a simple test for rain could be to randomly place particles across your viewport (width) and apply gravity to it..
if also could take wind into account so the particles gain some kind of angle. is that what u want?
i did not get the idea of the blob thing.. dont know that

how i manage the rotation of the particle?

import traer.physics.*;

Particle b;
ParticleSystem physics;

void setup()
{
 size( 400, 650 );
 frameRate( 24 );
 smooth();
 rectMode( CENTER );
 noStroke();
 noCursor();
 
 physics = new ParticleSystem( 0.5, 0.1 );
 b = physics.makeParticle( 0.4, 100, 0, 0 );
 b.makeFree();
}

void draw()
{
 handleBoundaryCollisions( b );
 physics.tick();

 background( 255 );
 
 fill( 0 );
 rect( b.position().x(), b.position().y(), 20, 20 );
}

// really basic collision strategy:
// sides of the window are walls
// if it hits a wall pull it outside the wall and flip the direction of the velocity
// the collisions aren't perfect so we take them down a notch too
void handleBoundaryCollisions( Particle p )
{
 if ( p.position().x() < 0 || p.position().x() > width )
   p.setVelocity( -0.9*p.velocity().x(), p.velocity().y(), 0 );
 if ( p.position().y() < 0 || p.position().y() > height )
   p.setVelocity( p.velocity().x(), -0.9*p.velocity().y(), 0 );
 p.moveTo( constrain( p.position().x(), 0, width ), constrain( p.position().y(), 0, height ), 0 );
}

got it!

another question:
can i get the attraction status from a particle?

Particle temp;
boolean status;
status = temp.isOn(); // ?

becaus the funktion boolean boolean isOn() ist just for Attraction objects?!?!

i splited the resolution 640*480 in a raster of 160 * 120 "pixel" with a size of 4*4 normal pixel. i've made an 2D array[][] to store boolan. now theire are pixel shapes to store into this array like a rect of 4*4 pixel (in the transformed resolution). when the pixel rain falls down the pixel should fill this shape. so i made an array for the rain filled with particles and an array for static particle to which the rain particle get attracted. now i want to loop through the rain particle to get the status if it already gets attracted or not.

this is what i've done till now:

import traer.physics.*;

Particle b, c;
ParticleSystem rain,statics;

boolean raster[][];

int numPixels_w;
int numPixels_h;
int blockSize = 4;

void setup()
{
 size( 640, 480 );
 frameRate( 24 );
 numPixels_w = width / blockSize;
 numPixels_h = height / blockSize;
 raster = new boolean[numPixels_w+1][numPixels_h+1];
 smooth();
 rectMode( CENTER );
 noStroke();
 noCursor();
 
 rain = new ParticleSystem( 0.5, 0.1 );
 statics = new ParticleSystem( 0, 0.1 );
 
 createStatics();
 createRain();
}

float i = random(PI*2); // random rotation (value between 0 - PI*2)

void draw()
{
 background( 255 );
 rain.tick();

 fill( 0 );
 
 checkStatics();
 drawStatics();
 checkRain();
 drawRain();

 i+=random(0.1,0.2);
}

void createRain(){
 for(int i = 0; i<100; i++){
   Particle temp = rain.makeParticle( random(0.2, 1), random(width), random(-1000), 0 );
 }
}

void drawRain(){
 for(int z = 0; z < rain.numberOfParticles(); z++){
   Particle temp = rain.getParticle( z );
   pushMatrix();
   translate(temp.position().x(), temp.position().y());
   rotate(i);
   rect( 0, 0, 4, 4 );
   popMatrix();
 }
}

void checkRain(){
 for(int z = 0; z < rain.numberOfParticles(); z++){
   Particle temp = rain.getParticle( z );
   if(temp.position().y() > (height + 4)){
     temp.moveTo( random(width), random(-1000), 0 );
   }
 }
}

void createStatics(){
 for(int i = 0; i<100; i++){
   Particle temp = statics.makeParticle();
 }
}

void drawStatics(){
 for(int z = 0; z < statics.numberOfParticles(); z++){
   Particle temp = statics.getParticle( z );
   pushMatrix();
   translate(temp.position().x(), temp.position().y());
   rect( 0, 0, 8, 8 );
   popMatrix();
 }
}

void checkStatics(){
 int z = 0;
 for(int j = 0; j < numPixels_h; j++){
   for(int i = 0; i < numPixels_w; i++){
     if(raster[i][j] == true){
       // Position des statics Particle -> temp
       float pos_s_x = i * blockSize; // x
       float pos_s_y = j * blockSize; // y
       Particle temp = statics.getParticle( z );
       temp.moveTo( pos_s_x, pos_s_y, 0 );
       for(int t = 0; t < rain.numberOfParticles(); t++){
         Particle r_temp = rain.getParticle( t );
         // Position des rain Particle -> r_temp
         float pos_r_x = r_temp.position().x(); // x
         float pos_r_y = r_temp.position().y(); // y
         float d = dist(pos_s_x, pos_s_y, pos_r_x, pos_r_y);
         if( d < 10 ){
           rain.makeAttraction( temp, r_temp , 100, 25 );
           break;
         }
       }
       z++;
     }
   }
 }
}

void clearRaster(){
 for(int j = 0; j<numPixels_h; j++){
   for(int i = 0; i<numPixels_w; i++){
     raster[i][j] = false;
   }
 }
}

void keyPressed() {
 if (keyCode == UP) {
   raster[75][60] = true;
   raster[76][60] = true;
   raster[77][60] = true;
   raster[78][60] = true;
   raster[79][60] = true;
   raster[80][60] = true;
   raster[81][60] = true;
   raster[82][60] = true;
   raster[83][60] = true;
   raster[84][60] = true;
   raster[85][60] = true;
 }
 if (keyCode == DOWN) {
   clearRaster();
 }
}