Read this tutorial to learn how to make it rain using particles in cocos2d.
We begin by adding the rain drop image to our project.
All you have to do is add the code bellow to your init meathod.
CCParticleSystem* emitter = [CCParticleRain node];
[self addChild: emitter z:10];
CGPoint p = emitter.position;
emitter.position = ccp( p.x, p.y);
emitter.life = 4;
emitter.texture = [[CCTextureCache sharedTextureCache] addImage: @"Raindrop.png"];
emitter.startSize = 10.0f;
Life is how long the particles in the emitter last. Texture is the image you want the particles to be. StartSize is the size of the particles to begin with. If you would like to see more of what you can do with particles run the particle test that comes with cocos2d.
Copy right Britney Lee Johnson August 2010.