No one has figured out the right formula for the weather app yet. Phones come stocked with basic forecast apps with cartoony icons, but today's mobile devices have more potential than that. One ambitious effort called Dark Sky recently hit the App Store, and I teamed up with weather guru and former RWW webmaster Jared Smith to test it out.

Dark Sky is a Kickstarter-launched app for iPhone and iPad that zeroes in on the basic use case for a weather app on a mobile device. It uses your current location by default, but you can view the weather anywhere (U.S. only for now). Dark Sky does away with the complex modeling (and guesswork) involved in forecasting the weather and focuses on the question you ask of your phone as you're walking out the door: Is it raining right now, and will it rain soon?

Its UI is totally novel. On the phone, it shows a timeline with a graph of the precipitation over the next hour. Below, in huge text, it says what's happening now and what will happen this hour. A tab at the top reveals a cool, animated radar view that gives a general sense of the precipitation, and you can scroll back and forth in time to see what it predicts for the short term. It's a pretty powerful interface for a quick little phone app. On the iPad, the radar is visible from the main screen.

"For our initial release, we're just focusing on precipitation and getting that right," says Jay LaPorte, one of the creators of Dark Sky.

"Most meteorologists use complicated mathematical models of the weather, which works fairly well over medium intervals, but don't tell you what's going to happen in the short-term." Dark Sky leaves the far-out forecasting to the geeks. "We take a statistical approach that they would find horrifying (in fact, several have emailed us expressing as much), but which works really well for short time-spans: we only try to tell you what the precipitation (if any) will look like for the next hour."

Since the app is all about precipitation, it adds a little entertainment when your skies are clear. "CLEAR SKIES ARE BORING," it says in sunny yellow letters. "Tap here to view a storm in Smicksburg, PA," or wherever else the action is.

For the basic question of whether to bring an umbrella with you, Dark Sky's interface gets everything right. All the right information is presented in all the right places, and the radar view is lovely. But how does Dark Sky do in the field?

Is It Accurate?

Jared and I each tested the app for a few days in our own environments. The "NOW" section was pretty accurate, but the short-term forecasting was uneven. You can even watch the trend lines fluctuate as the app makes its calculations.

We compared the radar images to RadarScope a serious weather radar app that Jared trusts. For simple, slow-moving weather systems, the images were comparable. However, as Jared pointed out, Dark Sky's radar colors are unusual. "I can tell this wasn't designed by meteorologists," he says.

The usual green, yellow, red scale is softened, replaced by a blue-to-violet scale that fits with the app's color schemes but seems to understate severe weather. The radar's prediction model also gets noticeably less realistic as the weather gets more severe.

In fact, it's extreme weather that gave us the most concern. There were serious storms in Minnesota the night we tested, with tornadoes on the ground and everything, and Dark Sky's readout said "NOW: Rain." It's one thing to build the crazy physics of unpredictable, high-velocity storms into the app, which is out of scope for Dark Sky right now. But why not just add the word "Tornado!" Why not put a warning icon on the screen?

"Dark Sky doesn't make any effort to identify and compensate for chaotic storm behavior," LaPorte says. There are no meteorologists on the team. They're just avid programmers going after an everyday use case. "Extreme weather warnings are definitely on our radar (pardon the pun), though, and they tie in perfectly with one of the features we're working on for our 2.0 release: push notifications."

As of now, Dark Sky's interface is wonderful, and its short-term precipitation tracking is worth a try. We do hope that it will incorporate severe weather notifications, even if it doesn't model the prediction of intense storms. Dark Sky concentrates on the right question - "Is it going to rain?" - but people are going to count on weather apps to keep them safe, too.

Dark Sky for iPhone and iPad is $3.99 on the App Store.