An altimeter measures a hot-air balloon’s altitude above sea level.
Altimeters are essential flight instruments because balloon pilots need to know their height while ascending, descending, and navigating different wind layers. A conventional aviation altimeter estimates altitude from atmospheric pressure, which decreases as height increases.
A barometer also measures air pressure, but it is primarily used to monitor weather and pressure changes rather than directly display altitude. A thermometer measures temperature, while an anemometer measures wind speed. These instruments may all be useful during ballooning, but they do different jobs.
Because atmospheric pressure varies with weather, a pressure altimeter is not perfectly exact unless it is calibrated to the relevant local pressure setting. Modern balloon equipment may also use GPS-based altitude information, but the standard instrument name for measuring height remains “altimeter.”