Data gathered from Android phones via OpenSignal app used in geoscience research

Did you know your Android phone’s battery temperature readings can tell researchers a lot about environmental temperatures? That’s what the makers of OpenSignal discovered about a year ago, and the company has the peer-reviewed scientific paper to prove it.

OpenSignal recently announced that its app helped to prove that temperature readings from smartphone batteries correlate strongly to outdoor temperatures. Or as OpenSignal put it, “With a mathematical transformation, the average battery temperature across a group of phones gives the outdoor air temperature.”

The correlation was so strong that it got OpenSignal to thinking about what would happen if the company repurposed other sensors in Android phones to detect weather conditions. That led to the introduction of WeatherSignal in May, an app that we recently featured in a collection of 13 awesome science apps for smartphones.

How it happened

OpenSignal began by offering an application (also called OpenSignal) that lets you find optimum cellular coverage near you as well as nearby Wi-Fi hotspots. The app relies on crowd-sourced data pulled from OpenSignal users. One reading that OpenSignal takes from Android phones is battery performance, which includes data on charge level, voltage, and battery temperature.

Applying a formula for linear transformation, OpenSignal discovered a correlation between smartphone battery temperatures and outdoor temperatures.

On a whim, the company decided to plot its database of smartphone battery temperatures pulled from OpenSignal users in the London area against historical temperature data from Weather Underground. The company quickly discovered that, with a few mathematical adjustments, there was a strong linear correlation between smartphone battery temperature and London’s air temperature.

After checking out eight additional cities using similar methods, the company discovered a correlation between battery and outdoor temperatures with a mean absolute error of 1.52 Celsius.

The findings based on OpenSignal data were recently published in Geophysical Research Letters, a bi-monthly peer-reviewed journal about geoscience. The paper was written in cooperation with researchers at the Royal Netherlands Meteorological Institute, Wageningen University in the Netherlands, and the Massachusetts Institute of Technology (MIT).

The data collected for the paper were drawn exclusively from OpenSignal readings, but if OpenSignal data gets published again the information probably will come from WeatherSignal. Unlike its predecessor, WeatherSignal is specifically designed to repurpose sensors in your phone to get all kinds of weather readings.

The WeatherSignal app

WeatherSignal uses a phone’s light sensors that are meant to adjust your screen’s brightness settings to record how bright the sun is in your area.

The app also provides information about magnetic flux, air pressure, and yes, temperature.

As smartphones increase in functionality with new sensors, OpenSignal plans on adding more readings to WeatherSignal.

A case in point is Samsung’s Galaxy S4, which has a hygrometer and ambient temperature sensor that WeatherSignal uses to take humidity readings.

Data taken from WeatherSignal users are uploaded to OpenSignal’s servers where the information is anonymized and turned into a database for weather research. OpenSignal plans on providing data dumps to researchers from time to time.

The appeal of WeatherSignal is that instead of getting temperature from immobile weather stations, the app records temperature readings from all over the place as users go about their daily lives.

WeatherSignal operates periodically in the background, a setting you can control. And if you don’t want WeatherSignal to automatically upload data, you can upload reports manually instead.

Getting personal with WeatherSignal

Sadly, while WeatherSignal may be able to accurately measure the external temperature in aggregate, the app’s temperature readings aren’t terribly accurate on individual phones.

Nevertheless, WeatherSignal can be used for all kinds of quantified self-tracking. The app outputs all individual data it collects into a CSV file that you can analyze yourself.

WeatherSignal can help collect information about daily activity levels. Migraine sufferers can use it to help find correlations between headaches and the weather. You could also track links between mood and weather, or the differences in lighting measurements between your home and the office. Check out OpenSignal’s blog post for more information about self-tracking.

You can download WeatherSignal from Google Play. And, if you think that app is cool, don’t forget to check out our collection of 13 science apps that can aid researchers or just enhance your own understanding and enjoyment of the world around you.

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