The sentence structure is considered more "literary" and can actually be rearranged to make it appear in the "normal" English word order: Seldom is the weather more dramatic than when thunderstorms strike --> The weather is seldom more dramatic than when thunderstorms strike. In Chinese, this is 天氣很少比雷雨交叉時更戲劇性/天氣很少比雷雨交叉時更讓人印象深刻 In English, you can move the adverb up front for dramatic effect or emphasis on the adverb. I rarely wake up before 9:00 a.m. in the morning (normal word order) --> Rarely do I wake up before 9:00 a.m. in the morning. English is a Germanic language and we have kept some of the Germanic word order. Like German, the verb has to take the second position when you move the adverb up front. (So you can't say Rarely I wake up before 9:00 a.m. in the morning. You must move the verb into the second position. But here you can't say "Rare wake I up before 9:00 a.m.", so "do" moves into second place after "rarely") In your original sentence, "is" moves into second position after you move "seldom" up front. The weather is seldom .... -> Seldom IS the weather ... I hope this is helpful.

just to add to these great answers the point of using "than": The things being compared here are states of the weather right? The criterion for comparison is "dramatic", as in "what is the most dramatic weather condition?" So the sentence poses the question "how often do you find weather more dramatic THAN thunderstorms?" To which the answer would be "seldom!"

When a sentence begins with a negative adverbial like never, rarely or seldom, the auxiliary verb comes before the subject. Note that these time expressions are normally used with a perfect verb form or modal auxiliary verbs. Comparative forms are also common. Examples Never have I seen anything stranger. Seldom have I heard anything more ridiculous.

Seldom means not often, or uncommon. So the sentence means it is uncommon for the weather to be more dramatic than a thunderstorm. This could mean that the area doesn't often have weather worse than a thunderstorm (for example worse weather could be a typhoon, hailstorm etc). The "most dramatic" or worst weather they get is a thunderstorm, and it is not often that there is worse weather.