Outtakes: Reading a Barometer Is Tricky
To be useful, barometer readings of atmospheric pressure must be very precise, but the temperature, a barometer’s location, and the atmospheric pressure all affect the readings.
In a mercury barometer, the metal housing and the mercury expand or contract at different rates with temperature changes. To allow for these differences, observers must adjust readings based on the temperature.
Any barometer is affected by differences in the earth’s gravity at different locations. The How Do I Adjust My Barometer page on the United Kingdom’s National Physical Laboratory Web site explains this and how corrections are made.
Even after all of the corrections for temperature and location are made, reporting an official atmospheric pressure involves more than writing down and reporting the numbers read directly from a barometer. The figure read directly from a barometer after the corrections described above is called the station pressure. It’s not reported with weather observations because, by itself, such a reading is not useful. Both the weather and a barometer’s height above sea level affect the atmospheric pressure acting on it, and both of these must be taken into account in reported barometric pressures.
Chapter 3 of The AMS Weather Book includes a chart showing how atmospheric pressure decreases with altitude. As described in Chapters 3 and 5, atmospheric pressure changes are an important part of weather changes. Since meteorologists are interested in weather changes, they need observations that don’t include the effects of a barometer’s height above sea level.
Weather observers do this by calculating sea-level pressure, which is defined as: “A pressure value obtained by the theoretical reduction of barometric pressure to sea level.” In effect, sea-level pressure is what the barometer would read at a particular time and place if it could somehow be at sea level, such as at the bottom of a hole that goes down to sea level. Sea-level pressure calculations use the station pressure, the elevation above sea level, and the average temperature for the 12 hours before the observation. Today, of course, computers handle the calculations at most weather stations.
Sea-level pressure reports make it possible to draw maps of surface atmospheric pressure patterns that are not distorted by elevation differences between weather stations. Otherwise, all high-elevation locations would be mapped as having low pressure.
Weather stations, especially those at airports, also report a pressure figure called the altimeter setting or altimeter reading, which pilots use. It's defined as “The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.” It is calculated using the station pressure and the elevation but not the temperature, as for the sea-level pressure.
Most aircraft altimeters, which measure the height of the aircraft above sea level, use an aneroid barometer to move the pointers on the altimeter. In other words, an altimeter is nothing more than a barometer, the most common ones an aneroid barometer. Altimeters are also being made with electronic devices that respond to air pressure. These are at the heart of digital barometers and altimeters. The University Corporation for Atmospheric Research’s Measuring Atmospheric Pressure Web page has a diagram showing how an aneroid barometer works.
Aircraft altitude is the height above sea level, even over land hundreds of miles from any ocean. Imagine what it would be like flying in an airplane as the pilot tries to stay exactly 10,000 feet above mountains, say while flying over the Rockies. All in all, using an altitude reading based on the pressure of the air an aircraft is flying through works better than basing altitude on the height above the earth’s surface.
Still, many aircraft also have radio altimeters, which beam radio signals down and measure the distance to the earth’s surface using the time it takes for the radio beam to reflect back to the aircraft, at the speed of light. Radio altimeters don’t replace pressure altimeters. Instead, they give pilots added information, such as a warning that they are flying dangerously near to a mountain peak.
In the February 2008 issue of Flight Training Magazine, an article intended for student pilots explains the basics of how pilots use altimeters.
You can use the Pressure Conversions section of the Weather Calculator on the El Paso Area NWS Forecast Office’s Web site to make the calculations described above, such as using station pressure to calculate sea-level pressure or converting pressure readings among the different measurement systems.