How does altitude affect air pressure and density? If you happen to be a climber, you will truly understand how a change in air pressure can affect you in many ways.
It is due to that change in pressure that you experience breathing difficulties and other issues.
But the question is, “how does increasing altitude affect air pressure and density in the first place?”
The air pressure becomes low as you move up, and it goes up at lower altitudes, and all this happens due to gravity forcing air down.
Fact: The average pressure at sea level is about 1013 millibars, or 29.92 inches of mercury, depending on the unit of measurement used for atmospheric pressure.
Understanding the Effect of Altitude on Air Pressure and Density
Altitude refers to the distance above sea level and is related to air pressure. Where the altitude is high, the air pressure is low.
This has to do with gravity forcing air down to the center of the earth, leaving a decreasing amount of gas molecules in the air.
Getting an Idea of Air Pressure and Density
To get a better idea of how altitude has an impact on air pressure as well as density, you need to learn more about these concepts:
Air Pressure
Air pressure is the weight of the air pressing against a given area.
When air pressure is measured at sea level, the pressure is always one atmosphere.
On a barometer, the instrument used to measure air pressure, one atmosphere equals 76cm of a mercury column.
For every 105m of altitude the air pressure falls by 1cm. Because this is a constant, air pressure can be used to measure attitude.
Air Density
Air density is defined as mass divided by volume.
It is related to air pressure and altitude and is a measure of how many gas molecules are present.
Like air pressure, air density decreases the higher you go.
Air at high altitudes with a low density and pressure is often described as rarified, meaning it contains less breathable gases.
Most people find it challenging to breathe rarefied air so the mountaineers exploring higher altitudes take portable tanks of oxygen with them.
Fact: By traveling to higher elevations, where there is less oxygen, the effects of altitude-related changes in density and air pressure on human physiology become more pronounced.
How Does Altitude Affect Air Pressure and Density?
Okay, so what happens to density when altitude increases or decreases?
Well, air density and air (atmospheric) pressure are directly connected forming a three with temperature.
What you need to understand is that density is directly proportional to pressure but only indirectly proportional to temperature.
In a constant temperature when air pressure increases, air density increases.
But, on the other hand, when the temperature changes but the pressure is constant, the density decreases.
The Impact of Altitude on Pressure and Density
Air pressure and density are both influenced by temperature which gets colder the higher up you go.
So, both air pressure and density change at each new layer of the atmosphere as it spreads out and the gas molecules have more room to move about.
We can feel this change when traveling by plane and air is trapped in the inner ear and presses uncomfortably on the eardrum.
Thankfully, the situation is temporary.
As the plane descends, the change in pressure causes the eardrum to push out and the ear to ‘pop’.
How Does an Increase in Altitude Affect Air Pressure?
Surface air pressure is equal to the mass of air in the column above it.
When at sea level, this means all the weight of the column of air above is pressed down to the ground by gravity.
As the altitude increases the air column is shorter and so there is less air mass in the atmosphere above to press down.
At that point, the air pressure is lower than on the ground because there is less air above.
Since the mass of the air is created by the density of gas molecules, where there are less gas molecules, such as high up in the atmosphere, there is less mass and hence, lower pressure.
Although the atmosphere is a thin band wrapping around the earth, within it differing temperatures create numerous layers so that each layer has its own density and atmospheric pressure.
The Density of Air at Higher Altitudes
If you want to learn more about how the density of air changes as you change altitudes, you have to learn more about the molecules in the air.
The thing you need to know is that the air is less dense at higher altitudes because it contains fewer molecules of oxygen, nitrogen and carbon dioxide.
Moreover, things work differently at different altitudes.
For instance:
At Higher Altitudes
Air also expands as it rises so that high in the atmosphere, each layer can spread out.
Those molecules that have escaped the pull of gravity to remain in the atmosphere are high in energy and able to move freely without dumping into each other.
At Lower Altitudes
At lower altitudes, the molecules are nearer the center of the earth and have more weight.
This drags them closer together increasing the pressure between them. Here, the gravitational attraction is less.
Collectively the mass of the molecules causes pressure by compressing the molecules below them.
And those below them are compressed and so on down the column increasing the compression and therefore the pressure.
The Role of Water Vapor in the Air
The amount of water vapor in the air also has an effect on its density and pressure.
In the atmosphere, there is more water vapor than other gases.
It is one of the greenhouse gases and has more of an effect at high altitudes than it does at low.
The more water vapor there is, the lower the air density becomes.
An Important Consideration
The ideal gas law describes the relationship between gases and pressure, volume and temperature.
It states that a given volume of air at a given pressure will have a set number of molecules.
However, any of these gas molecules can be displaced from the air column by water vapor.
Whenever and wherever a lighter water vapor molecule is added to the air, it displaces another molecule and lowers the density.
Fact: Changing pressure systems influence wind flow and precipitation, and the air pressure and density fluctuations with altitude also have a role in these patterns.
The Importance of Air Pressure and Density in the Real World
Okay, so you might have gathered the basics of how air pressure, density and changing altitudes work together.
But, you need to know that both density and air pressure have practical applications in the real world as well.
Examples of density in use include:
- How ships float and submarines sink
- How liquids flow through pipes
- How oil floats in salad dressings and at sea
How the density of the fluid affects the fluid’s velocity, pressure and height is set out in Bernoulli’s equation.
Air Pressure and Density in Aerospace Industry
It also accounts for how planes fly using pressure and velocity, rather than density although density can still affect flight.
Pilots have to make constant adjustments to counter the uneven density of the plane as it uses fuel and lowers its mass during flight.
Air Pressure and Density for Mountaineers
Because air pressure decreases at a predictably constant rate, mountaineers can use air pressure measurements to find out exactly how high they are.
But ultimately, anyone can make use of air pressure just by drinking liquid through a straw. There are other real world examples of how air pressure is used.
For instance:
- In tires and other inflatables such as balloons
- In a vacuum cleaner, to create the suction that lifts dirt
- When flushing a toilet
- When making a meal using a pressure cooker
- When getting a note out of a wind instrument
- In flight, as the difference in pressure between the top and bottom of the wing creates the force that lifts the wing
Fact: The lift and thrust required to keep an aircraft aloft vary greatly with altitude, making air pressure and density crucial considerations for air and space travel.
Takeaway
How does altitude affect air pressure and density? At higher altitudes, air pressure and density drop.
This is because the air pressure drops as you ascend since the weight of the air molecules drops.
When atmospheric pressure decreases, there are fewer air molecules in a given volume.
Therefore, atmospheric pressure and density decrease with increasing altitude.
