. . . uses a barometer for measuring air pressure both to determine altitude, and to track changes in air pressure over time as a way of predicting changes in the weather. There is a valve that you open "on demand" so to speak, at 4:00. You calibrate the barometer as a first step and then set the weather gauge to the zero position and the next time you open the valve, it shows whether the barometric pressure is rising or falling, and thus, if better weather or worse weather should be expected. You can also at the same time calibrate the barometer for the right altitude so you get an accurate reading even if air pressure has changed (as it will have) from the last calibration. The barometer is a dual-capsule, aneroid type. A very cool complication that offers a lot of adaptability and accuracy. Breva has launched 2 models; one a weather gauge/altimeter and the other a pure altimeter with a very high precision altitude read-off system. They are also very cool looking