When is it appropriate to choose にわたって or を通じて in regard to time?

Question

I seem to have a hard time recognizing when to choose either にわたって or を通じて in the following question:

この地方は一年（　　）暖かく、とても過ごしやすい。

The correct answer is を通じて. My idea is that を通じて would make this say "throughout this year". With にわたって, I thought that it would be "Over the course of one year". Yet, I'm thinking that these are incorrect. What is the difference between these two words with respect to time? Also, what would be the correct translation for this phrase?

Answer 1

• にわたって expresses the duration of a single ongoing project/activity.

試験のために、一年にわたって勉強した。
'I spent an year studying (possibly taking holiday breaks) for the exam.'

ここは40000年に渡って氷河期で、凍っていた。
'This place has been in ice age and was frozen for 40000 years.'

• を通じて means that multiple instances of a single event or a single state are repeated consistently throughout the time span.

試験のために、一年を通じて勉強した。
'I studied throughout the year without taking a holiday break (possibly for a few years) for the exam.'

ここは1年を通じて凍る。
'This place is frozen for the whole year.'

Answer 2

…にわたって specifies the length of a period for which an event or a state continues. 一年にわたって暖かい would mean “warm for one year,” and maybe it is not warm after one year.

一年を通じて means “all the year round” (≈ no matter which day in the year it is), and it does not specify the length of a period for which an event or a state continues. 一年 just specifies a period of a single unit. We also say 一日を通じて, 一月を通じて, 一週間を通じて, 四季を通じて, and so on.