<style>
#canvas {
position:fixed;
top:0px;
left:0px;
z-index:-1;
}
</style>

<canvas id="canvas"></canvas>
<script>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

//canvas dimensions
var W = window.innerWidth;
var H = window.innerHeight;
canvas.width = W;
canvas.height = H;

//snowflake particles
var mp = 25; //max particles
var particles = [];
for (var i = 0; i < mp; i++) {
  particles.push({
  x: Math.random() * W, //x-coordinate
  y: Math.random() * H, //y-coordinate
  r: Math.random() * 4 + 1, //radius
  d: Math.random() * mp //density
  })
}

//Lets draw the flakes
function draw() {
  ctx.clearRect(0, 0, W, H);

  ctx.fillStyle = "rgba(255, 255, 255, 0.8)";
  ctx.beginPath();
  for (var i = 0; i < mp; i++) {
  var p = particles[i];
  ctx.moveTo(p.x, p.y);
  ctx.arc(p.x, p.y, p.r, 0, Math.PI * 2, true);
  }
  ctx.fill();
  update();
}

//Function to move the snowflakes
//angle will be an ongoing incremental flag. Sin and Cos functions will be applied to it to create vertical and horizontal movements of the flakes
var angle = 0;

function update() {
  angle += 0.01;
  for (var i = 0; i < mp; i++) {
  var p = particles[i];
  //Updating X and Y coordinates
  //We will add 1 to the cos function to prevent negative values which will lead flakes to move upwards
  //Every particle has its own density which can be used to make the downward movement different for each flake
  //Lets make it more random by adding in the radius
  p.y += Math.cos(angle + p.d) + 1 + p.r / 2;
  p.x += Math.sin(angle) * 2;

  //Sending flakes back from the top when it exits
  //Lets make it a bit more organic and let flakes enter from the left and right also.
  if (p.x > W + 5 || p.x < -5 || p.y > H) {
  if (i % 3 > 0) //66.67% of the flakes
  {
  particles[i] = {
  x: Math.random() * W,
  y: -10,
  r: p.r,
  d: p.d
  };
  } else {
  //If the flake is exitting from the right
  if (Math.sin(angle) > 0) {
  //Enter from the left
  particles[i] = {
  x: -5,
  y: Math.random() * H,
  r: p.r,
  d: p.d
  };
  } else {
  //Enter from the right
  particles[i] = {
  x: W + 5,
  y: Math.random() * H,
  r: p.r,
  d: p.d
  };
  }
  }
  }
  }
}

//animation loop
setInterval(draw, 33);
</script>