snow.js

var snowMax = 50;

var snowColor = ["#DDD", "#EEE"];

var snowEntity = "•";

var snowSpeed = 0.75;

var snowMinSize = 8;

var snowMaxSize = 24;

var snowRefresh = 50;

var snowStyles = "cursor: default; -webkit-user-select: none; -moz-user-select: none; -ms-user-select: none; -o-user-select: none; user-select: none;";

var snow = [],
    pos = [],
    coords = [],
    lefr = [],
    marginBottom,
    marginRight;

function randomise(range) {
    rand = Math.floor(range * Math.random());
    return rand;
}

function initSnow() {
    var snowSize = snowMaxSize - snowMinSize;
    marginBottom = document.body.scrollHeight - 5;
    marginRight = document.body.clientWidth - 15;

    for (i = 0; i <= snowMax; i++) {
        coords[i] = 0;
        lefr[i] = Math.random() * 15;
        pos[i] = 0.03 + Math.random() / 10;
        snow[i] = document.getElementById("flake" + i);
        snow[i].style.fontFamily = "inherit";
        snow[i].size = randomise(snowSize) + snowMinSize;
        snow[i].style.fontSize = snow[i].size + "px";
        snow[i].style.color = snowColor[randomise(snowColor.length)];
        snow[i].style.zIndex = 1000;
        snow[i].sink = snowSpeed * snow[i].size / 5;
        snow[i].posX = randomise(marginRight - snow[i].size);
        snow[i].posY = randomise(2 * marginBottom - marginBottom - 2 * snow[i].size);
        snow[i].style.left = snow[i].posX + "px";
        snow[i].style.top = snow[i].posY + "px";
    }

    moveSnow();
}

function resize() {
    marginBottom = document.body.scrollHeight - 5;
    marginRight = document.body.clientWidth - 15;
}

function moveSnow() {
    for (i = 0; i <= snowMax; i++) {
        coords[i] += pos[i];
        snow[i].posY += snow[i].sink;
        snow[i].style.left = snow[i].posX + lefr[i] * Math.sin(coords[i]) + "px";
        snow[i].style.top = snow[i].posY + "px";

        if (snow[i].posY >= marginBottom - 2 * snow[i].size || parseInt(snow[i].style.left) > (marginRight - 3 * lefr[i])) {
            snow[i].posX = randomise(marginRight - snow[i].size);
            snow[i].posY = 0;
        }
    }

    setTimeout("moveSnow()", snowRefresh);
}

for (i = 0; i <= snowMax; i++) {
    document.write("<span id='flake" + i + "' style='" + snowStyles + "position:absolute;top:-" + snowMaxSize + "'>" + snowEntity + "</span>");
}

window.addEventListener('resize', resize);
window.addEventListener('load', initSnow);