If you ever see an Airbus A380 superjumbo take off with a full load of 525 people (in three classes) and a full fuel load, at its maximum takeoff weight of 1,250,000 pounds, pause a second and think: “Nothing but air is holding that thing up.”
Of course, you could think the same thing as you leave the ground in your Cessna 152.
All pilots know that air flowing over the wings creates enough aerodynamic lift to more than balance the airplane’s weight, which is what gets an airplane in the air and keeps it there. Obviously, even though it’s invisible, air is as “real” as rocks. It has mass and thus weight. Air isn’t heavy, however. On a day when the temperature is 59 degrees Fahrenheit and the atmospheric pressure is 29.92 inches of mercury, a cubic foot of air at sea level weighs approximately 0.08 pounds.
Although pilots don’t think constantly about air pressure, it’s part of the basic science of several things pilots must know to be safe. For example, a pilot who knows how an aircraft’s altimeter converts air pressure readings into height indications will know how to interpret altimeter readings. A pilot who understands what is going on in areas marked on weather maps as having either high or low air pressure will have a better mental picture of the weather for a flight.
BASIC SCIENCE OF AIR. Approximately 78 percent of the Earth’s air is nitrogen, about 21 percent is oxygen, and the remaining 1 percent is a host of other gases, including water vapor. The term “air molecules” refers to this mixture. Since the air’s molecules have mass, gravity pulls them toward the center of the Earth. If gravity is pulling them down, you might ask, “What keeps air molecules from piling up on the ground like sand dumped from a truck?”
They don’t pile up in a big heap on the ground because they are zipping around at high speeds, colliding with other air molecules and everything else on Earth, including you and your airplane. As gravity pulls air molecules down, collisions with upward-moving air molecules are pushing them back up. The resulting balance of forces accounts for the pressure at any altitude.
In air with a 59-degree-F temperature and atmospheric pressure of 29.92 inches of mercury, the pressure is 14.7 pounds per square inch. As you ascend, fewer air molecules are pushing down from above; this is why the air becomes less dense (thinner) and the pressure decreases with altitude.
You don’t feel air pressure because it’s the same inside and outside your body—most of the time. When your ears pop as you go higher, you are feeling the air pressure inside your ears adjusting to equal the lower outside pressure. If the openings between your sinus cavities are clogged, you begin to feel pain when the higher pressure inside can’t escape as you ascend and the higher outside pressures pushes against tissue.
AIR PRESSURE AND PERFORMANCE. The decrease of air pressure and density with altitude affects the performance of both airplanes and the people in them. An aircraft’s engine, whether it’s a piston engine or a jet, creates power by burning fuel, and this requires oxygen from the air. As you go higher, oxygen continues to make up about 21 percent of the air, but since there are fewer molecules of all kinds in each cubic foot of air, engines have less oxygen than at lower altitudes. Eventually you will reach an altitude at which the air no longer has enough oxygen to produce the power needed to take the aircraft higher. Decreasing pressure also reduces lift.
Human performance, too, suffers when the air grows too thin. Just like an airplane’s engine, the cells of our bodies need oxygen to burn our fuel— food—to keep us functioning. This is why most airplanes that normally fly much higher than 10,000 feet are pressurized. Air is pumped into the cabin, making the pressure and density the same as a lower altitude where our bodies receive enough pressure to work well. High-flying airplanes that aren’t pressurized need to be equipped with oxygen masks, which supply added oxygen to those using them.
AIR PRESSURE AND WEATHER. Compared to the air pressure changes when an airplane climbs or descends, the pressure differences that create winds and storms are small. But these relatively small changes have huge consequences. When you climb from sea level to 4,000 feet, the air pressure around your airplane drops by around four inches of mercury. This is greater than the change at the surface from the outermost edge of a strong hurricane to its low-pressure center.
Air pressure differences cause the winds to blow as air moves from high pressure toward low. The bigger the difference, the faster the winds blow. The lower a storm’s central pressure, the stronger the storm. It’s also why measurements of the air pressure and how it is changing are an important part of all weather observations and forecasts.
An “L” on a surface weather map locates the center of an area of surface low pressure. An “H” locates a center of high pressure. Rising air creates low pressure at the Earth’s surface because air is rising there. As air rises, it cools, and unless it’s very dry, clouds and maybe precipitation will form. Sinking air creates surface high pressure. Since sinking air warms, it tends to evaporate clouds. This is why you are more likely to find clear—or nearly clear—skies in areas of high pressure.
INTO THE AIR. On average, air pressure decreases as you go up, and by the time you reach 18,000 feet, the pressure is approximately seven pounds per square inch. At this altitude about half of the Earth’s air, by weight, is above you and half is below.
Aircraft altimeters sense the air pressure around the airplane and convert these pressure readings into feet above mean sea level. This works because air pressure changes at a regular rate with height. However, weather-related air pressure changes affect the rate of change. This is why pilots need to adjust their altimeters regularly as they fly into changing weather.
AIR PRESSURE MEASUREMENTS. Meteorologists (and pilots) don’t use pounds per square inch to measure air pressure because this value isn’t measured directly. The surface weather data the National Weather Service issues for the public and for altimeter settings is in inches of mercury. However, the NWS uses millibars, a metric measurement, for all upper-air pressures.
Inches of mercury refers to how far the air pressure will push mercury up into a tube with the top of the tube closed and the open bottom in a container of mercury exposed to the air pressure.
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