Under Pressure

Tim Vasquez

Thursday, January 21, 2021 at 12:12 PM ET

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Evangelista Torricelli is credited with the invention of the barometer in the mid 17th century.

Key Takeaways:

The article highlights the critical importance of atmospheric pressure and accurate altimeter settings for flight safety, tracing the historical development of barometers and understanding pressure.
It distinguishes between various pressure expressions vital for aviation and meteorology: Station Pressure (QFE), Altimeter Setting (QNH), Sea Level Pressure (QFF/SLP), and Pressure Altitude (QNE), each serving specific purposes and reference levels.
QNH (Altimeter Setting) is presented as the primary value for pilots below 18,000 feet, allowing altimeters to display accurate altitude relative to sea level or an airfield, and emphasizing that even small errors in setting can pose significant safety risks.
"Pressure reduction," the complex process of converting observed station pressure to sea-level equivalents like QNH and QFF, is crucial for accurate altimetry and forecasting, as it accounts for variables like elevation, temperature, and moisture.

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Dialing in the altimeter setting is perhaps one of the most universal, timeless rituals of aviation, spanning decades, continents, and cultures, and tying all pilots to one another. This underscores the importance of atmospheric pressure and its effect on flight safety. But how much do you know about where those numbers come from?

In this issue we’ll take an introductory look at barometers, pressure, and get a better look at where QNH, QFE, and other values come from. If you’ve found them mysterious or confusing, this article is for you.