Weather nouns become plural

Rain can be used as a verb or noun. An intransitive verb is commonly used as a verb, meaning "rain". The noun is "rain".

First of all, rain is an uncountable noun, so rain is only a three-syllable form, not a word.

It often rains. It often rains.

It is raining outside. It is raining outside.

It in these two sentences refers to the weather, which is usually used as the official subject when talking about the weather.

Secondly, rainy is an adjective used to modify nouns, usually used with day. It rained continuously.

Today is a rainy day. Today is a rainy day.

It is rainy in spring. It is rainy in spring.

The same is true for other weather words, but not necessarily for verbs. You have to see if you can "rain" like Sun, so you can only use clear as the noun "Sun" and sunny as the verb "Sunny"

Orz explained it as simply as possible, because he thought of it himself, so the example is a bit short, but the explanation should be no problem. Please cheer for the LZ exam.