An altimeter is an instrument that measures air pressure and uses that to determine the altitude of an object with respect to a fixed level. Before we get into the details, we need to review the history of pressure measurement.

A barometer is used to tell us the pressure of the air at a particular location. Galileo was puzzled about the question of whether air has weight and if it exerts pressure on surrounding objects. Air weighs about 4.7 pounds per square inch at sea level. The average air pressure, at sea level, is about 1013.2 millibars or 29.92 inches of mercury. At the average air pressure, the air is pushing a thin tube of mercury to a height of 29.92 inches.

Galileo’s student, Evangelista Torricelli, finally overcame the difficulty of measuring the height of mercury in a glass tube and invented the first mercury barometer on June 11, 1644. His description of the barometer was in a letter to Michelangelo Ricci. Some historians claim that Rene Descartes actually proposed the first barometer on June 2, 1631 in a letter to one of his pupils. Torricelli had a problem with his barometer. Mercury, like all metals, expands and contracts with changing temperatures. This expansion and contraction of the mercury made accurate measurements impossible. His barometer was acting like a thermometer. It wasn’t until the 19th century that corrections for temperature were correctly applied to mercury barometers.

Gottfried Wilhelm Leibniz proposed a second type of barometer around 1700. He described small-enclosed bellows, which would be compressed and dilated as the weight of the air increased or decreased. There was a question about humidity and how it would affect the instrument, so Leibniz proposed attaching a steel spring inside which would measure the resistance of the spring, against the air pressure. Although it was many years before this type of barometer was invented, he had virtually described the Aneroid Barometer.

Altimeters are aneroid barometers. They are used to find the barometric pressure of your location and convert it into your altitude above sea level when referenced to the existing measured sea level barometric pressure. It can also be converted into a pressure altitude, which is the measurement of the pressure at your particular altitude.

Pressure altimeters are used on aircraft so they can fly and land safely in all types of weather.
Aircraft are separated vertically and horizontally. If one aircraft is not using the correct altimeter setting, it could end up too close vertically to aircraft flying below or above it. If they are off by too much altitude, this could result in a disastrous midair collision.

Landing with good visibility usually is not a problem, but landing in poor weather can be disastrous. A pilot can land by a constant glide approach to the runway, which requires a correct altimeter setting. The constant glide approach is basically a nicely sloped line down to the end of the runway. If the altimeter is set incorrectly there are two things that can occur. The first is overshooting the end of the runway. The second result is you could fly into the ground before the runway.

There have been many well-documented airline disasters directly related to incorrect altimeter settings and monitoring.

Pressure altimeters not only need to be periodically reset for sea level pressure but also for corrections between the actual temperature and standard atmospheric temperature profile. They also need to be calibrated for mechanical errors in the instrument.

Michael Eckert

I grew up an Air Force Brat. Traveled the country and lived in Georgia, Maine, New York, Hawaii and Oklahoma.

I fell in love with the weather in Oklahoma. My father was transferred to Tinker AFB in 1973. While in Temporary housing (a mobile home, which is the standard in Oklahoma) I experienced my first severe thunderstorm with strong winds and hail the size of baseballs. The next day I was in the base library looking up books on weather. The rest is history.

I graduated from the University of Oklahoma in 1983 with a Bachelor’s Degree in Meteorology. The first two years we took Calculus, Differential equations, Physics, Chemistry and Computer science classes with the Engineering Students. It was a grind. My degree is actually from the College of Engineering. The last 2-3 year’s focus was on Meteorology including Observational networks (Satellite, Radar, Surface), Physics, Thermodynamics, Dynamics, Synoptic, Winter Weather, Severe Weather and Climatology.

My first job out of college was with a small forecasting company in Oklahoma City. I was immediately put on TV (OETA) and Radio (WKY) as their broadcast Meteorologist. After two years in broadcasting, I decided to pursue the National Weather Service route and got a position in Toledo, OH as an intern. After a couple of years, I was promoted to a forecaster position at the Cleveland Forecast office. I quickly moved into the Weather Preparedness position and was responsible for all the preparedness activities in the state of Ohio.

In 1992 I decided to pursue other forecast opportunities and moved to the Meteorological Operations Division of the National Meteorological Center in Washington, DC. This group is now called WPC (Weather Prediction Center). There I fine-tuned my forecasting of Synoptic Weather with my focus on Heavy Convective Rainfall and Winter Storms, under the supervision of Dr. Louis Uccellini. He has written several books on East Coast Winter storms. I was promoted to a Senior Branch Forecast position during my tenure at MOD. Part of my job was to teach weather classes at COMET (Cooperative Program for Operational Meteorology, Education, and Training).

In 2012 I was given the opportunity to start up a new weather support group with the FAA (Federal Aviation Administration) in Warrenton, VA at the ATCSSC (Air Traffic Control System Command Center). The ATCSCC is where the FAA identifies solutions to air traffic inefficiencies in the NAS (National Air Space) for the CONUS (Continental United State). Weather impacts are the biggest impact on Aviation with yearly losses over 20 billion dollars. My job was to help lower these inefficiencies/costs by providing weather impact briefings and forecasts in order to keep the air planes moving as safely and efficiently as possible.

I retired in 2022 and now am running Lake Anna Weather, LLC.