What does an altimeter measure?

An altimeter measures the pressure and height above sea level. It is an essential instrument in aviation that helps pilots determine their altitude relative to sea level, which is crucial for safe navigation and maintaining appropriate flight levels. The altimeter operates based on the principle that atmospheric pressure decreases with an increase in altitude. As an aircraft ascends, the pressure outside the aircraft decreases, causing the altimeter to indicate a higher altitude. Conversely, as the aircraft descends, the pressure increases, leading to a lower altitude reading.

Understanding the correct function of an altimeter is vital for pilots, as accurate altitude readings are necessary for avoiding obstacles, complying with air traffic control instructions, and ensuring safe flight operations. Other options do not pertain to the altimeter's measurement capabilities, as it does not measure speed, temperature, humidity, or fuel levels, which are monitored by different instruments in the aircraft.