.snow-box {
    width: 100%;
    height: 100%;
    position: absolute;
    z-index: 999;
    left: 0;
    right: 0;
    bottom: 0;
    transform: translateY(-100%);
    animation: 12s snow_moving linear forwards infinite;
}

.snow-box:nth-of-type(2n) {
    animation-delay: 6s;
}

@keyframes snow_moving {
    0% {
        transform: translateY(-100%);
    }

    100% {
        transform: translateY(100%);
    }
}

/* 雪片 */
.snowflake {
    width: 5px;
    height: 5px;
    background-image: radial-gradient(circle, #fff, rgba(255, 255, 255, 0.4));
    border-radius: 50%;
    position: absolute;
    left: 0;
    top: 0;
    box-shadow: 0 0 1px 2px rgba(255, 255, 255, 0.2);
    animation: 5s flake_moving linear forwards infinite;
}