## PSN-L Email List Message

Subject: Re: BBC news article about infrasound
From: John Hernlund hernlund@............
Date: Mon, 17 Feb 2003 02:29:11 -0800

```Hey Chris,
In the ideal gas treatment (this is where the approximate comes in),=20=

the equation of state is just PV=3DnkT where P is pressure, V is volume,=20=

n is number of molecules, k is Boltzman's constant, and T is=20
temperature. This is the same as P=3DrkT where r is the number density.=20=

While this is quite approximate, it does capture the gross features of=20=

the atmosphere surprisingly well. The hydrostatic part of the=20
atmospheric equilibrium (force balance) is determined by dP/dz=20
proportional to -r*g, where g is the gravitational acceleration. At=20
constant T this gives you:

dP/dz proportional to -P*g/kT or upon integrating just P proportional=20
to exp(-z*g/kT) where z is the height. Since P is proportional to r in=20=

this case, then density also falls off exponentially with height.

The temperature of the atmosphere does decrease slightly up to a=20
certain height, but then rockets way up to thousands of Kelvins in the=20=

ionosphere. You can see from the equation of state that at constant=20
pressure, the number density also decreases as temperature increases.=20
At constant temperature, the pressure decreases in proportion to the=20
density. There is obviously a tradeoff between these two end members,=20
which is reflected in the adiabatic (constant entropy) vs isothermal=20
(constant temperature) solutions. But you can certainly see that, no=20
matter what, a decrease in temperature will never lead to a decrease in=20=

density unless pressure is increased in larger proportion, which does=20
not occur in the hydrostatic atmosphere. This is all reflected in a=20
term called the "scale height" of the atmosphere, which is the height=20
at which the density decreases by a factor of 1/e =3D 1/2.7182818...  =
For=20
the Earth's atmosphere this is well measured and is (if my memory suits=20=

me well) around 9 kilometers (it is proportional to kT/g from the above=20=

considerations).

Anyways, this is all old news, and was proposed by Pascal some time=20
ago. The idea was that pressure is primarily due to the weight of the=20
atmosphere above you, so that the hypothesis could be tested by=20
carrying a barometer to the top of a mountain and comparing its reading=20=

with the value at the bottom of the mountain. Tests did, of course,=20
confirm this hypothesis.

So why all this fuss about density? Density is the amount of mass=20
contained in a volume, and according to Sir Newton if the mass=20
decreases then the acceleration increases given the same forcing. This=20=

is what really leads to the large amplitudes for vertically propagating=20=

waves...

important is not applicable in this case. I would note, however, that=20
it could be appropriate for sound waves traveling horizontally at a=20
constant altitude where the pressure is about constant, and this is=20
probably what you are thinking about. Horizontal temperature changes=20
will then induce density changes in inverse proportion. This is not=20
part of the hydrostatic atmosphere, but rather is part of the dynamic=20
atmosphere which drives winds, etc.. However, this dynamic part is=20
quite small compared to the static part when vertical directions are=20
considered, much like in the deep Earth. The only exception might be=20
when a tornado comes by, and pressure plummets horizontally very=20
rapidly.

Also note that the ionosphere is quite a bit higher up there beyond the=20=

tropopause, and that the atmosphere does not ever really end but merges=20=

in a complicated way with the solar wind and its nature up there is=20
governed mostly by magnetic field effects. Dispersion in the ionosphere=20=

is complicated by the polarizing effects of moving cations and anions=20
vertically apart creating a virtual capacitance that damps these=20
oscillations.

Cheers!
John

On Sunday, February 16, 2003, at 06:49 PM, ChrisAtUpw@....... wrote:
> In a message dated 16/02/03, hernlund@............ writes:
>
> Because the atmosphere becomes less dense approximately exponentially=20=

> with height, the sound wave velocity decreases.
>
> Hi John,
>
> =A0=A0=A0=A0=A0=A0The velocity of sound in air is primarily =
proportional to the=20
> square root of the absolute temperature, which of course decreases=20
> with height up to the stratosphere. Pressure has relatively little=20
> effect over the atmospheric range. Dispersion goes roughly with f^2,=20=

> where f is the frequency, so waves of a few Hz or less can propagate=20=

> around the globe. Volcanic explosions may be detected going around the=20=

> Earth several times.
>
> =A0=A0=A0=A0=A0=A0Regards,
>
> =A0=A0=A0=A0=A0=A0Chris Chapman=
Hey Chris,

In the ideal gas treatment (this is where the approximate comes
in), the equation of state is just PV=3DnkT where P is pressure, V is
volume, n is number of molecules, k is Boltzman's constant, and T is
temperature. This is the same as P=3DrkT where r is the number density.
While this is quite approximate, it does capture the gross features of
the atmosphere surprisingly well. The hydrostatic part of the
atmospheric equilibrium (force balance) is determined by dP/dz
proportional to -r*g, where g is the gravitational acceleration. At
constant T this gives you:

dP/dz proportional to -P*g/kT or upon integrating just P proportional
to exp(-z*g/kT) where z is the height. Since P is proportional to r in
this case, then density also falls off exponentially with height.

The temperature of the atmosphere does decrease slightly up to a
certain height, but then rockets way up to thousands of Kelvins in the
ionosphere. You can see from the equation of state that at constant
pressure, the number density also decreases as temperature increases.
At constant temperature, the pressure decreases in proportion to the
density. There is obviously a tradeoff between these two end members,
which is reflected in the adiabatic (constant entropy) vs isothermal
(constant temperature) solutions. But you can certainly see that, no
matter what, a decrease in temperature will never lead to a decrease
in density unless pressure is increased in larger proportion, which
does not occur in the hydrostatic atmosphere. This is all reflected in
a term called the "scale height" of the atmosphere, which is the
height at which the density decreases by a factor of 1/e =3D
1/2.7182818...  For the Earth's atmosphere this is well measured and
is (if my memory suits me well) around 9 kilometers (it is
proportional to kT/g from the above considerations).

Anyways, this is all old news, and was proposed by Pascal some time
ago. The idea was that pressure is primarily due to the weight of the
atmosphere above you, so that the hypothesis could be tested by
carrying a barometer to the top of a mountain and comparing its
reading with the value at the bottom of the mountain. Tests did, of
course, confirm this hypothesis.

So why all this fuss about density? Density is the amount of mass
contained in a volume, and according to Sir Newton if the mass
decreases then the acceleration increases given the same forcing. This
is what really leads to the large amplitudes for vertically
propagating waves...

important is not applicable in this case. I would note, however, that
it could be appropriate for sound waves traveling horizontally at a
constant altitude where the pressure is about constant, and this is
probably what you are thinking about. Horizontal temperature changes
will then induce density changes in inverse proportion. This is not
part of the hydrostatic atmosphere, but rather is part of the dynamic
atmosphere which drives winds, etc.. However, this dynamic part is
quite small compared to the static part when vertical directions are
considered, much like in the deep Earth. The only exception might be
when a tornado comes by, and pressure plummets horizontally very
rapidly.

Also note that the ionosphere is quite a bit higher up there beyond
the tropopause, and that the atmosphere does not ever really end but
merges in a complicated way with the solar wind and its nature up
there is governed mostly by magnetic field effects. Dispersion in the
ionosphere is complicated by the polarizing effects of moving cations
and anions vertically apart creating a virtual capacitance that damps
these oscillations.

Cheers!

John

On Sunday, February 16, 2003, at 06:49 PM, ChrisAtUpw@....... wrote:

ArialIn a message dated
16/02/03, hernlund@............ writes:

Because the atmosphere becomes less dense
approximately exponentially with height, the sound wave velocity
decreases.

=
Arial0000,0000,0000Hi
John,

=A0=A0=A0=A0=A0=A0The velocity of sound in air is primarily proportional =
to the
square root of the absolute temperature, which of course decreases
with height up to the stratosphere. Pressure has relatively little
effect over the atmospheric range. Dispersion goes roughly with f^2,
where f is the frequency, so waves of a few Hz or less can propagate
around the globe. Volcanic explosions may be detected going around the
Earth several times.

=A0=A0=A0=A0=A0=A0Regards,

=A0=A0=A0=A0=A0=A0Chris Chapman=
```