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Name: Matt Thomas
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 2000-2001


Question:
HI. I am writing to ask you whether sound waves travel:-

a) Faster at night because of less dense air due to temperature changes? and/or

b) Further at night for the same reason as in (a)

I think that the waves in the two states (day and night) have the same characteristics.


Replies:
The speed of sound is higher in denser media, that is, sound moves faster in solids than in liquids, and it moves faster in liquids than in gases. This is why one hears the rumbling sound of an incoming train first from the metal rail tracks and then in the air.

If one considers a dry climate where air in devoid of water moisture, then due to heating, expansion, and reduction in air in density during daytime, one would expect faster sound velocity at night because.

The issue gets complicated for humid air. Humid air is lighter than dry air because dry air is mostly nitrogen while humid air contains lower density water vapor. During the early part of day, the sun heats up the air at higher altitudes first. Heated dry air descends and replaces the moist air, and in that sense increases air density at the ground level. This is the reason why land and vegetation aroma is most pronounced early in the day and diminished as sun pushed the dry air to the ground and the lighter moist air up. Therefore, one has to look the density and humidity numbers to see what effect is more if water moisture is involved. However, as I mentioned, if the air is dry, it is denser at night due to it lower temperature, leading to higher sound velocity.

AK

Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Argonne, IL 60439



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