.cloud {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: url('../img/cloud.png');
    background-size: cover;
    opacity: 0.9;
    /*
    animation: cloud-reverse 180s linear infinite;
    */
    animation: cloud-reverse 13s ease-in-out 0s 1 forwards;
}

@keyframes cloud-reverse {
    0% {
        background-position: 5440px;
        opacity: 1;
    }
    100% {
        background-position: 0px;
        opacity: 0;
    }
}