Weather

A pressure altimeter—the kind found in all airplanes—measures the pressure of the atmosphere around the aircraft. But its readings are in feet above mean sea level, not inches of mercury, or millibars—the units used to measure atmospheric pressure in the United States.

This works 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 and works well enough for examples, such as one below, or for answers to FAA knowledge test questions.

The Kollsman window The window on an altimeter’s face that shows the setting is named for Paul Kollsman, who invented an altimeter in the 1920s that was accurate enough for safe “blind” flying—we call it instrument flying.

If atmospheric pressures and temperatures at each altitude never changed, altimeters wouldn’t need to be adjusted using the Kollsman window. Weather changes, however, regularly cause atmospheric pressure to increase or decrease, affecting altimeters. In addition, the atmospheric pressure often changes along a flight’s route.

To see how these pressure changes affect an altimeter, imagine an airplane at a sea-level airport. On a day when the air pressure happens to be exactly 29.92 inches of mercury—the sea-level pressure in the standard atmosphere—the correct reading in the Kollsman window is 29.92, and the altimeter reads 0, the airport’s sea-level elevation.

If an area of low atmospheric pressure moves in, lowering the sea-level pressure to 28.92, and no change to the 29.92 setting, the altimeter would read 1,000 feet. The altimeter senses only the atmospheric pressure around the airplane. It doesn’t “know” whether the pressure is 28.92 because the airplane is 1,000 feet above sea level or because the surface pressure is that low.

An altimeter that’s in good working order and adjusted to the correct altimeter setting will read within 75 feet of the airport’s elevation when an airplane is on the ground, which means a pilot can set an altimeter to read the airport elevation when the altimeter setting isn’t otherwise available.

Obtaining the altimeter setting When you’re in the air you need to obtain altimeter settings along your route and at the destination via radio using Flight Watch on 122.0 MHz from ATC, or from airports that have automated weather reports. Manned and automated weather stations measure the local atmospheric pressure, called the station pressure, and calculate the altimeter setting. You can think of the altimeter setting as being what the atmospheric pressure at a particular time and place—such as an airport 2,134 feet above sea level— would be if it could be measured at sea level there and then.

High to low, look out below Figure 1 (below) illustrates the altimeter memory aid: “High to low, look out below.” When the airplane took off from Airport A the altimeter setting was 30.74. The pilot climbed until the altimeter read the planned cruise altitude of 5,000 feet. Using one inch less of mercury equals 1,000 feet, the altimeter read 5,000 feet when the pressure around the airplane was 25.74.

The pilot is flying from an area of high pressure to low pressure. The yellow line shows the 25.74 pressure surface, which slopes down from the high pressure to the low pressure.

At Airport B, if the pilot doesn’t set the altimeter to the local setting of 29.74, the airplane will be 1,000 feet lower than indicated—a dangerous place to be if any high towers or hills are around. “High to low look out below” also applies to temperatures as shown in Figure 2. Here, the altimeter setting could be the same all of the way from left to right, let’s say 29.92. However, in the warm air on the left the column of air expands, pushing up the 24.92 inches of mercury pressure surface. In the cold air on the right the air column contracts, pulling down the 24.92 inches of mercury pressure level.

What’s inside an altimeter Most of today’s analog altimeters use an aneroid—a closed disk or series of disks that senses air pressure. The airplane’s static port is connected to the back of the altimeter, thereby allowing ambient pressure to fill the instrument. Because the aneroid is closed, it expands with reduced ambient pressure and collapses with increased pressure. Since it’s directly connected to a series of gears and rods, it manually moves the hands on the face of the instrument.

The FAA is certifying digital altimeters that use solid-state devices that sense pressure, and digital barometers are used in automated weather stations. Scientists first measured atmospheric pressure by reading how far up into a closed tube the air’s pressure pushed mercury—thus “inches of mercury” in U.S. measurements.

While it could be marked in feet above sea level rather than inches, a mercury barometer wouldn’t work as an airplane altimeter. First, it would have to be at least 32 inches long and it has to sit upright. Also, turbulence would slosh mercury out of the cistern at the bottom of the tube and tiny drops of toxic mercury flying around a cockpit would distract pilots.