The poem pictures a crowd pouring into the streets, and it borrows the language of weather to do it. The protesters move "like running water," suddenly and all at once. That image sets up the rest: the people are rain, and the place they come into is a dry season, parched and in need of relief.
The third stanza shifts to something heavier. The "life-time pain of people" is so deep that even sleep can't answer it, so the hurt follows them through the night as well as the day. Out of that grows the protest. The speaker seems to see it as both a release and a kind of renewal, a "green weather" that might bring growth after drought.
The ending is less hopeful. People "swear and swear / swallowing dust," which suggests anger that keeps choking on its own surroundings. The mood is a mix of urgency, weariness and stubborn hope. Judging by the title, survival depends on speaking out anyway.