var posy = 0;
function snow() {
  posy += 1;
  document.getElementById("gr1").style.backgroundPosition = "0px " + posy + "px";
  if (posy%2 == 0) document.getElementById("gr2").style.backgroundPosition = "0px " + posy/2 + "px";
  setTimeout("snow()", 1000);
}

window.onload = snow;