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.