iOS 9.3 is coming in the Spring and will introduce Night Shift as well as a number of other new features.
Evidence of iPhone 6 Atmospheric Pressure Sensor Found in iOS 8
A barometer is used to measure atmospheric pressure, giving indications on current and future weather trends as well as altitude data. The report says new APIs in iOS 8 reference altitude data, including one called "isRelativeAltitudeAvailable" that tells apps if a device "supports reporting relative altitude changes". For an iPhone 5s, the API returns "no".
According to sources at Apple, the Cupertino-company has been working on an updated version of the Compass application that includes altitude tracking, so perhaps that application, which was originally added in 2009 alongside the iPhone 3GS, is what customers will utilize to track their altitude.iOS 8 is expected to be launched this fall alongside new, larger iPhones. The altitude and pressure sensors could make an appearance in the phone as well as new iPads that are expected later this fall.