Cool weather experience - echos off clouds

[DaveC426913]

TL;DR

Suddently I can hear the highway!

Sitting out in my shed with the door open. Warm, sunny skies. And quiet.

I suddenly look up from my reading because I can hear the highway traffic. I couldn't a moment ago.

I see that a fast, thick white cloud layer has moved overhead and blotted out the sun in less than five minutes. If it weren't 20C out here, I'd think it was a snow sprinkle coming in.

I know air conditions can change what we hear, but I'd never heard it change so rapdily and overtly.


Question: is it an echo off the clouds? Or is it a change in humidity?

(It's also very weird that the weather map shows zero cloud cover anyhwere within 50 miles. And it is usually accurate up-to-the minute).

Gosh, I hope it's not a toxic gas leak! '79 Mississauga Rail Disaster all over again!

 

[berkeman]

It could be that an inversion layer formed at the same time (I don't know if inversions create clouds or vise-versa of if they are even associated). Inversion layers are well known for bending sound waves back down toward the Earth's surface:

https://en.wikipedia.org/wiki/Inversion_(meteorology)

 

[Baluncore]

Question: is it an echo off the clouds? Or is it a change in humidity?

Both. The flat base of the clouds marks the dew point for the local atmosphere. There is only gas below the cloud base, while above the cloud base there are fine droplets of liquid water, falling, and then evaporating again, circulating. That is a form of density inversion, in that saturated air below the cloud base is less dense than dry air, (H₂O has a lower molecular weight than N₂ or O₂), while the cloud above is cooler, and contains a greater mass of liquid water. I expect it is the step change in acoustic impedance at the cloud base, that acts as the acoustic mirror.

Way back in the 1980s, on a still Saturday night, there was a Johnny Cash concert about 20km away from me. At odd moments, I could hear single frequency thumps, coming down from that direction in the sky, not enough to identify the song, but sufficient to recognise it as being from loud music. To be so narrowband filtered, I believed it must have been ducted by a waveguide in the atmosphere. An alternative explanation is that the high frequencies were attenuated by distance, leaving only the very lowest bass note, like thunder, but without the rolling echos.

 

[muzzammilhussain]

Both. The flat base of the clouds marks the dew point for the local atmosphere. There is only gas below the cloud base, while above the cloud base there are fine droplets of liquid water, falling, and then evaporating again, circulating. That is a form of density inversion, in that saturated air below the cloud base is less dense than dry air, (H₂O has a lower molecular weight than N₂ or O₂), while the cloud above is cooler, and contains a greater mass of liquid water. I expect it is the step change in acoustic impedance at the cloud base, that acts as the acoustic mirror.

Way back in the 1980s, on a still Saturday night, there was a Johnny Cash concert about 20km away from me. At odd moments, I could hear single frequency thumps, coming down from that direction in the sky, not enough to identify the song, but sufficient to recognise it as being from loud music. To be so narrowband filtered, I believed it must have been ducted by a waveguide in the atmosphere. An alternative explanation is that the high frequencies were attenuated by distance, leaving only the very lowest bass note, like thunder, but without the rolling echos.

So according to your analysis it is both echo and humidity. I am I right?

 

[Baluncore]

So according to your analysis it is both echo and humidity. I am I right?

It is only an echo, when energy is reflected back to a listener at the source.

The bulk properties of the atmosphere, with water droplets in the base of a cloud, differs from the bulk properties of the near-saturated air below the cloud. Those volumes will have different acoustic impedances.
https://en.wikipedia.org/wiki/Acoustic_impedance

Where there are different impedances in contact, there is an impedance mismatch, some incident energy will be transmitted, while the remaining energy is reflected, with the angle of incidence equal to the angle of reflection.

We do not see inside the bulk or body of anything. The only things in the universe that we can sense, touch, see, or hear, are the impedance mismatches between those things.

 

[DaveC426913]

So according to your analysis it is both echo and humidity. I am I right?

It is only an echo, when energy is reflected back to a listener at the source.

The options would be reflection, humidity and transmission.
1] The sound is reflected off the cloud layer, just like it might do off the side of a building.
2] The humid layer of cloud improves reflection of sound.
3] The sound is transmitted directly through more humid air.

 

[.Scott]

The flat base of the clouds marks the dew point for the local atmosphere. There is only gas below the cloud base, while above the cloud base there are fine droplets of liquid water, falling, and then evaporating again, circulating. That is a form of density inversion, in that saturated air below the cloud base is less dense than dry air, (H₂O has a lower molecular weight than N₂ or O₂), while the cloud above is cooler, and contains a greater mass of liquid water. I expect it is the step change in acoustic impedance at the cloud base, that acts as the acoustic mirror.

The two keys here are "flat base" and that "step change". Typical human hearing is most sensitive in range of 1000 to 5000Hz. That's a wavelength of 7 to 34cm. The ability of the bottom surface of a cloud to reflect sound relies on how well-defined that surface is.

From my experience (with sail planes, power planes, and sky-diving), the transition distance across the bottom surface of a cloud is most often meters, but can be as little as 10 or 20 cm.

What's interesting is that I recall the most distinctly formed cloud bottoms from building cumulous clouds - where the bottom is not just well-defined but also concave. What's makes that interesting is that opens up the possibility of a cloud reflecting sounds from on spot on the ground and refocusing it to another.

I suddenly look up from my reading because I can hear the highway traffic. I couldn't a moment ago.

I see that a fast, thick white cloud layer has moved overhead and blotted out the sun in less than five minutes. If it weren't 20C out here, I'd think it was a snow sprinkle coming in.

This description allows for some interpretation. The fact the you describe it as a potential source of "sprinkle" suggests to me that it is a cumulous. Since it may just crossed over (or even been generated by) a warm sun-lit highway could have led to a well-defined concave base.
So, even though most clouds would not reflect sound very well - or even at all - you may have found a situation where things were far from typical.

 

[DaveC426913]

The ability of the bottom surface of a cloud to reflect sound relies on how well-defined that surface is.

I suspect that the well-definedness of a boundary has more to do with the strength of the gradient than its physical granularity.

A thermocline of 10 degrees F will be a much better reflector than a thermocline of 3 degrees F. The variance of the surface, by cm or m, will affect the clarity of the signal - i.e. it will be nore ir less a rumble.

 

[.Scott]

I suspect that the well-definedness of a boundary has more to do with the strength of the gradient than its physical granularity.

I gave this problem to Google:

How much dB reflection will I get with 1KHz sound passing through a continuous temperature gradient of 5, 10, and 20 degrees centigrade across 5, 10, 20, 50, and 100 centimeters?

Gradient Thickness	($\Delta$T = 5$^\circ$C)	($\Delta$T = 10$^\circ$C)	($\Delta$T = 20$^\circ$C)
5 cm	-48.71 dB	-42.76 dB	-36.86 dB
10 cm	-52.97 dB	-46.96 dB	-40.98 dB
20 cm	-65.05 dB	-59.31 dB	-53.86 dB
50 cm	-76.87 dB	-69.93 dB	-62.40 dB
100 cm	-77.31 dB	-70.48 dB	-63.22 dB

1KHz is about 34cm. As you can see, there's a huge penalty for going over that wavelength. In contrast, the reflection seems to change by only the square of the acoustic impedance.

It also mentioned that if the transition was a logh function (so that the gradient built up and levelled off smoothly across the thickness) there would be about zero reflection.

 

[DaveC426913]

I gave this problem to Google:

Whose response is a chatbot. (Not that I have any reason to doubt the figures, except on general principle.)

 

[.Scott]

Whose response is a chatbot. (Not that I have any reason to doubt the figures, except on general principle.)

I did several sanity checks. The biggest problem I had with the Google ChatThing was that it would always find a few ways of interpreting my questions and a few ways of answering.
Based on how it responded in several cases, I take it that there is an integration that needs to be evaluated to generate those table values - an evaluation done stepwise analytically. So, now you have a reason to doubt.
It also offered me Python code - which I did not bother to review in detail.