A variometer for your Garmin watch — climb rate, audio vario, altitude tape, flight levels and thermal assist. Buy once, install directly, fly offline.
Requires a Garmin watch with a barometer (Forerunner 935/945, fēnix 5/6 and other 240×240 models). After paying you get an activation code and the installer; enter the code in the app once, with your phone nearby, and it runs offline after that.