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I understand that ことになる has the meaning "It has been decided that" and can be
used to point to a ritual or habit of a society like taking off your shoes.

However, I don't get what it means when it is like this:

> あすで１週間雨が降りつづくことになる。

How is that different from say,

> あすで１週間雨が降りつづくようになる。
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2014-07-25T06:04:23.120
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grammar
How do ことになる and ようになる differ? (When ことになる cannot = "It has been decided that")
Where did you see the first example? It doesn’t make sense to me unless you
believe in the Greek gods and are saying it has been decided (by them) that it
will continue to rain.

The verb that precedes ことになる must be one that is capable of being decided and
done. Natural phenomena out of the hands of human beings, cannot ことになる.

You are likely getting confused because in English, “to decide” can mean
either “decide to take an action” or “decide some fact will come to pass.”

In the case of it continuing to rain, this would be a decision (conclusion,
actually) that some fact will come to pass. This is _not_ what you use ことになる
for. You use it for the “decide to take an action” one.

Think of ことになる as a passive form of ことにする

ピザを食べることにした = I decided to eat pizza. ピザを食べることになった = It was decided that
(we/I/he) would eat pizza.
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2014-07-25T11:12:58.583
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> １週間雨が降りつづくことになる。 is totally fine.  
>  "it will end up raining for a week"  
>  １週間雨が降りつづくようになる。 is very strange.  
>  "it will come to be that it rains for a week"  
>  １週間雨が降りつづくようだ。 is fine.  
>  "it appears that it will rain for a week"

Using user1713450's pizza example:

> ピザを食べることにした  
>  "I decided to eat pizza"  
>  ピザを食べることになった  
>  "I ended up eating pizza"  
>  ピザを食べるようになった  
>  "I came to eat pizza"  
>  ピザを食べるようにした  
>  "I made an effort to eat pizza"
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2014-07-25T14:54:06.837
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First, I must clearly state that " **it has been decided that ~~** " is a
highly overrated translation of 「～～ことになる」 among J-learners. Truth is that that
is not what it means even half the time.

「あすで１週間雨{しゅうかんあめ}が降{ふ}りつづくことになる。」 means " **It will have ended up raining a
whole week**." It has already been raining for 6 straight days now (at the
time of utterance) and the weather forecast says that it will rain again
tomorrow.

A more natural or less stiff way to say this would be
「もし明日{あす}も雨だったら、（これで）一週間雨が降り続{つづ}いたことになる。」, but virtually all native speakers
would understand 「あすで１週間雨が降りつづくことになる。」.

Here, 「ことになる」 means " **to end up (in a certain way)** " and that is already a
very common usage of the expression.

Your second sentence 「あすで１週間雨が降りつづく **よう** になる。」, however, makes little sense,
I am afraid.

「ことになる」 refers to (and focuses on) the final result whether it happened
naturally or someone made it happen on purpose.

「ようになる」 focuses on the change from one situation to another. "It was like A
before but it is like B now." e.g. 「去年{きょねん}までは泳{およ}げなかったが、今年{ことし}からは泳げる
**ようになった** 。」 In this example sentence, you cannot replace the 「ようになった」part
with a 「ことになった」 because it talks about a **change** in one's swimming ability.
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2014-07-25T15:10:18.017
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