There is default weather gadget for Windows 7, by which you can know the weather conditions of your city. But if you want more, in the form of wallpaper, here is good software to do that. “Weather Desktop Background Changer” is a better app for Windows 7, which lets you pick up set of custom wallpapers which will be automatically applied to your desktop depending on the weather conditions of your state or city (US only).
Using this app, you can get the weather report in your taskbar and the weather forecast for the next couple of days. This will also show the current temperature, humidity and wind speed.
The cons for this application is that you have to manually add the backgrounds and add each one individually. On the other side, the pro side is that it allows you to customize the background to your liking.
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Enjoy! Windows 7 Rocks