Weather

At first glance an aircraft’s altimeter seems pretty simple. It tells you how high you are. What could be easier? It’s not as simple as that. An aircraft altimeter is supposed to show the height above mean sea level. A new pilot could ask: “I’m flying out of Scottsbluff, Nebraska, where the runway is 3,967 feet above sea level. Why should I care about how high I am above sea level?”

Using mean sea level as the zero altitude is more than an arbitrary rule; it makes the most sense. You would never want to try to remain at a particular distance above the ground, especially when flying over mountains. And the air traffic control system would not work without controllers being able to assign particular altitudes that aren’t changing based on a distance from the hills and valleys below.

To measure the height above the ground, you need a radar altimeter. Its antenna sends microwaves down and uses the time it takes for them to reflect back at the speed of light to calculate height above the ground. Radar altimeters work well only relatively close to the ground. Air carriers use radar altimeters for low-visibility landings and as a part of autoland systems. Here a radar altimeter “tells” the autopilot when to begin flaring for a landing and controls the autothrottles. Military pilots use radar altimeters to follow terrain as they fly mere feet above the ground or water to avoid radar detection. The frequencies used for radar do not follow the curve of the Earth. The lower an aircraft is flying the closer it can approach a radar antenna while remaining below the radar’s horizon.

Ordinary airplane altimeters are called “pressure altimeters” because they use air pressure to measure height above sea level. This gives accurate altitudes—as long as needed adjustments are used—because atmospheric pressure decreases with height, but not at a steady rate. The abbreviated Standard Atmosphere chart shows how.

Nevertheless, when you’re near the ground, the rule of thumb that says atmospheric pressure decreases by one inch of mercury for each 1,000 feet in altitude gained works pretty well. In fact, you should use this figure for altimeter questions on FAA knowledge exams.

An altimeter is a barometer that’s calibrated to read in feet above sea level instead of inches or mercury or millibars as barometers do. Altimeters have traditionally been sensitive aneroid barometers. An aneroid is a flexible metal bellows from which some air was removed before it was sealed. Pressures lower than the inside pressure allow the bellows to expand; higher pressures squeeze it, making it shorter. A lever attached to one end of the bellows moves as the pressure changes to point to a number on a dial indicating the surrounding air pressure.

Altimeters are available that replace mechanical aneroids with pressure transducers, which converts pressures into electrical signals that can be used to give altitude in digital form instead of with the traditional pointers.

Atmospheric pressures lower than the surrounding pressure are associated with foul weather. The centers of storms are areas of low atmospheric pressure, while areas of high pressure are associated with clear, calm weather.

For an altimeter to give accurate indications, adjustments must be made for these pressure changes. To do this you need a figure that all weather stations report called the altimeter setting. Before taking off for a flight your checklist should include altimeter setting, which you can obtain via radio if the airport has automated weather reporting or a tower with automated terminal information service—ATIS.

An altimeter setting from a nearby airport would work fine. Except in the centers of strong storms, atmospheric pressures don’t change much over a few miles. When you have the setting you turn the knob to the setting number next to the pointer in the Kollsman window. When you set your altimeter to the correct setting it should read the elevation of where you’re sitting. If it doesn’t, your altimeter might need calibration. If a nearby altimeter setting isn’t available, set your altimeter to show the airport elevation when you are still on the ground.

The figure shows what happens when you don’t set your altimeter when flying into an area of low atmospheric pressure. At Airport A, you set your altimeter to 30.74 and since you’re at sea level, it reads O altitude. As you climb, the altimeter senses lower and lower pressure until the pressure is 25.74 inches of mercury. The altimeter reads 5,000, and you level off and head for Airport B. Since the surface pressure is lower at B than at A, all of the levels aloft at a particular pressure slope down from A to B following the line labeled “Indicated 5,000 feet.”

You’re so busy holding an indicated altitude of exactly 5,000 feet and enjoying the view that you neglect to obtain the altimeter setting for Airport B and do not reset your altimeter. As you arrive over B, your altimeter still reads 5,000 feet. But, how high are you actually above the sea level airport? When you reset your altimeter to Airport B’s 29.74 reading, you see that you are actually 4,000 feet above the ground.

You can remember this example with the saying, “high to low, look out below.” If you fly toward an area of lower pressure you need to look out below because things such as mountaintops and television towers could be closer than you think.

If in this example you landed, and then reset your altimeter to the airport’s setting of 29.74 and flew back to Airport A without setting your altimeter at an indicated altitude of 4,000 feet, you would again follow the orange line to be higher than indicted at A. You could remember what happens in such a case with something like: “Low to high, head for the sky.” This case isn’t as dangerous as the trip from A to B. If you entered the traffic pattern that’s supposed to be 1,000 feet above the airport without setting your altimeter, you’d be 2,000 feet above the airport on the downwind leg.

The "high-to-low, look-out-below" rule applies to temperature as well as to air pressure, but unless you are flying into extremely cold weather you aren’t as likely to run into trouble as from not setting your altimeter correctly.

As air warms, the molecules of its various gases begin moving faster and spread out. When the air is colder, the air molecules are closer together. Since the pressure at any altitude depends on the weight of the air above, an area of warm air can have the same surface pressure as cold air, but the column of warm air will be higher. If your barometer is measuring an atmospheric pressure 25.74 inches of mercury, you will be closer to the ground in cold air than in warm air.

Altimeters have no way of adjusting for temperature, but you can use a flight computer to estimate how much lower than indicated altitude you are in cold air.You use only the temperature at your flight level for the calculations, and this assumes that the air is cooling at a uniform rate between you and the ground.

Cold air that lowers pressure levels is one reason why you need to worry more about altimeter readings during the winter than in the summer, especially in the cold parts of the world.

You’re not likely to encounter situations where this makes a difference unless you’re flying in some very cold regions. For example, during the extremely cold (even for Alaska) temperatures in that state in early February 1999, the FAA prohibited instrument landings at some airports because the cold made altimeter readings too unreliable.