in
ひゃく, ひゃっ should be the natural choice when writing ひゃっキロ in kana and I would
not be too surprised to see

> [100]{ひゃっ} [km]{キロメートル}

as _furigana_ in a textbook. (That said, in all textbooks for beginners I
could find, there actually seem to be no _furigana_ at all — try to search the
web for images with something like `単位 教科書`.)

Note that numbers are usually not written in _kana_ , so one should not expect
any reliable corpus data to confirm this — e.g. the Balanced Corpus of
Contemporary Written Japanese has 0 hits for both ひゃっキロ and ひゃッキロ.

However, @a20 has found a paper titled "Morphology of numerical expressions in
contemporary Japanese", where the pronunciation of numbers plus counters in
written in kana and you can find the following:

> [![enter image description
> here](https://i.stack.imgur.com/DY5f8.png)](https://i.stack.imgur.com/DY5f8.png)

Also interesting is the following:

> [![enter image description
> here](https://i.stack.imgur.com/qXrIl.png)](https://i.stack.imgur.com/qXrIl.png)

which says that for 6 and 100 (which end in く), gemination essentially always
occurs, but for 1 and 8 (which end in ち), there is some variation.




I agree with @Simon in the comments: since the gemination comes from the く in
ひゃく, ひゃっ should be the natural choice when writing ひゃっキロ in kana and I would
not be too surprised to see

> [100]{ひゃっ} [km]{キロメートル}

as _furigana_ in a textbook. (That said, in all textbooks for beginners I
could find, there actually seem to be no _furigana_ at all — try to search the
web for images with something like `単位 教科書`.)

Note that numbers are usually not written in _kana_ , so one should not expect
any reliable corpus data to confirm this — e.g. the Balanced Corpus of
Contemporary Written Japanese has 0 hits for both ひゃっキロ and ひゃッキロ.

However, @a20 has found a paper titled "Morphology of numerical expressions in
contemporary Japanese", where the pronunciation of numbers plus counters in
written in kana and you can find the following:

> [![enter image description
> here](https://i.stack.imgur.com/DY5f8.png)](https://i.stack.imgur.com/DY5f8.png)

Also interesting is the following:

> [![enter image description
> here](https://i.stack.imgur.com/qXrIl.png)](https://i.stack.imgur.com/qXrIl.png)

which says that for 6 and 100 (which end in く), gemination essentially always
occurs, but for 1 and 8 (which end in ち), there is some variation.

