Weather icons are very handy when displaying the weather status, generating a weather based content & more.
You can even customize the look of your website according to the weather conditions (CSS-Tricks has a nice tutorial on this).
There are beautiful free weather condition icons that can be used. Here is a collection of them:
Glossy Weather Icons
Set contains 22 glossy weather icons.
Sizes are 16*16px to 128*128px. They are in both ICO & PNG format.