Same bug from Ubuntu : https://bugs.launchpad.net/ubuntu/+source/gnome-icon-theme/+bug/298665

[Original description]
weather item icon on the panel is broken, look at the screen-shot, thanks!

[Better description of the problem from Ubuntu Brainstorm]: http://brainstorm.ubuntu.com/idea/14731/
I know this is a very small thing to suggest, but the fog/mist icon used by the built in Ubuntu weather applet is VERY ugly.
The square icon looks out of place, and whenever I see it my first though isn't "oh look, fog" but rather "great!, why isn't my weather applet working" -- It looks more like an image isn't loading then a representation of the current weather.

[Comments]
The original bug report shows how much this icon is confusing; the bug reporter thought that the applet was not working properly! And some comments in Brainstorm report the same idea ("Is it fog??! I thought it was a bug", etc).

Two possibilities: either this is a Wishlist bug (the icon is ugly, change the icon), either it is a low importance bug (people think the applet is not working).

Reassigned to 'gnome-icon-theme', source of icons.

Fixed in master branch.