Effect of Humidity on Air Density & Coefficient of Friction

• ritwik06
In summary, when humidity increases, the density of air decreases due to the lower molecular mass of water compared to oxygen and nitrogen molecules. The amount of water vapor in the air can vary, but the overall number of molecules in a set volume remains constant, causing a decrease in density. The coefficient of friction is the ratio of frictional force to normal reaction, typically the weight of an object on a horizontal surface.
ritwik06
1. What is the effect on the density of air if humidity increases?
2. Define co-efficient of friction.

ritwik06 said:
1. What is the effect on the density of air if humidity increases?
2. Define co-efficient of friction.
What do you think?

Hootenanny said:
What do you think?
well, I think that:
1. with increase in humidity the density should increase.
2. co-efficient of friction is the ratio of the force of friction to normal reaction(the weight of the body in most cases)

Please will anybody tell me whether I am right?

ritwik06 said:
Please will anybody tell me whether I am right?
Air is primarily made up of diatomic molecules of oxygen (~21%) and nitrogen (~78%), with water ranging from 0 to 7%. Now 21% + 78% is 99%, so obviously these numbers apply to dry air. When the humidity is high, water molecules are replacing the diatomic oxygen and nitrogen molecules. How does the mass of a water molecule compare to the masses of oxygen and nitrogen molecules?

#2 looks good, except I wouldn't say weight in most cases. I would perhaps say weight for horizontal surfaces.

OlderDan said:
Air is primarily made up of diatomic molecules of oxygen (~21%) and nitrogen (~78%), with water ranging from 0 to 7%. Now 21% + 78% is 99%, so obviously these numbers apply to dry air. When the humidity is high, water molecules are replacing the diatomic oxygen and nitrogen molecules. How does the mass of a water molecule compare to the masses of oxygen and nitrogen molecules?

#2 looks good, except I wouldn't say weight in most cases. I would perhaps say weight for horizontal surfaces.

Well, of course the density water molecules is less than the density of nitrogen and oxygen molecules. But this doesn't seem to be a genuine reason. the amount of nitrogen and oxygen is fixed in air that is 99%. They arent replaced. Only the water vapour molecules take the intermolecular spaces. This should increase the density. Dont you think so?

I agree with you for the second one! Thanx

ritwik06 said:
They arent replaced. Only the water vapour molecules take the intermolecular spaces.
This is not so. Remember that a gas of a set volume always contains the same number of molecules (one mole of an ideal gas at STP occupies about 22 litres).

Hootenanny said:
This is not so. Remember that a gas of a set volume always contains the same number of molecules (one mole of an ideal gas at STP occupies about 22 litres).
Well what do you mean to say whether the density increases or decreases?

ritwik06 said:
Well what do you mean to say whether the density increases or decreases?
Since the molecular mass of a water is less than the molecular mass of air and the number of molecules in a set volume of a gas is constant, it follows that making the air more humid (substituting water molecules for air 'molecules') would decrease that mass contained in the set volume. Hence, the density would decrease.

Hootenanny said:
Since the molecular mass of a water is less than the molecular mass of air and the number of molecules in a set volume of a gas is constant, it follows that making the air more humid (substituting water molecules for air 'molecules') would decrease that mass contained in the set volume. Hence, the density would decrease.
I can't see any reason to agree to both of your posts at once. Please try to look from my point of view. You said that 1 mole of air has 99% of Nitrogen and Oxygen! 0.03% is always Carbon dioxide. Now the rest portion 0.97% consists of water vapours, dust, traces of other gases, pollen grains. etc. etc. The ratio of the last category of substances is not fixed. Now tell me if all the 0.97% of air has water vapours, ie. now the air is saturated with water. Will the density not increase?

ritwik06 said:
I can't see any reason to agree to both of your posts at once. Please try to look from my point of view. You said that 1 mole of air has 99% of Nitrogen and Oxygen! 0.03% is always Carbon dioxide. Now the rest portion 0.97% consists of water vapours, dust, traces of other gases, pollen grains. etc. etc. The ratio of the last category of substances is not fixed. Now tell me if all the 0.97% of air has water vapours, ie. now the air is saturated with water. Will the density not increase?
No, for the reasons I stated above, the density will decrease. The number of molecules in a volume of air remains constant. Therefore, if we introduce water vapor into air these molecules of water must displace air 'molecules'. Therefore, the density must decrease since the molecular mass of water is less than the molecular mass of air. I never mentioned the proportions you quote and neither me nor OlderDan stated that any of the proportions are fixed.

Last edited:
Well thanks

ritwik06 said:
Well thanks
Do you understand why this occurs?

1. How does humidity affect air density?

Humidity is the measure of water vapor in the air. As humidity increases, the amount of water vapor in the air also increases. This increase in water vapor causes the air molecules to spread out, resulting in a decrease in air density. This decrease in air density can affect the performance of objects that rely on air density, such as airplanes.

2. What is the relationship between humidity and air density?

Humidity and air density have an inverse relationship. As humidity increases, air density decreases. This is because the water molecules in the air take up space and displace the air molecules, leading to lower air density.

3. How does humidity affect the coefficient of friction?

The coefficient of friction is a measure of the resistance between two surfaces as they move against each other. Humidity can affect the coefficient of friction by altering the properties of the surfaces. In high humidity, the surfaces may become more slippery due to the presence of water on the surface. On the other hand, in low humidity, the surfaces may become drier and stickier, resulting in a higher coefficient of friction.

4. Can humidity affect the accuracy of air density measurements?

Yes, humidity can affect the accuracy of air density measurements. This is because the amount of water vapor in the air can vary, which directly affects air density. Therefore, when measuring air density, it is essential to also consider the humidity level to ensure accurate results.

5. How does humidity affect the performance of aircraft?

Humidity can significantly impact the performance of aircraft. As mentioned earlier, humidity affects air density, which, in turn, affects the lift and drag forces on an aircraft. In high humidity, the air density is lower, resulting in less lift and more drag, which can affect the aircraft's performance and stability. Pilots must consider humidity when planning flights to ensure safe and efficient operation of the aircraft.

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