Smoke by Glen Murphy.

Drag the mouse across the image to move the particles. Code has not been optimised and will run fairly slowly.

 
int res = 2;
int penSize = 30;
int lwidth;
int lheight;
int pnum = 30000;
vsquare[][] v;
vbuffer[][] vbuf;
particle[] p;
int pcount = 0;
int mouseXvel = 0;
int mouseYvel = 0;

int randomGust = 0;
int randomGustMax;
float randomGustX;
float randomGustY;
float randomGustSize;
float randomGustXvel;
float randomGustYvel;

void setup() 
{
  size(200, 200);
  lwidth = width/res;
  lheight = height/res;
  v = new vsquare[lwidth+1][lheight+1];
  vbuf = new vbuffer[lwidth+1][lheight+1];
  p = new particle[pnum];
  noStroke();
  for(int i = 0; i < pnum; i++) {
    p[i] = new particle(random(width/2-20,width/2+20),random(height-20,height));
  }
  for(int i = 0; i <= lwidth; i++) {
    for(int u = 0; u <= lheight; u++) {
      v[i][u] = new vsquare(i*res,u*res);
      vbuf[i][u] = new vbuffer(i*res,u*res);
    }
  }
}

void draw() 
{
  background(#cccccc);
  
  int axvel = mouseX-pmouseX;
  int ayvel = mouseY-pmouseY;

  mouseXvel = (axvel != mouseXvel) ? axvel : 0;
  mouseYvel = (ayvel != mouseYvel) ? ayvel : 0;

  if(randomGust <= 0) {
    if(random(0,10)<1) {
      randomGustMax = (int)random(5,12);
      randomGust = randomGustMax;
      randomGustX = random(0,width);
      randomGustY = random(0,height-10);
      randomGustSize = random(0,50);
      if(randomGustX > width/2) {
        randomGustXvel = random(-8,0);
      } else { 
        randomGustXvel = random(0,8);
      }
      randomGustYvel = random(-2,1);
    }
    randomGust--;
  }

  for(int i = 0; i < lwidth; i++) {
    for(int u = 0; u < lheight; u++) {
      vbuf[i][u].updatebuf(i,u);
      v[i][u].col = 0;
    }
  }
  for(int i = 0; i < pnum-1; i++) {
    p[i].updatepos();
  }
  for(int i = 0; i < lwidth; i++) {
    for(int u = 0; u < lheight; u++) {
      v[i][u].addbuffer(i, u);
      v[i][u].updatevels(mouseXvel, mouseYvel);
      v[i][u].display(i, u);
    }
  }
  randomGust = 0;
}

class particle 
{
  float x;
  float y;
  float xvel;
  float yvel;
  float temp;
  int pos;

  particle(float xIn, float yIn) {
    x = xIn;
    y = yIn;
  }

  void reposition() {
    x = width/2+random(-20,20);
    y = random(height-10,height);

    xvel = random(-1,1);
    yvel = random(-1,1);
  }

  void updatepos() {
    int vi = (int)(x/res);
    int vu = (int)(y/res);

    if(vi > 0 && vi < lwidth && vu > 0 && vu < lheight) {
      v[vi][vu].addcolour(2);

      float ax = (x%res)/res;
      float ay = (y%res)/res;

      xvel += (1-ax)*v[vi][vu].xvel*0.05;
      yvel += (1-ay)*v[vi][vu].yvel*0.05;

      xvel += ax*v[vi+1][vu].xvel*0.05;
      yvel += ax*v[vi+1][vu].yvel*0.05;

      xvel += ay*v[vi][vu+1].xvel*0.05;
      yvel += ay*v[vi][vu+1].yvel*0.05;

      v[vi][vu].yvel -= (1-ay)*0.003;
      v[vi+1][vu].yvel -= ax*0.003;

      if(v[vi][vu].yvel < 0) v[vi][vu].yvel *= 1.00025;

      x += xvel;
      y += yvel;
    } 
    else {
      reposition();
    }
    if(random(0,400) < 1) {
      reposition();
    }
    xvel *= 0.6;
    yvel *= 0.6;
  }
}

class vbuffer
{
  int x;
  int y;
  float xvel;
  float yvel;
  float pressurex = 0;
  float pressurey = 0;
  float pressure = 0;

  vbuffer(int xIn,int yIn) {
    x = xIn;
    y = yIn;
    pressurex = 0;
    pressurey = 0;
  }

  void updatebuf(int i, int u) {
    if(i>0 && i<lwidth && u>0 && u<lheight) {
      pressurex = (v[i-1][u-1].xvel*0.5 + v[i-1][u].xvel + v[i-1][u+1].xvel*0.5 - v[i+1][u-1].xvel*0.5 - v[i+1][u].xvel - v[i+1][u+1].xvel*0.5);
      pressurey = (v[i-1][u-1].yvel*0.5 + v[i][u-1].yvel + v[i+1][u-1].yvel*0.5 - v[i-1][u+1].yvel*0.5 - v[i][u+1].yvel - v[i+1][u+1].yvel*0.5);
      pressure = (pressurex + pressurey)*0.25;
    }
  }
}

class vsquare {
  int x;
  int y;
  float xvel;
  float yvel;
  float col;

  vsquare(int xIn,int yIn) {
    x = xIn;
    y = yIn;
  }

  void addbuffer(int i, int u) {
    if(i>0 && i<lwidth && u>0 && u<lheight) {
      xvel += (vbuf[i-1][u-1].pressure*0.5
      +vbuf[i-1][u].pressure
      +vbuf[i-1][u+1].pressure*0.5
      -vbuf[i+1][u-1].pressure*0.5
      -vbuf[i+1][u].pressure
      -vbuf[i+1][u+1].pressure*0.5
      )*0.49;
      yvel += (vbuf[i-1][u-1].pressure*0.5
      +vbuf[i][u-1].pressure
      +vbuf[i+1][u-1].pressure*0.5
      -vbuf[i-1][u+1].pressure*0.5
      -vbuf[i][u+1].pressure
      -vbuf[i+1][u+1].pressure*0.5
      )*0.49;
    }
  }

  void updatevels(int mvelX, int mvelY) {
    float adj;
    float opp;
    float dist;
    float mod;

    if(mousePressed) {
      adj = x - mouseX;
      opp = y - mouseY;
      dist = sqrt(opp*opp + adj*adj);
      if(dist < penSize) {
        if(dist < 4) dist = penSize;
        mod = penSize/dist;
        xvel += mvelX*mod;
        yvel += mvelY*mod;
      }
    }
    if(randomGust > 0) {
      adj = x - randomGustX;
      opp = y - randomGustY;
      dist = sqrt(opp*opp + adj*adj);
      if(dist < randomGustSize) {
        if(dist < res*2) dist = randomGustSize;
        mod = randomGustSize/dist;
        xvel += (randomGustMax-randomGust)*randomGustXvel*mod;
        yvel += (randomGustMax-randomGust)*randomGustYvel*mod;
      }
    }
    xvel *= 0.99;
    yvel *= 0.98;
  }

  void addcolour(int amt) {
    col += amt;
    if(col > 196) col = 196;
  }

  void display(int i, int u) {
    float tcol = 0;
    if(i>0 && i<lwidth-1 && u>0 && u<lheight-1) {

      tcol = (+ v[i][u+1].col
      + v[i+1][u].col
      + v[i+1][u+1].col*0.5
      )*0.3;
      tcol = (int)(tcol+col*0.5);
    }

    fill(255-tcol, 255-tcol, 255-tcol);
    rect(x,y,res,res);
    col = 0;
  }
}