We've heard in the past that Apple plans to fit the upcoming iPhones with a wide range of sensors pertaining to health, but today 9to5Macrelayed news that suggests the new units will also support a barometer. Similar information popped up in vague rumors last March, but new evidence suggests there might be some truth to them.
The clues are found in new APIs for iOS 8 that identify the capabilities of the device running the system, most notably in the form of one called "isRelativeAltitudeAvailable." Enter it, and it responds with a "no" when activated on the iPhone 5s. That's not much to go on, but the very fact that the option exists hints that the feature should be available for one of the two rumored iPhone 6 units.
The earlier rumor relayed by G for Games claimed that we'd have measurements for other data as well, such as atmospheric pressure (thus the barometer), temperature, and humdity. If there's truth to the rumor, it's likely that we'll see most of the data conveyed through an upcoming update to the Compass app that's supposedly on the way. (Again, rumors upon rumors.)
We'll get to see if there are any truth to these claims sometimes this fall, when Apple is expected to launch two large iPhones alongside iOS 8. The units are thought to boast powerful new A8 processors, which should easily be able to handle the multiple sensors in question.