i have a rain Gameobject and a tornado Gameobject, they are all created by particles in the unity..but how can i combine them together so that i can do a script to make them happened at same time..
And is there a way to render them on and off randomly so that when user pass this area will loose points..
You have two seperate particle emitters, so you need to put them both into parent object. Create an empty object in Unity, Rename it "Weather" or something, and then drag and drop your two particle emitters into it.
Now you can create references to both of those emitters with a script and call
tornadoParticle.emit = false;
rainParticle.emit = false;
to turn the Particle (or true for on.)
To turn them on and off randomly, you need to use Random.value which you can read more bout here: http://unity3d.com/support/documentation/ScriptReference/Random.html
You need to generate a random number and use the Monobehavior "Update" function to check Time.deltaTime against this random number and flicker it on and off.
Hopefully you can follow, if not, I recommend going through a few scripting tutorials to pick up the basics.