If we sit in a boat on a lake we can often hear people talking on the shore clearly in contrast to sitting in an empty field and hearing the people talk over the same distance.

I heard that this effect comes due to temperature differences of the air over land and over water and that the sound waves bend somehow resulting in amplifying the sound. But I do not really understand how or why we hear the sound better.

Does anyone know the correct/detailed physical explanation for this?

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    $\begingroup$ Additionally the water surface will dissipate much less sound (as it is even and compact) in comparison to a field which has a soft, uneven surface that muffles sound. $\endgroup$ – Sebastian Riese Sep 13 '15 at 15:24

Air nearest the water is cooler than air farther above the water. As sound travels slower in cool air, if sound waves from warmer air enter the cooler layer they are refracted downward toward the ear of someone in a boat.

If the water is calm, its flat surface allows sound waves to travel unobstructed and to reflect from the surface. Instead of dissipating in tall grasses and other obstructions on land, sound waves retain their coherence for longer distances over calm water. Sound waves also may reflect from calm water's surface, bouncing up to the ear.

Addtionally, if you are sitting quietly in a still boat on calm water, there is little or no ambient noise to interfere with sound waves coming to you from a distance. So sounds from shore may seem to be more clear, which you may confuse with loudness.

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