To add this effect on your blog:

Go to your Customize Page and press the Edit Custom HTML button.
Next, look for the <body> part of your codes.
Paste this codes below that(below the <body>):

Код:
<script type="text/javascript" src="http://static.tumblr.com/qsifglb/mZjl87ts1/f8.js"></script>

<script type="text/javascript">
snowStorm.snowColor = '[b]#bbbbbb[/b]'; //
snowStorm.flakesMaxActive = 30; 
</script>

snowStorm.snowColor = ‘#bbbbbb’; // changes the color of snow by changing the value of the Hexadecimal color. For example, change #bbbbbb(grayish) to #ff0000 to make it red. You can pick a color at ColorPicker.com.

snowStorm.flakesMaxActive = 30; controls the amount of snowflakes showing in the page at the same time. 30 means there would be at most 30 snowflakes falling. Don’t go crazy with this. Better leave it as it is or don’t make it more than 50.

4.  After you’ve made the changes you want, hit the Update Preview Button, Appearance button, then Save button.

5. Check your blog if it worked.

This effect also follows your mouse’s movement. If your cursor is in the left part of the page, the snow would fall to the right and vice versa. This is also one of those snow effects that loads fast.