.snow {
  position: absolute;
  height: 100%;
  width: 100%;
  background: url("images/snow500.png"),
    url("https://user-images.githubusercontent.com/7342669/50293049-ad189c00-0440-11e9-8cae-a57ad65c22ca.png"),
    url("images/snow300.png");
  animation: letItSnow 20s linear infinite;
  z-index: 0;
}

@keyframes letItSnow {
  0% {
    background-position: 0px 0px;
  }
  100% {
    background-position: 500px 1000px, 400px 400px, 300px;
  }
}