Weather

When you’re mastering the information and skills needed to fly an airplane, you can easily lose sight of the medium that makes flying possible: the atmosphere. Imagine that after earning your private pilot certificate you offer to take a friend for a ride. What do you say when he asks: “What keeps this thing in the air?” You’d probably launch into an explanation of how the air flowing over and below the wings create lift.

“I still don’t see anything that’s holding us up,” your passenger comments.

You know the flow of air around an airplane’s wings keeps it in the sky. But since air is invisible, you should be able to explain that this invisible substance is as real as a rock; it has mass that well-designed wings or rotor blades harness to take heavier-than-air vehicles aloft and keep them there.

Let’s take a look at the atmosphere: the air that not only keeps your airplane in the air but supplies the oxygen needed to keep the propeller spinning.

Empty space. 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. (When we refer to “air molecules” below, we’re talking about this mixture.)

What you’ve probably not heard is that the air around us is mostly empty space; in fact, it’s in the neighborhood of 99 percent empty space—which makes the fact that air can hold up fully loaded Boeing 747s even more amazing.

The air exerts enough pressure to keep airplanes in the air because the billions of molecules that constitute air are moving very fast—something like 1,000 mph at the temperatures near the Earth’s surface. The pressures exerted by the molecules of the gases in air depend on the masses of the molecules and the speed at which they are traveling. The speeds, in turn, vary with temperature. The higher the temperature, the faster the molecules are moving and the more pressure they exert.

If you heat air that is confined in a closed container, the pressure of the air against the sides, top, and bottom of the container increases as the molecules speed up. Cool the container and the pressure of the air inside decreases.

Since the atmosphere isn’t a closed container, what happens when air is heated or cooled is more complicated. Heated air expands and becomes less dense. Cooled air contracts and becomes denser. Both have important consequences for aircraft performance and effects on altimeters.

If you’re just beginning to wrestle with understanding the weather, one point might confuse you. We say above that cold air is denser than warm air. As you go aloft, however, the air usually grows colder. Why doesn’t this colder air sink into the less dense warm air below?

The answer is that air density depends on its pressure as well as its temperature. As you go aloft, the air’s pressure decreases, making the air less dense. This pressure decrease is more than enough to overcome the density increase from cooling the air.

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.

Pressure decreases with altitude because there are fewer air molecules above exerting downward forces.

In ground school or self-study for the FAA’s knowledge tests, you learned that atmospheric pressure decreases by one inch of mercury for each 1,000 feet in altitude gained. While the FAA expects you to use this formula on its tests, it’s only a rule of thumb or an approximate formula that works well enough for the first few thousand feet of the atmosphere. To use the real formula you would have to know it, be handy with calculus, and have a scientific calculator.

The chart above shows the average atmosphere pressure in the metric units called millibars in the United States. (They are called hectopascals [hPa] in most of the rest of the world, including in Canada). The chart clearly shows how the air’s pressure decreases with height. You’ll see that roughly half of the air’s weight is at the 500-millibar level, which is 18,000 feet above sea level on the average.

Differences in the air’s pressure have important consequences for your health at high altitudes, as well as your aircraft’s performance.

Any aircraft’s engine, whether it’s a piston engine or a jet, creates power by burning fuel. Like any kind of combustion, this requires oxygen from the air. (Rockets are an exception. Solid rocket fuels include oxygen. Liquid-fuel rockets carry a tank or tanks of liquid oxygen.) As you go higher oxygen continues to make up approximately 21 percent of the air, but since there are fewer molecules of all kinds in each cubic foot of air, there is less available oxygen than at lower altitudes. Eventually you will reach an altitude at which the engine is using all the available oxygen. At this point the engine can’t produce more power to take the aircraft higher. Even with a turbocharger—which increase the altitude at which the engine continues to produce power—eventually you’ll reach an altitude at which the engine won’t get enough oxygen.

As you climb into thinner air, your airplane’s wings will produce less lift. Basically, a certain number of air molecules must flow around your airplane’s wings each second to create the lift that keeps the airplane in the air. As the air grows less dense with altitude, added speed can make up for the thinner air, but only up to a point. Thinner air aloft also reduces drag, but this help can carry you only so far.

Like your airplane, your body uses fuel—food—for chemical reactions that require oxygen to release the energy that keeps you running. Your body’s performance suffers from a lack of oxygen.

A lack of oxygen is known as hypoxia and it is especially dangerous because it can sneak up on you. In fact, an early symptom could be a feeling of euphoria: “I’m feeling great and enjoying the view and life.”

This is why the FAA requires pilots of unpressurized aircraft to use oxygen when flying higher than 12,500 feet for more than 30 minutes or all of the time when flying above 14,000 feet.

You sometimes hear that high-altitude air “has less oxygen.” This isn’t the case; no matter how high you are flying in the atmosphere, roughly 21 percent of the air’s molecules are oxygen—there are fewer molecules of all kinds.

Many general aviation pilots rarely, if ever, fly higher than 10,000 feet; hypoxia isn’t a real hazard. Nevertheless, you don’t have to fly higher than maybe 2,000 or so feet to feel the effects of unequal air pressure.

Most of us generally don’t feel the air’s pressure because it’s the same inside and outside your body. When your ears pop as you climb or descend in an aircraft, or even in the elevator of a very tall building, you are feeling the air pressure inside your ears adjusting to equal the 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 pressures pushes against tissue. You can feel these “sinus block” pains above your eyebrows, or in your upper cheeks, maybe even your teeth. The pain can be excruciating and interfere with your flying.

If you have a head cold, you’re better off not flying. The FAA’s Pilot's Handbook of Aeronautical Knowledge notes that, “Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the sinus openings. Oral decongestants have side effects that can impair pilot performance.”

A sinus infection or a bad head cold is the only time natural atmospheric air pressure changes with altitude should get in the way of your flying. Sinus pain is nature’s way of convincing you that the air and its pressure are real. This is a lesson in atmospheric science you’d probably not want to learn while trying to concentrate on landing an airplane.