The Imaginary Air That Makes Altimeters Work: The Standard Atmosphere

The real atmosphere is messy: pressure and temperature vary by the hour, the front, and the airmass. Yet altimetry, engine performance charts, and aircraft certification all need a fixed reference. The answer is a shared fiction — the International Standard Atmosphere (ISA), a globally agreed model of how the atmosphere "should" behave that everyone in aviation calculates against. The ISA defines sea-level conditions of 15 degrees Celsius and 29.92 inches of mercury (1013.25 hectopascals), with temperature falling at a standard lapse rate of about 2 degrees Celsius per thousand feet through the lower atmosphere. A pressure altimeter is, at heart, a barometer with its dial marked in feet against this standard model: it reads height by assuming the air thins at the ISA rate. Because real pressure differs from the standard, pilots constantly correct for it. Below 18,000 feet they set the local altimeter setting (the current sea-level-equivalent pressure) so their altimeter reads true field elevation. At and above 18,000 feet in the United States, everyone sets a common 29.92 and flies "flight levels," so that even as the real pressure varies, every aircraft uses the identical reference and their altitudes stay correctly stacked relative to one another — the whole point being separation, not absolute truth. Temperature matters too: on a cold day the real atmosphere is "shorter" than standard, so a pressure altimeter reads higher than the aircraft actually is — a hazard near terrain that demands cold-temperature corrections. The ISA also underlies density altitude, performance charts, and engine ratings. It is one of aviation's foundational abstractions: a pretend atmosphere precise enough that two airplanes a thousand feet apart on their altimeters are genuinely a thousand feet apart in the sky.