'Fog bends sound waves' Myth?

[jerrytug]

Hello, I have read in books and YM articles,and heard anecdotally from sailors,that sound signals are altered by fog to become "misleading","deceptive" etc. Why? I know fog is confusing. But I don't believe it diverts sound waves,so I call BS on this old wives' tale. Unless there is scientific proof,cheers Jerry

 

[boguing]

Well, I'm pretty certain that it is correct advice. I've ended up in thick fog in the Channel on several occasions.

Down below is the best place to pick up propeller noise. In the cockpit it really does seem to come from behind/port/stbd/in front on rare occasions.

The only definite is the wash when you meet it.

The density of fog is significantly higher than drier air. I'd suggest that you Google that and reflect. Excuse the pun.

 

[elton]

The fog height indicates a layer of air at a different temperature to the one above. Differing temperatures means different air density. Two layers of air of different densities will cause refraction, and possibly reflection of sound waves, resulting in effects such as you describe. Something about the effect here

 

[sarabande]

the speed of sound is faster in dense material. If a sound wave in clear air hits a fog bank (where the air is denser) it will speed up. Depending on the angle of incidence, the angle of refraction will change. Likewise , coming out of a fog bank. And other changes will be made to direction and velocity by the thickness of the fog, and the height above sea level

It is not like light passing through plate glass.

 

[elton]

It is not like light passing through plate glass.

Did you mean "is not unlike light passing through plate glass"?

 

[sarabande]

thinking about that, I mean both

a) in the same way that light is distorted by refraction when passing through glass (or any other translucent medium)

and

b) in a different way from light passing through glass, which has parallel sides, as a fog back has complex multicurved interfaces with non-foggy areas.



I hope that make it clear; sort of ....

 

[Cruiser2B]

If you throw a pebble into a still pond the waves ripple out from the plop in an even circular pattern - imagine that is how sound radiates in clear air. When sound hits a water drop, it radiates out of the drop in all directions - so fog being a lot of water droplets is like throwing a handful of pebbles into the pond; the ripples radiate in all directions, causing constructive and destructive interference. This makes it very hard to distinguish the direction from which a sound signal is heard.