.snow{
    background-image:
            url(../media/snow.png),
            url(../media/snow1.png);
    width: 100%;
    height:100%;
    top: 0;
    left: 0;
    z-index:1;
    animation: snow 20s linear infinite;
}
@keyframes snow {
    from {background-position: 0px 0px, 0px 0px, 0px 0px;}
    to {background-position: 500px 1000px, 200px 400px, -100px 300px;}
}