When a pilot uses the term altitude, the reference is usually to altitude above sea level as read on the altimeter, but it is important that the distinction is made to understand what types of altitude exist. When meteorologists refer to the height of the cloud layer above an airfield, they are usually referring to the altitude above the field elevation. When air traffic control refers to an altitude at FL180 and above, they are referring to pressure altitude. A flight crew in an aircraft approaching the runway during a low ceiling CAT III instrument approach will need to understand the value of a radar altimeter. Understanding what “altitudes” are important at different periods of flight, and the effect of temperature, pressure, and moisture on those altitudes, is imperative for safe flight.
