The following is a closeup of an altimeter. This instrument mearsures the vertical elevation of an object above a given reference point. The reference may be the surface of the earth, mean sea level (MSL), or some other point. There are several different types of altitude, depending on the reference point used! The pilot needs to know what reference point is used so that he/she will know what altitude applies.
The variometer provides the balloon pilot with information as to how fast/slow the balloon is changing altitude. Where the altimeter provides the pilot his actual altitude reading, the variometer will let him know if he's dropping like a rock or gently bringing the balloon down. This is also important in navigation, since the balloon will drift with winds at a given elevation, and as such, one way to control the direction of the craft, is by staying at a given altitude for a length of time before proceeding to another altitude.
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