I included my own free developer Weather Underground API key to get you started, no guarantees on how long it lasts. Sign up for your own free developer API key here:
https://www.wunderground.com/weather/api/
Salesforce has introduced Locker-Service since this was originally written, the update needs to be disabled for this app to run as-is. A few instances of 'event.target.id', etc, need to be replaced with the 'getSource().getLocalId()' equivalents for this to be Locker-Service safe.
The app is intended as a learning tool for Lightning Components and Lightning Design Tokens - not as a supported app. Have fun!