What does an altimeter typically measure?

An altimeter is an instrument designed to measure the altitude of an aircraft relative to a specific pressure level, typically the standard atmospheric pressure at sea level. It operates based on the principles of atmospheric pressure changes with altitude; as an aircraft ascends, the air pressure decreases. The altimeter converts this change in air pressure into an altitude reading.

When using altimeters, pilots generally set a reference pressure, known as the altimeter setting, which is crucial for ensuring accurate altitude readings and safe separation from other aircraft. The universal standard reference level is the standard atmosphere, where 29.92 inches of mercury (Hg) is considered sea level pressure. This ensures that altitude readings are consistent and allow for safe navigation and flight operations.

The other options do not accurately describe the function of an altimeter. For example, measuring the weight of the aircraft pertains to load management and is influenced by other instruments, techniques, or scales. Similarly, distance from the ground is not a direct measurement provided by typical altimeters, which are focused on altitude relative to a pressure level rather than direct ground distance. Lastly, speed measurement relates to the aircraft's velocity, typically recorded by a separate instrument known as an airspeed indicator, which is not the function of an altimeter