That sound during the storm was absolutely frightful.

frightful 是由名词 fright（惊吓、恐惧）加上形容词后缀 -ful 构成的形容词，意思是"可怕的、令人恐惧的"。后缀 -ful 表示"充满...的"，所以 frightful 就是"充满恐惧感的"。