Air pressure plays an important role in your flying
Most people are perfectly happy going through life without ever thinking about air and the pressure that it's always exerting on us. Anyone who operates an aircraft, however, needs to know at least a little about the basic science of air pressure and how it affects humans, flying machines of any kind, and the weather.
While pilots don't have to be thinking constantly about air pressure, it's often the background to the things you do think about before and during a flight. This is why a basic understanding of how air pressure works could help you to make better flying �ecisions. 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 planned flight.
Let's begin with a look at air and a bit of theory about its pressure.
Air is a gas, which means it's invisible. But it's no less real just because we can't see it. It's "stuff" just as much as the wing of your airplane is made of "stuff." To be more scientific about it, the gases that make up the air have mass.
Anyone who doesn't believe that air is as real as rocks has a hard time explaining how aircraft can leave the ground and stay up much, much longer than a thrown object ever could. For some people, a fear of flying could arise in part because they don't really see how air keeps an airplane aloft. As far as such people are concerned, some kind of magic keeps airplanes from falling out of the sky. Magic that you don't understand can be scary.
About 78 percent of Earth's air is nitrogen, about 21 percent is oxygen, and the remaining 1 percent is a host of other gases, including water vapor. In this article, when we talk about air molecules, we are using the term as shorthand for all of the molecules - nitrogen, oxygen, and so on - that make up 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?
Air molecules don't pile up on the ground because they are zipping around at high speeds, colliding with other air molecules and anything else that's in the air, such as you and your airplane. As gravity pulls air molecules down, collisions with upward-moving air molecules are pushing them back up. The result is a balance of forces that leads to fewer and fewer molecules in any cubic foot of air as you go higher. In other words, the air becomes less dense and its pressure decreases with altitude.
Think of it this way: The pressure of the air at any altitude depends on the weight of the air above that altitude pushing down on the air. Unlike water, gases such as air are compressible; applying pressure to them increases the density. The result is that at sea level, the air is pushing on everything with a pressure of about 14.7 pounds per square inch. We can go through our life never realizing that the air is pushing against us, however, because the pressure is the same inside and outside our bodies - 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, say by a head cold, you begin to feel pain when the higher pressure inside can't escape as you ascend.
On average, air pressure decreases at a regular rate as you go up, and by the time you reach about 18,000 feet, the pressure is around 7 pounds per square inch. In other words, at this altitude about half of Earth's air, by weight, is above you and half is below you.
By now, you might be wondering why you never see air pressure reported in pounds per square inch, but in inches of mercury in the United States and something called hectopascals or millibars. The only reasons for using inches of mercury to measure air pressure are history and habit. Inches of mercury refer 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. The first measurements of air pressure were made this way and weather stations still use mercury barometers to measure air pressure because they are the most accurate method. The metric equivalent of inches of mercury is centimeters of mercury.
Scientists and engineers, including those who design aircraft and those who study the weather, measure the air's pressure in terms of pounds per square inch in the U.S. system and Newtons per square meter in the metric system. Unlike inches (or centimeters) of mercury, scientists and engineers can use pounds per square inch or Newtons per square meter in mathematical formulas that describe the weather and aircraft performance.
Weather scientists and forecasters - almost all of whom work in the metric system these days - use a different form of Newtons per square meter. The older metric system term is the millibar, with 1,000 millibars equal to a force of 100,000 Newtons per square meter. In most of the world, the term millibars is being replaced by hectopascals (hPa), with 1,000 hectopascals being the same as 1,000 millibars. But the U.S. National Weather Service uses millibars for upper air pressure reports and forecasts.
The fact that air pressure and density decrease with altitude has important consequences for aircraft and the people in them. An aircraft's engine, whether it's a piston engine or a jet, creates power by burning fuel, and that combustion 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 engine is using all available oxygen; no oxygen remains to produce more power to take the aircraft higher.
As you gain altitude, you also lose lift in the thinner air. In very basic terms, a certain number of air molecules have to 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.
Humans, too, run into trouble 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 our bodies 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.
If the Earth didn't have weather, air pressure would be a lot easier to understand. Pressure would decrease at a regular rate as you ascended, and that would be it. But, as we know, the weather brings changes in air pressure and even in the rate at which it decreases with altitude.
Compared to the changes in air pressure seen 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, as measured by a barometer. 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. This is why the lower the pressure at its center, the stronger a storm will be. It's also why measurements of the air pressure and how it is changing are an important part of all weather observations. Forecasters need to know what the current air pressures are doing to forecast what the pressures will be in the future. When forecasters have a good handle on future air-pressure patterns, they are able to accurately predict wind speeds and directions. This, in turn, helps them to predict where cooler or warmer air should go.
When you see an L on a weather map, you know it locates the center of an area of low pressure. An H locates the center of high air pressure.
Rising air creates low pressure at the Earth's surface because air is actually moving away from the location. As air rises, it cools, and unless it's very dry, clouds and maybe precipitation will form. Sinking air creates high pressure at the Earth's surface. 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.
In a way, the air can be compared to an ocean in which airplanes swim. The more pilots know about this ocean, the better they'll be at using it to their advantage.
Air pressure corrections
Reporting the air pressure at a weather station is more complicated than reading the number indicated on a barometer. Below are the different kinds of surface air pressure:
Station pressure: The reading directly from a barometer. It is not included in regular weather reports. For any station at an elevation higher than mean sea level, this reading will be lower than the ones reported.
Sea level pressure: In brief, this is a calculation of what the pressure would be at the station if sea level pressure could be measured directly. To calculate sea level pressure, the observer assumes that the properties of the theoretical atmosphere extending down to sea level would be the same as at the station. The calculation uses the current station pressure and temperature as well as the station's elevation. Sea level pressure is used to draw weather maps with areas of high and low pressure. If station pressures were used for this, the lowest pressures would always be over the highest elevations.
Altimeter setting: This calculation is similar to the one used to calculate sea level pressure, but the temperature is not used. You can use your airplane's altimeter when it's on the ground to find the altimeter setting, if the altimeter is accurate. Just adjust the altimeter so the altitude that it reads is the elevation where the airplane is located. The number that shows up in the Kollsman window is the altimeter setting for that location.
Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth
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