This page lists an example sentence for 交替:
雨天與晴天交替更迭
According to the site, 交替 means to replace; alternately; in turn.
However, 更迭 means to alternate; to change.
Isn't 交替更迭 then redundant?
If 交替更迭 is redundant, could you change the example sentence to 雨天與晴天交替?
How would you say the example sentence in Cantonese?
交替 and 更迭 are two similar terms that complement each other in an four characters idiomatic phrase. Using four characters idiomatic phrase is a very common practice in Chinese speech.
For example:
小心 (careful) and 謹慎 (cautious) have similar meaning, it is often put together as an idiomatic phrase (but not a true idiom)
You can say "你要小心" or "你要謹慎", they deliver a similar message, and both are to the point.
You can also combine the two adjectives into one idiomatic phrase "小心謹慎" and say "你要小心謹慎" to illustrate a more detailed message and add literary flair to your speech.
"雨天與晴天交替更迭" is grammatically correct and detailed. However, it would be more elegant to say "晴雨交替" or "晴雨更迭" (another way to make idiomatic phrase is reduce a long phrase into four characters phrase)
More examples:
縱横 (intersecting) 交錯 (crossing) --> 縱横交錯
聰明 (intelligent) 伶俐 (smart) --> 聰明伶俐
We do it in English too, e.g. "He is a clever, smart man"
Despite their meaning being very similar to one another, I don't think they are the same, especially in your example.
更迭 is more about people, time, dynasty and such. E.g. 朝代更迭，人事更迭 etc. Synonyms would be 更替 or 变迁。
交替 is more about natural phenomena or the alternation between one of a few states. E.g.
明暗交替
在正弦曲线中，波峰与波谷交替出现
So in your example, I think 交替 would be more suitable. And 晴雨交替 suggested by Tang Ho is a very good answer in my opinion.
But using them as 交替更迭 is definitely redundant. It's something we're taught to avoid since primary school.
