National Emblem and National Flag of the Republic of China Act Article 13

Unless there is a situation where the national flag must be hung, the national flag shall be lowered when it rains. After the rain stops, the national flag shall be raised again. No ceremony is required outside the official raising and lowering times.
中文原文 中華民國國徽國旗法第13條
天雨時,除特殊情形,必須懸掛國旗外,應將國旗降下。
雨停後,再行升上。
其不在規定升降時間內,不必舉行儀式。

English translation for reference only; the Chinese text prevails. Not legal advice.