i {
    position: absolute;
    
    background: #fff;
    border-radius: 50%;
    box-shadow: inset 0 -1px 1px rgb(114, 114, 114);
    top: -10px;
    animation: nieve linear infinite;
}
@keyframes nieve {
    0% {
        top: -10px;
    }
    100% {
        top: calc(250vh + 100px);
    }
}