As mentioned earlier, the particle が is used when the subject is a piece of new information. This causes the following two sentences to be different.
In the first sentence, the rain is the topic, already known to the speaker and the listener. In the second sentence the rain is a new piece of information.
Then look at the sentences below. In the sentences using は, the subject is not new information. In the sentences using が, the subject is new information.
It's possible for the particle が to appear in a sentence multiple times.