The weather is always one of the great thoughts for those who go to the mountains, whether you want to take a simple trip or think of something more challenging. As long as you are at home or otherwise you can use your smartphone there are good apps for weather forecasts, but problems start when you are in areas where the phone does not take and you still have to somehow predict the time it will take.
It is possible in these cases to forecast the weather using an altimeter and a barometer, which are often found in watches or GPS navigators for outdoor and hiking, or knowing the atmospheric pressure (often in the shelters a barometer is there) and l 'altimetry of the point where you are located, which is marked on the maps or on the signals that are found in the mountains.
The Altimeter GPS app uses the integrated barometer of your iPhone (available from iPhone 6S or higher). It shows useful indications such as atmospheric pressure and weather forecast based on the barometer.
The data displayed are the result of the sensors in the mobile device, ie the GPS and the barometer. If the latter is not available in the terminal, the data will be less accurate as the app will be based on data from local weather stations, if available.