The iPhone 6, 6 plus and the iPad Air 2 added a barometer sensor. Ostensibly it is supposed to help the device better track your steps for the health app by knowing when you've gone up and down stairs. But there is no reason it can't also be used for weather purposes. That's exactly what Pressur does. It reads the data from the sensor and displays it on your screen.
The app is simple enough. The main screen shows you the current reading. And the Log shows you a list of the most recent readings. I haven't completely figured out how they are supposed to work. Sometimes I see a lot of recent readings and sometimes it acts like I've just run it for the first time and it only shows me the most recent one. So, often the Trends diagram only shows me one or two data points making for a very boring graph.
The app offers an in-app purchase to remove ads and adds some additional options (and maybe the ability to track more than a few data points?). It also allows you to add a trend graph as a Today widget.
Pressur is a free app with ads or $0.99 to remove ads as an in-app purchase. It requires the iPhone 6, iPhone 6 Plus, or an iPad Air 2 and iOS 8 and up.