Re: Do temperature and humidity affect the transmission of sound and static?

Josephine,

Thanks for the questions.  Let's go through them and see what we can do.

First, we should be clear on what "travels with ease" means for sound.  I 
am taking it to primarily mean that the speed of sound is higher for the 
specified conditions.  An alternate perspective which could require 
different explanations is from the view point of how far sound carries.

You are right, as the air gets colder, the molecules tend to get closer 
together.  In science parlance, we say that the density increases.  A bag 
weighs more when filled with cold air instead of warm air.

Generally, the solid form of something is colder than its liquid form, and 
so the molecules are closer together in the solid compared to the liquid 
form.  Water is not a good example of this however, it is one of the few 
substances where the molecules are farther apart in the solid form.

Lower temperature in a gas results in the speed of sound decreasing.  This 
might not make sense from the point of view that the molecules are close 
together at low temperature and so they should have less distance to travel 
to bump into each other.  Don't forget that each bump between the cold 
molecules "carries" the sound wave a shorter distance than when they are 
spread out.  Also, there is a greater mass of air in a given volume when 
the air is cold.  This means that the sound wave has to vibrate more mass 
to travel through that volume. Also, at low temperature the molecules are 
moving more slowly. This means the sound wave travels more slowly in the 
cold dense air.

Another weird thing I should mention is that humidity in the air makes the 
air lighter, less dense.  This is because a molecule of water only masses 
about 18 atomic mass units, while air is about 29 AMU, which is an average 
between the molecules of nitrogen (28) and oxygen (32).  Airplane pilots 
know that when the humidity is high, it takes longer for them to lift off a 
runway because the air is less dense.  This is also why low air pressure, ( 
a falling barometer), means a storm is on the way, because of all the moist 
air of a storm being lighter than plain old dry air.

The presence of water vapor (humidity) in the air lowers the average mass 
of the molecules that are in the air.  This has the effect of increasing 
the speed of sound in the air.

So I conclude that cold air should make sound travel slowly, while humid 
air should decrease the density of air, and also make sound travel faster. 
I am not sure what happens when air is cold and humid at the same time, the 
effects would seem to cancel each other out.

In any case, I agree with you that sound seems to travel better, if not 
faster, when the air is cold.  I used to live in Salcha, Alaska, and when 
the temperature was minus 30 F, my sister and I could hear the school bus 
from almost 2 miles ( 3 kilometers ) away.  I think what was happening was 
that there was an inversion, with very cold air near the ground with much 
warmer air in a layer about 50 meters up.  When the sound reached the warm 
air layer its direction of travel would be bent back down toward the 
ground.  So sounds made on the ground were trapped near the ground and 
could be heard for long distances. However, someone in a balloon floating 
over the inversion layer would have had a hard time hearing sound coming 
from the ground underneath, I would guess.
Another reason that sound might seem to travel better in really cold air is 
that fewer people and animals are stirring about, so there is less 
background din to interfere with hearing what you are listening for. :-)

Regards and Best Wishes,

Everett  (fellow northerner)