The BME280 is a great new chip from Bosch, which was originally designed for the next generation of smartphones. It is made up of a very accurate pressure sensor and an associated temperature sensor which helps calibrate the pressure readings. And just for fun they threw in a pretty solid humidity sensor in there as well!

So with an I2C connection you have access to enough weather data to make some pretty good predictions for your local area. Or you can just use the pressure sensor with it's abililty to discern the difference in 7.5cm in altitude.  The chip contains smarts to smooth out measurements (since it's so sensitive you don't want slamming doors to mess with your pressure readings, for example) which are fully under your control.

We have a full implementation of an Arduino driver to take advantage of this sensor out of the box.