"I opened up the radar app on my iPhone and sure enough, a big ominous red splotch hovered over my location. But the thing was, I had no idea how long it would last. I had the map right in front of me, but I couldn't figure it out! The animation wasn't any help, it was too jumpy and splotchy to tell whether it'd be ten minutes or an hour," he wrote in an e-mail to CNN.
"There had to be a better way. And if it didn't exist, we'd have to build the damn thing."
So the two engineers downloaded free radar data from the National Oceanic and Atmospheric Administration and built a software program that would read the storm blotches on the map more precisely against locations on the ground. Then they devised computer algorithms to map the velocity of storms and predict their paths.
Finally, they added tools to let them monitor and update their predictions in real time. Radar stations only take new weather images every 5 or 10 minutes, which makes most radar animations look like stuttery slideshows. Dark Sky's algorithm predicts what storm patterns look like between the radar frames, creating an animated weather map that flows more smoothly.
"We take a sort of statistical approach to predicting the weather, rather than using meteorological modeling," Grossman said. "So it works well over short periods of time, where the fluid dynamics of the atmosphere are approximately linear. But over longer periods of time it breaks down. The reason we do it this way is that the statistical approach actually works better for the immediate future, and that's what we're primarily concerned with."
Some weather experts are skeptical, however. CNN meteorologist Sean Morris warns the app should be used with caution.
"Because it relies solely on statistical information, it may not be able to predict significant changes in the weather due to complex atmospheric dynamics – changes that could happen in a matter of minutes," he said.
"The app assumes that areas of precipitation do not dissipate, change in size, intensity or shape. But this is rarely the case," Morris continued. "Small rain showers can grow into gigantic severe thunderstorms in a matter of minutes in volatile atmospheric situations. Even today's supercomputers can't accurately predict the onset or ending of precipitation down to the minute."
Grossman and Turner hope to lanch Dark Sky next March for the iPhone, iPad and iPod Touch. (They expect it to cost somewhere in the $5 to $10 range; an Android version may follow.) The two developed a prototype of the app, then turned to Kickstarter
, a site that crowdsources funding of innovative projects, to raise the $35,000 needed to finish it.
"I think most weather people are stuck with the same mindset of ten or twenty years ago when it comes to how weather data is delivered," said Grossman, who believes "the current apps out there are just slightly more advanced versions of the TV weatherman or newspaper forecast."
Grossman put a prototype of Dark Sky to use on his wedding day last June, when storm clouds were threatening his outdoor ceremony. Everyone was planning to take shelter when Grossman consulted the app on his phone, which told him the rain would hold off for an hour. It did, and the couple got married outdoors as planned.