The Altimeter GPS app uses the integrated barometer of your iPhone (available from iPhone 6S or above). Show useful indications such as barometric pressure and weather forecasts.
The data displayed is the result of the sensors present in the mobile device, namely the GPS and the barometer. In case 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.
What to do to forecast the weather in the mountains with altimeter and barometer
To make weather forecasts with altimeter and barometer, first of all it is important to know precisely the altitude you are at and then do some simple mathematical calculations starting from atmospheric pressure at sea level, which is 1013.25mb. On average, atmospheric pressure drops by 1 millibar every 10 meters of altitude, so knowing the altitude you are at, it is possible to calculate the average atmospheric pressure at that altitude.
For example at 2500 meters of altitude the atmospheric pressure will be 1013.25mb – 250mb = 763 mb
Once the average atmospheric pressure has been calculated at the altitude at which you are located, you need to know the atmospheric pressure updated at the moment, and this can be done with the barometer. From the fact that there is high pressure or low pressure it is therefore possible to make a forecast on how the weather is evolving.
High pressure means good weather, low bad weather pressure
In general, high atmospheric pressure means good weather, while low air means bad weather. So, in the example of the 2500 meters of altitude, if the pressure is higher than 763mb there is good weather, if it is lower there is bad weather. This however does nothing but photograph the current situation and is not very useful. Instead, it is more useful to predict the time that will be in a few hours, and this can be achieved by making several measurements: if indeed there is good weather, then with atmospheric pressure higher than the average of the altitude you are at (in our example 763 mb) but the trend is decreasing up to 763 mb or even less it is foreseeable that the weather will worsen; vice versa in the case in which the measured pressure has the tendency to grow and exceed that average of the quota to which one is found, more so if these dynamics of increase or lowering of the pressure are fast or sudden.
Thanks to the GPS altimeter and the barometer that both work using your smartphone’s sensors, you won’t need an internet connection to view the values.