In reference to Cecil’s answer to the sea level question (September 5, 1980), I had a thought.
What happens as the ice caps melt and raise the sea level? I mostly doubt they would willingly change the current standard values for the purposes of finding altitude by barometric reading unless they absolutely HAD to. But would they have to?
If as Cecil said “the ocean surface serves as a floor for the atmosphere,” and the floor happens to raise a few feet over a certain amount of time… if we’re still reading altitude in planes using a barometer, wouldn’t the plane altitude start measuring incorrectly at some point?
I’m thinking if a plane tries to land at an airport that is charted as being 1000 ft above sea level, but sea level has risen 20 ft since that airport was charted, then it’s now barometrically at 980 ft above sea level. The plane will also be seeking to land at an altitude 20 ft lower than what it’s altimeter reads. When its altimeter reads 1000 ft baromatrically it will still be at 1020 ft on the old charts unless the altimeter has been adjusted for the new sea level. So what happens when the plane THINKS it’s at the same height as the runway but is actually 20 ft above it?
Or am I not properly understanding atmospheric pressure and its planetary causes?
A link to the column you’re commenting on is appreciated. Providing one can be as simple as pasting the URL into your post, being sure to leave a blank space on either side of it. Like so: What is sea level? - The Straight Dope
The pilot looks out the window and says, “Oh, so that’s where the runway is,” then lands.
My question has more to do with using a barometer to measure atmospheric pressure, then using that to determine altitude. Weather strongly affect barometric pressure - TV weather folk talk about “highs” and “lows”, they’re referring to pressure. So how is that accounted for on the fly when flying? Er, you know what I mean.
When you synchronize an earth bound barometer according to a meteorological report, the adjustment is effectively setting it to what it would read at sea level. This is the same principle by which changes in atmospheric pressure, caused by weather and atmospheric tides, are corrected for when using a sensitive barometric altimeter. This correction may be carried out for nautical height (QNH; i.e. sea level) or for the airfield elevation (QFH; i.e. runway height). This information is internationally relayed in aviation weather reports in millimetres of mercury (mmHg), except for America and Canada, who like to be obstreperous, and persist with inches.