Thank you, Alco!
I'm using similar code for rain, this is cut from it:
(you have to edit this before using, I cut it unchanged from my proj.)
Code:

---

...
bmap rain_mist_pic = <smoke.tga>;
function rain_mist_()
{
	my.alpha -= 5 * time;
	if(my.alpha < 0) 
	{
		my.lifespan = 0;
		return;
	}
	my.size += 2*time;//*0.5
}
function rain_mist()
{
	temp.x = random(2) - 1;
	temp.y = random(2) - 1;
	temp.z = random(0.5) + 0.5; // play with this value
	vec_set(my.vel_x,temp);
	my.bmap = rain_mist_pic;
	my.size = 5+random(5);//5
	my.flare = on;
	my.bright = on;
	my.move = on;
	my.gravity = 0;
	my.alpha = 70;//70
	my.function = rain_mist_;
}

bmap raindrop = <drop5.tga>; 

//RAIN effect function
Function eff_rain_()
{
	if(content(my.x) == content_solid) // the particle is inside a solid?
	{
		effect(rain_mist, 1, my.x, nullvector); // generate mist
		 // generate mist
		my.lifespan = 0; // don't allow the particles to continue their movement if they have hit something solid
		return;
	}
  my.skill_b += 0.06;//0.03
  my.skill_a = 1  - my.skill_b;//6
  if(my.skill_a < 0) {
    my.lifespan = 0;
  	}
}

Function eff_rain()
{
    if(my.skill_a < 0) { my.skill_a = 0; my.skill_b = 0; }
    my.bmap = raindrop;
    my.vel_x = random( 10 )-5;
    my.vel_y = random( 10 )-5;
    my.vel_z = -40 ;
    my.size =14 ;
    my.move =1 ;
    my.alpha = 100 ;
    my.x += random(CELL_SIZE * 16)-CELL_SIZE * 8;
    my.y += random(CELL_SIZE * 16)-CELL_SIZE * 8;
    my.z += random(CELL_SIZE * 2)-CELL_SIZE;
    my.transparent = on ;
    my.gravity =0 ;
    my.bright =on;
    my.function = eff_rain_;
    my.flare =1 ;
}
...
...
	var myPos[3];
	while(1) {
		while(RAIN_NEEDED==on)&&(MAP_VIEW_ACTIVE==off) {
			vec_set(myPOS,camera.x);
			temp.x = COS(camera.pan);
			temp.y = SIN(camera.pan);
			temp.Z = 600*COS(camera.tilt);
			MyPos.X = myPos.X + temp.Z*temp.X;
			MyPos.Y = myPos.Y + temp.Z*temp.Y;;
//			MyPos.Z = myPos.Z + 600*SIN(camera.TILT);
			MyPos.Z = Camera.Z + 100;//800;200
			effect(eff_rain,50 ,MyPOS,nullvector);
			if (moon.alpha > 50) { moon.alpha-=time; }
			moon.alpha = max(moon.alpha,0); 
			if (moon_flare.alpha > 50) { moon_flare.alpha-=time; }
			moon_flare.alpha = max(moon_flare.alpha,0); 
			wait(1);
		}
...

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