This app is a thermometer which displays the temperature of the air (celcius or fahrenheit degree scale) by using a built-in temperature sensor. However some devices don't have this sensor and then the application use the sensor of device's electronical subassembly (e.g. the phone battery). Such temperature can be different from the ambient air temperature so to obtain the most accurate temperature you should leave your phone untouched for an hour (the best way of getting the correct temperature is to check it just after waking up after your phone hasn't been used for the whole night). Moreover to get more proper result you can use calibration menu.
How to do it?
Check outside temperature in the weather forecast. Run calibration. Adjust device temperature to fit measured outside temperature.

That not all!We have bonus. The app is also a live wallpaper and widget (you can place direct link to this app on your home screen)