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A barometer and an altimeter both function by variations in atmospheric pressure. How do changes in barometric pressure affect an altimeter and how is compensation accomplished so that pilots don't crash due to variations in their altimeters related to barometric pressure? Is there a constant that allows conversion from millibars to feet in altitude, and if so, what is it?
William B. ***********************************************************
Both of these instruments are useful. Barometers measure pressure while altimeters can measure the altitude of an object in reference to the ground. It uses the scale in altitude instead of pressure.
Barometric pressure when it is measured at the airport or anywhere else is reduced to sea level. This is called the "station pressure". Such as, as I'm writing this, the pressure at KSLC is 30.14 inches of mercury. If we didn't adjust for sea level, the the pressure in Salt Lake and other high up locations (we're at about 4200 feet above sea level) would constantly be lower. Then, when analyze a weather map, we'd see constant lower pressures over places with higher elevations, that's not good! So pressure is adjusted to sea level so we can analyze weather data across the globe. The equation used for this adjustment is called the Hypsometric Equation. It involves the Ideal Gas Law and one of my other favorites, the hydrostatic equation which relates pressure, height, density and gravity. The constant in these equations would be R in the Pv=NRT of the Gas law. R is the gas constant in dry air.
Deep breath everyone, yet another example of why mathematics and weather go hand and hand!
So yes, when the barometer changes, the altimeter changes too and if you weren't a pilot, you might not know what I just learned by answering this. When the plane is on the ground, the altimeter is adjusted, it does not adjust itself. The pilot will set the alitmeter based on the station pressure that is given which has already been adjusted for the sea level correction as we mentioned above. The altimeter is then adjusted again every 50 nautical miles below 18,000 feet. Big jet planes fly above this level, so the barometer will stay at a constant while it's above 18,000 feet and once they get below it, it will be adjusted again so they don't crash into anything.
As far as converting between all the different units like millibars or inches of mercury or hectopascals (the same as a millibar) there are various places you can do this. One inch of mercury is about 1000 feet but you can find some of the common conversions in some of the links on the right.
Answered by KSL Meteorologist Dina Freedman.
