If sound travels faster in water due to the increased density and more tightly packed molecules, I would have thought the reason sound travels faster in humid air is due to the added moisture, but I’ve read that humidity leads to a decrease in density and that this is the reason sound travels faster in humid air. The two facts seem at odds with each other. Can anyone explain this?
In: Physics
So you general thinking is correct, more molecules means more bumping of molecules means faster sound travels. Buuuuut:
But, humid air means a lot of water molecules in the air, which displaces normal N2 and O2 molecules.
Basically, at the same pressure, for every H2O molecule you add, a N2 or O2 molecule get displaced and pushed away. And guess what. one H2O molecule has less mass than one O2 or N2 molecule, because hydrogen (the H) is VERY light. Just one proton and one electron.
One hydrogen is 1 AMU (atomic mass unit). 1 oxygen is 16 AMU. So one O2 molecule is 16+16=32 AMU. While one water molecule (H2O) is 1+1+16=18 AMU. 18 amu is less than 32 amu, so one molecule of H2O replacing one molecule of O2 means that goes form 32 to 18 AMU, which is less mass, which is less dense.
But, lighter things (less dense things) can move faster. It’s way easier to throw a tennis ball really fast than it is to throw a bowling ball really fast. So those sound vibrations will travel through less dense air (where the air molecules can move much faster) quicker than more dense air (where the air molecules move slower).
Water is heavy – or dense – because it is a liquid. Atoms and molecules in liquids are packed close together.
Water in humid air is a gas. Molecules in gasses are much more spread out – and so it is less dense.
A different question is when the air cools and the humidity condenses into fog or cloud – tiny droplets of water suspended in the air. Sound gets badly messed up in fog!
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