Your Galaxy S5 is jammed to the brim with all sorts of electronic sensors. These are capable of reading humidity levels, ambient temperature, air pressure, and much more. Yet, while this data is constantly being collected by your phone, there aren't any built-in apps that are capable of displaying much of it.
So developer YD Visual created an app that will display readings from your S5's internal sensors. With quick access to several weather-related readings and even battery temperature, you'll be more in tune with your surroundings and your Galaxy S5.
YD Visual's app, S5 Sensors and Battery, is available for free on Google Play in most European countries. But, if you live in the United States, the app is not listed on that version of the Play Store.
As such, you'll need to download the app from a different source. By default, only apps from official sources (like Google Play) can be installed on your Galaxy S5, but a setting is available to allow for installation of apps from any source.
To enable this option, refer to Nelson's guide on Unknown sources.
From your Galaxy S5's web browser, tap this link to download the installer file. The download should begin automatically.
When that's done, tap the Download Complete notification to launch Android's installer interface. From here, press Install, then when that's done, tap Open.
The app will perform a calibration when you first launch it to get a good baseline for all of its data. When that's done, you'll be presented with all of the readings.
There's info on ambient temperature, humidity, pressure, light level, and even battery temperature and voltage. With all of this data, your Galaxy S5 just morphed into the Swiss Army Knife of smartphones.