Measuring Air Temperature With Phone Batteries

2013-08-13

Mike Gerwitz

OpenSignal—a company responsible for mapping wireless signal strength by gathering data using mobile device software—noticed an interest correlation between battery temperature on devices and air temperature[0].

Aggregating daily battery temperature readings to city level revealed a strong correlation with historic outdoor air temperature. With a mathematical transformation, the average battery temperature across a group of phones gives the outdoor air temperature.[0]
Note: Graph renderings on their website require proprietary JavaScript, but the article does describe it in detail, so it is not necessary. In particular, note that, from their provided equation[0], their scaling factor `m' implies that there is a smaller variance in battery temperature in the graph than there is in the actual air temperature, but that there is still a correlation.

This is an interesting find. The article further states that “[...] we have one data point where the Android data is actually more reliable than the traditional source.”

Such data can be very useful in providing decentralized data, so long as issues of privacy[1] are addressed. Doing so is not terribly difficult, but would have a number of factors. In particular, the user would need the means to submit data anonymously, which could be done via software/networks such as Tor[2]. GPS location data is certainly a privacy issue when it is tied to your mobile device, but fortunately, it's unneeded: you can trust your users to let you know where they reside by either (a) opting into using location services or (b) allowing them to specify a location or approximate location of their choosing (approximations would be important since a user may not wish to change their location manually while they travel, say, to and from work). If enough devices submit data, then legitimate data would drown out those who are trying to purposefully pollute the database. Such an example can be seen with Bitcoin, in which networks will reach a consensus on correct blockchains[3] so long as “a majority of computing power is controlled by nodes that are not cooperating to attack the network”. Of course, users would be able to pollute the network by sending false data as it is, and the data is already tarnished from various factors such as body heat.[0]

Of course, I do assume that mobile devices will contain temperature sensors in the future; some already do[4] (but I cannot encourage their use, as they use proprietary software[5]). However, this is still a clever hack (I suppose that term is redundant). In my searching while writing this article, I did notice prior examples of ambient temperature readings using Android software[6] (proprietary[5]), but the software does not aggregate data for purposes of determining weather patterns.

Finally, please do not download OpenSignal's app; it too is proprietary[5]; this discussion was purely from a conceptual standpoint and does not endorse any software.

[0] http://opensignal.com/reports/battery-temperature-weather/

[1] http://mikegerwitz.com/2013/08/London-Trashcan-Spies

[2] https://www.torproject.org/

[3] http://en.wikipedia.org/wiki/Protocol_of_Bitcoin

[4] http://stackoverflow.com/a/11628921

[5] http://www.gnu.org/philosophy/free-sw.html

[6] https://play.google.com/store/apps/details?id=androidesko.android.electronicthermometer&hl=en