Introduction

A barometric altimeter (pronounced alt-i-me-ter or alt-im-eter depending on your place of origin) is theoretically an exceedingly simple device. The first accurate barometric altimeter, known as the Kollsman Window was invented by the German engineer Paul Kollsman in 1928. The altimeter measures air pressure in order to determine its height above sea level.

If only the practice was so simple - as we are aware, this is real life, not theory...

The Atmosphere

The earth's atmosphere clings to the surface by gravity, meaning that there is plenty of air down here on the surface for us to breathe. As we go higher and higher, the air thins out. At around 3000 meters, people who are not acclimatised to high altitudes will struggle to get along efficiently. In fact, airplanes travelling over 3000 meters are required to have pressurised cabins.

At around 5300 meters, half of the atmosphere is now below you; with the other half stretching out some 80 kilometers (50 miles) above. At this altitude, a human will become unconscious within 30 minutes without a supplemental oxygen supply. This is known as hypoxia.

The Barometer

We can use the thinning of the atmosphere to measure our altitude by using a barometer - which simply measures the air pressure around it. We just need to put the correct numbers in the correct places around the dial.

Complications

Here comes the science bit...

If a barometer can be used to measure height or the weather, then how do we remove variables in the equation? If the barometer is kept at a steady altitude, then we can watch changes in the weather over time. If the weather (or more accurately the mean-sea-level air pressure and temperature) remain the same, then we can accurately measure the altitude.

The altimeter needs to be calibrated with a known altitude at the start of a trip. Unfortunately, the weather changes while we're mountaineering or flying, due to the amount of time taken and/or distance moved during the trip. The accuracy will therefore become worse over time. There are solutions to this problem.

A mountaineer can recalibrate their altimeter whenever they are at a known altitude. The map of their area will show spot heights and contours. When they are in a known location, such as a trig point or summit, they can reset the altitude. It will then read accurately for a short time, slipping again as the weather changes.

Aviators don't have the luxury of spot heights, unless they want to clip mountaintops with their undercarriage. Each time they contact a new radio tower, they obtain that tower's mean sea level pressure reading and can then recalibrate the altimeter. This is especially important at the airport where the plane will land.

Altitude Types

The altitude given by a barometric altimeter is the height above mean sea level. The indicated altitude is that shown on the altimeter, and immediately after calibration, this is the same as the true altitude.

More important to aviators is the absolute altitude - the height above land. This can be calculated given a recently calibrated indicated altitude and a map showing the contours of the ground.

In the event that an aircraft cannot obtain a sea-level pressure reading for calibration, they can set an average atmospheric pressure value on their altimeter. That way, they know that there could be error, but can predict how far out the error could be in each direction - the weather can only affect the air pressure to a certain extent. This value is known as pressure altitude and is reasonable for high-altitude flight, but completely useless for landing.

Density altitude takes into account the temperature and humidity of the surrounding air. Given a sea-level pressure, the column of air above it is not all a uniform temperature and humidity. When taking off and climbing, the altimeter can read incorrectly due to these factors.

Conclusion

In conclusion - barometric altimeters are not simple to use, and in order to retain accuracy must be recalibrated many times on a single trip. You can never be totally sure that the reading is correct - and the longer you leave between calibrations the worse it will become. A reasonable knowledge of weather and physics are required to use one accurately, as well as good reference sources of recent sea-level pressure readings, or spot altitudes.

Sources:
http://www.space.edu/projects/book/chapter3.html
http://virtualskies.arc.nasa.gov/weather/tutorial/tutorial2e.html
http://www.todayinsci.com/2/2_22.htm