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Why is the air less dense higher up you go?

by sameeralord
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sameeralord
#1
Jun18-09, 03:15 AM
P: 640
Hello everyone

I have a quick question why is the air less dense higher up you go. I can understand air expands and the volume is bigger hence less dence but that only happens if it is warm, higher you go it is cooler so how does air expand. Thank you!!
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Borek
#2
Jun18-09, 03:42 AM
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Think gravitation.
sameeralord
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Jun18-09, 05:02 AM
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Quote Quote by Borek View Post
Think gravitation.
Thanksfor the reply. I can understand how this would create higher pressure lower down but how is this related to density. Warm air is more dense than cold air so how is cold air at the top be less dense.

Andre
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Jun18-09, 05:26 AM
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Why is the air less dense higher up you go?

Temperature is more or less equivalent to the total kinetical energy of the individual molecules. Density is the total mass (amount) of the molecules in a certain volume.
sameeralord
#5
Jun18-09, 05:44 AM
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Quote Quote by Andre View Post
Temperature is more or less equivalent to the total kinetical energy of the individual molecules. Density is the total mass (amount) of the molecules in a certain volume.
Thanks a lot for your reply it makes sense but now I'm bit confused about density and I'll ask my question in form of a picture

fleem
#6
Jun18-09, 06:06 AM
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Pressure is average force per area. If you have fewer molecules hitting an area over a given time, and/or the molecules are traveling more slowly, then there is less pressure.
Borek
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Jun18-09, 07:44 AM
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Quote Quote by sameeralord View Post
Warm air is more dense than cold air
Quite the opposite - assuming constant pressure, warm air is less dense.

But you can't directly compare densities, not taking pressures into account. The higher you go, the lower the pressure - so even if the air is colder, its density is lower.
sameeralord
#8
Jun18-09, 08:00 AM
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Quote Quote by Borek View Post
Quite the opposite - assuming constant pressure, warm air is less dense.

But you can't directly compare densities, not taking pressures into account. The higher you go, the lower the pressure - so even if the air is colder, its density is lower.
Yes that is what I meant I just wrote it wrong way around. Why is density lower with lower pressure. For example in my picture. The gas molecules at the red area, if I want to look at the density of these molecules do I take the blue box or the yellow box as my volume.

Thanks for the help so far
Borek
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Jun18-09, 08:13 AM
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Quote Quote by sameeralord View Post
Why is density lower with lower pressure.
Start with ideal gas equation:

[tex]n=\frac {PV} {RT}[/tex]

Density is mass per volume, mass is number of moles times molar mass. Combine it all.
qraal
#10
Jun19-09, 07:02 AM
P: 775
What we feel as air pressure is the weight of the air above us, thus the higher you go the less air there is above you. To remain in equilibrium the force must be equalised in all directions, thus why it's even in the horizontal directions. Remember that air is compressible - it occupies less volume (gets denser) the more it is squeezed, thus it's denser where the pressure is higher near the ground and as you get higher the amount of weight from a given volume of air decreases, so the pressure squeeze eases off increasingly rapidly with altitude.
Astronuc
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Jun20-09, 07:39 AM
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Quote Quote by sameeralord View Post
Hello everyone

I have a quick question why is the air less dense higher up you go. I can understand air expands and the volume is bigger hence less dense but that only happens if it is warm, higher you go it is cooler so how does air expand. Thank you!!
Air is less dense with altitude because there is less mass above a given elevation. It is the weight (force due to gravity) of the mass of air above a given elevation that produces the pressure.

At sea level, the air has a pressure of 1 atmosphere (1 atm, or ~14.7 psia, or 0.10132 MPa). With increasing elevation, the pressure decreases to the vacuum in space.

Heating occurs near the earth's surface via conduction from the earth's surface and direct heating, which is proportional to the density.

http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html
Variations in air properties extend upward from the surface of the Earth. The sun heats the surface of the Earth, and some of this heat goes into warming the air near the surface. The heated air is then diffused or convected up through the atmosphere. Thus the air temperature is highest near the surface and decreases as altitude increases. The speed of sound depends on the temperature and also decreases with increasing altitude. The pressure of the air can be related to the weight of the air over a given location. As we increase altitude through the atmosphere, there is some air below us and some air above us. But there is always less air above us than was present at a lower altitude. Therefore, air pressure decreases as we increase altitude. The air density depends on both the temperature and the pressure through the equation of state and also decreases with increasing altitude.
. . . .
Heating is also a function of composition, which also varies with altitude.
http://www.physicalgeography.net/fundamentals/7b.html
From an altitude of 20 to 50 kilometers, temperature increases with an increase in altitude. The higher temperatures found in this region of the stratosphere occurs because of a localized concentration of ozone gas molecules. These molecules absorb ultraviolet sunlight creating heat energy that warms the stratosphere. Ozone is primarily found in the atmosphere at varying concentrations between the altitudes of 10 to 50 kilometers. This layer of ozone is also called the ozone layer.
http://en.wikipedia.org/wiki/Ozone_layer
http://www.nas.nasa.gov/About/Educat...zonelayer.html

Notice that the lowest 10% of the atmosphere holds 90% of the air. This is because gases are compressable. In a huge pile of feathers the bottom-most feathers become compressed under the weight of the feathers above them. Likewise the lower levels of the atmosphere are filled with compressed air while the upper levels, such as the stratosphere, contain very 'thin' uncompressed air. Although the stratosphere layer is over four times thicker than the lower atmosphere, the stratosphere holds so little gas that ozone is still considered one of the minor trace-gases of the overall atmosphere.


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