App turns smartphone sensors into weather stations

Weather reports typically only tell you what the weather should be doing in given places ? not what it is doing. But that could soon change if an online service that uses people's phones as tiny weather stations works out. Launched today, WeatherSignal will crowdsource meteorological data from Android smartphones and map it on a website.

Samuel Johnson, spokesman for OpenSignal, the UK company behind WeatherSignal, says the project is launching now owing to the fact that phone-makers are packing ever more sensors into phones. In particular, he cites the commercial launch last week of the Samsung Galaxy S4, which has the largest sensor suite of any smartphone. "It's about unlocking the potential of such phones," Johnson says.

The S4 has a thermometer, barometer, hygrometer and a magnetometer to measure ambient temperature, air pressure, humidity and the Earth's local magnetic field strength, respectively. Combined with light levels from the sensor used to calculate screen brightness, Johnson and colleagues reckon their free WeatherSignal app ? available on the Google Play app store from today ? will feed a pretty good assessment of local weather conditions, minus any info on precipitation, to their servers.

Balmy batteries

It is not all about the S4. All Android phones monitor their lithium battery temperature to prevent it getting too hot, and OpenSignal's engineers have developed an algorithm that estimates the ambient temperature by interrogating the battery's temperature sensor.

"For each user this temperature measurement will be a bit approximate," says Johnson. "But we've discovered that there is a very strong correlation between battery and ambient temperature when spread over a crowd of users."

That means the whole population of Android phones running the app could potentially supply local ambient temperature information ? though it is not clear how the system will filter data taken from phones that are indoors or in people's pockets. There are no current plans to port the app to Apple's iOS operating system, because Johnson says obtaining iPhone battery temperature readings is not easy.

Big WOW

Charles Ewen, chief information officer at the UK's Met Office in Exeter welcomes WeatherSignal. Indeed, he hopes OpenSignal might agree to feed data from its app into the Met Office's own crowdsourced Weather Observation Website.

WOW collects, analyses and maps data from schools, weather enthusiasts and amateur weather stations, but it is not fed directly by a phone app in real time like WeatherSignal.

While it is an experimental system, WOW is already making an impact. "Using amateur observations from WOW we can get real-time ground truth on extreme weather events like blizzards," says Ewen. "It's beginning to play its part."

The WeatherSignal team does not yet know what to do with the data on local magnetic field strength that will be supplied by the Galaxy S4. Small variations in magnetic field can be used for indoor positioning systems, and Ewen suggests it could be good for space-weather forecasting, too. "Magnetic data might give you ground truth for the effects of coronal mass ejections," he suggests.

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