先建立一个名为"fly.js"的JS文件，代码为：

var no = 8 ;
var oTime = new Date();
var lastTime = oTime.getTime();
var x_sin, x_point, y_point;
var x_length, x_increased, y_increased;
var i, body_width = 932, body_height = 538;
var lastTime = 0;
var fallInterval = 2.4;

x_sin = new Array();
x_point = new Array();
y_point = new Array();
x_length = new Array();
x_increased = new Array();
y_increased = new Array();
var oBody = document.body;
var d=document;

/*body_width = oBody.clientWidth;
body_height = oBody.clientHeight;*/

for (i = 0; i < no; ++ i) {
       snowFlakeImgNo = Math.round(Math.random()*(snowflake.length-1));
       x_sin[i] = 0;
       x_point[i] = Math.random()*(body_width-250)+100;
       y_point[i] = Math.random()*body_height;
       x_length[i] = Math.random()*20;
//       x_increased[i] = 0.02 + Math.random()/10;
//       y_increased[i] = 0.7 + Math.random();
       x_increased[i] = 0.08 + Math.random()/5;
       y_increased[i] = 2.7 + Math.random()*2;

       d.write("<div id=\"snowFlakeDiv_"+ i +"\" style=\"position:absolute; z-index:11; visibility:visible; top: 15px; left: 215px;\"><img src=\"" + snowflake[snowFlakeImgNo] + "\" border=\"0\" id='snowImg_"+ i +"'></div>");
}

function snowFlakeFloat() {
       var now = new Date();
       var second = now.getTime();
       var scrollTop = document.body.scrollTop;
       var snowflakeDiv_0 = d.getElementById("snowFlakeDiv_0");
       var snowflakeDiv_1 = d.getElementById("snowFlakeDiv_1");
       var snowflakeDiv_2 = d.getElementById("snowFlakeDiv_2");
       var snowflakeDiv_3 = d.getElementById("snowFlakeDiv_3");
       var snowflakeDiv_4 = d.getElementById("snowFlakeDiv_4");
       var snowflakeDiv_5 = d.getElementById("snowFlakeDiv_5");
       var snowflakeDiv_6 = d.getElementById("snowFlakeDiv_6");
       var snowflakeDiv_7 = d.getElementById("snowFlakeDiv_7");
       for (i = 0; i < no; i++) {
              if (y_point[i] >= 0) {
                     y_point[i] += y_increased[i];
              }
              if (y_point[i] > scrollTop + body_height - 10 || y_point[i] < scrollTop || y_point[i] < 0) {              // 移出窗口，或在等待队列
                     if ((second - lastTime) / 1000 > fallInterval) {       // 从队列中下落
                            snowFlakeImgNo = Math.round(Math.random()*(snowflake.length-1));
                            if (document.getElementById('snowImg_'+i)) {
                                   document.getElementById('snowImg_'+i).src = snowflake[snowFlakeImgNo];
                            }
                            x_point[i] = Math.random()*(body_width-x_length[i]-250) + 100;
                            y_point[i] = scrollTop;
                            x_increased[i] = 0.05 + Math.random()/2;
                            y_increased[i] = 2.7 + Math.random()/2;
//                            x_increased[i] = 0.05;
//                            y_increased[i] = 2.7;
                            body_width = oBody.clientWidth;
                            body_height = oBody.clientHeight;
                            lastTime = second;
                     }else {
                            y_point[i] = -50;
                     }

              }
              x_sin[i] += x_increased[i];

              eval("snowflakeDiv_"+i).style.pixelTop = y_point[i];
              eval("snowflakeDiv_"+i).style.pixelLeft = x_point[i] + x_length[i]*Math.sin(x_sin[i]);

       }
}

window.setInterval("snowFlakeFloat()",100);

呵呵，做好这几步，一个漂亮的漂浮就搞定了。