[kate (OP)]

Can you eplain using proper physics with waves and sound why you can clearly hear evening conversations of two people across the lake.

[gsmollin]

Good evening, Kate. Your first post throws down the gauntlet! You must be a young girl, (I'm getting a pretty picture here.) standing by the lakeside on a cool evening. Crickets chirp. Frogs garrump. Then silence. Your keen ears pick up a distant conversation. How? Well first of all, I envy your keen ears. Take care of them.

The lake is wide open, and it must be very still, like a mill pond. The water holds the day's warmth, while air above the lake is getting colder. This forms a density inversion, with higher air density above, in the colder air, and lower density in the warmer air at the lake's surface. This is a duct for sound waves, and they travel great distances to your ears.

A similar thing can happen with microwaves, where it is commonly known as "ducting".

[chris]

So why should the temperature of the air make a difference to the quality or amplitude of the conveyed sound ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[gsmollin]

The higher density air acts like a wall of a waveguide, focusing the sound waves towards the water. The lake water also reflects sound waves. Of course there are no obstructions on the lake. There can be no wind. This happens on a summer evening. The sound propagates with much less attenuation than normally.

Refraction and reflection of sound by temperature-induced density changes is common. I could look for a couple of links.