$d: 70;      // density
$w: 600;     // grid size
$s: 3s;       // speed

// Generate the snow image using radial gradients
$grad: ();
@for $i from 0 to $d {
  $v: random(4) + 2;
  $a: random(5) * .1 + .5;
  $grad: $grad, radial-gradient(
      $v+px $v+px at
      (random($w - $v * 2) + $v)+px
      (random($w - $v * 2) + $v)+px, 
      rgba(255,255,255,$a) 50%,
      rgba(0,0,0,0)
  ) !global;
}