How does it work? You can read about the science behind that app at "How Dark Sky Works", but the creators put it simply,
We’ve found that precipitation bands are even more coherent and behave approximately linearly over the course of minutes, and in many cases up to an hour or more. So how do we go about quantifying this linear motion? Math, that’s how!The developers are programmers and mathematicians, not meteorologists. Weather data from NOAA is cleaned up and the short term precipitation forecast is extracted and then displayed in a clean and concise manner as shown in the screenshot at the right. The yellow band at the top shows expected precipitation amounts and it will move to indicate the confidence given to the predictions. There's also a simple radar display that allows you to scroll forward to see the expected short term movement.
This isn't a full-fledged weather app. It has one purpose; to forecast short term precipitation. After a few days of testing Dark Sky, I found it quite accurate. One evening I even spent some time outside watching the clouds roll in and waiting for the rain to start. Dark Sky seemed to be accurate for my location to within minutes.
Dark Sky for the iPhone is available in the iTunes Store for $3.99.