Q: Can you help me understand in very simple terms altimeter error when flying from high to low pressure areas and from low to high areas?  I am studying it but its just not clicking.—Mark, Noblesville, Ind.

A: To answer  your question I’ll begin with some basic information about aircraft altimeters and then use the diagram below to help you see how all of this works to create altimeter errors.

An altimeter is a barometer

A pressure altimeter—the kind found in almost air airplanes—measures the pressure of the atmosphere around the aircraft with the measurements given in feet above mean sea level.

In other words, a pressure altimeter is nothing more than a barometer (an instrument that measures atmospheric pressure), but its readings are in height, not not inches of mercury, or millibars—the units used to measure atmospheric pressure in the United States.

This works well—but with potential errors such as those this question is about—because atmospheric pressure decreases at a generally steady rate with altitude.

The pressure falls by approximately one inch of mercury for each 1,000 feet of altitude gained. This figure isn’t exact—it’s a rule of thumb—but it’s close enough for the first 10,000 or so feet above sea level.

These errors arise because, among other reasons, changes in atmospheric pressure through all levels of the atmosphere depend on the weather and change with the weather. For example, the atmospheric pressure in the center of a storm is lower than the pressure in the surrounding atmosphere.

High to low, look out below

What happens when an airplane flies from high to low atmospheric pressure without setting the altimeter to the lower pressure.

The image here shows what happens when a pilot flies from a place with a relatively high atmospheric pressure to a place with a lower pressure, without changing the altimeter setting.

It shows that an airplane has departed from an airport on the left side of the image where the altimeter setting was 30.74 inches of mercury and climbed to an indicated altitude (what the altimeter reads) of 5,000 feet.

It reads 5,000 feet because the altimeter senses an atmospheric pressure of 25.74 inches of mercury around the airplane.

As the airplane flies toward the area of lower pressure on the right side of the drawing the 25.74 inches of mercury pressure surface slopes toward the ground. The yellow line shows the route that the airplane will follow if the pilot continues to hold an indicated altitude of 5,000 feet and does not change the altimeter setting to the sea-level pressure of  29.74 inches of mercury.

Since the sea-level pressure at the airport on the left is only 29.74 inches of mercury, the pressure level of 25.74 inches of mercury is only 4,000 feet above the ground.

If a pilot followed the path shown in the yellow line, he or she would have to “watch out below” because the airplane will be closer to high objects such as television towers or hills than the pilot thinks.

Altimeter setting

This is why when flying more than a few miles away from the takeoff airport, a pilot should check on the altimeter setting by radioing Flight Watch or listening to radio messages from automate weather stations or control towers that give the altimeter setting.

For airports that are above sea level, the altimeter setting is calculated using the actual atmospheric pressure to produce a figure for what the sea-level pressure would be at the location if the air’s temperature isn’t taken into account.

Temperature effects on pressure levels

The air’s temperature also affects pressure levels. When air warms it like other things, expands, which push the pressure levels up. When air cools, pressure levels descend. As shown in a diagram I’ve published here, this means that the altimeter of an airplane flying in cold air could be lower than the indicated altitude.  That is, “high to low look out below” applies to flying from warm air into cold air as it does when flying from high to low pressure.

My AMS Weather Book: The Ultimate Guide to America’s Weather, should also help  you learn more about weather, which is a vital subject for pilots.

Chapter 3 is about atmospheric pressure, winds, and related topics. Among other things, this chapter has a two-page, full-color graphic explaining how air temperatures affect pressure levels, like those in the diagram above, and how this determines the speeds and directions of upper-altitude winds, including jet streams.

The book also has a supplemental Web site with material usually found in a book’s footnotes, short pieces exploring some topics in more detail, and a few “Outtakes,” which are sections that we had to cut from the original draft of the book to keep it from being too long. One of these,Reading a barometer is tricky, will help you better understanding altimeter settings.

I have an article, which you should find useful, on altimeters scheduled for the August issue of Flight Training magazine, published by the Aircraft Owners and Pilots Association. The magazine offers a free six-month subscription to student pilots.

My Flight Training February 2008 Altimetry story has more details on using an altimeter.

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