According to 9to5Mac, the source code for iOS 8 and Xcode 6 contains several references to a new sensor that would be a barometer.
The barometer is a measuring instrument that can measure atmospheric pressure, but which can also be used as an altimeter.
The sensor could provide the current altitude and weather information and warn of an impending change in the weather.
The updated CoreMotion APIs show clues to a possible height measurement but without giving information about the required hardware.
This type of integrated iPhone sensors could be useful in professional sport, but also for hikers, mountain climbers, bike riders and enthusiasts.