# Does humidity affect air density?

• khaichou
In summary, the conversation discusses the relationship between humidity in the air and its effect on density. The participants mention the need for a partial pressures analysis and using a ratio of densities to determine the overall density. It is mentioned that humid air is less dense than dry air due to the displacement of heavier diatomic molecules with lighter water molecules.
khaichou
Hi, as the topic stated, i would like to find out, does the humidity in the air affect the density??
thanks

I wouldn't see how it couldn't. I would think you would need to do a partial pressures analysis to see the water content (prolly involving a psychometric chart). Then using a ratio of densities of water/air at that temperature to get the overall density.

But this is definitely just a guess.

The molar mass of air is almost 29. The molar mass of water is 18. The more 'less dense' gas you have intermixed into the air, the less dense the air. So, yes, humid air is less dense than dry air.

My gut tells me that humid air is more dense than dry air.
(edit)
My gut, apparently lies:

http://www.usatoday.com/weather/wdensity.htm
"Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air."

BobG seems to be right on the money.

BobG said:
The molar mass of air is almost 29. The molar mass of water is 18. The more 'less dense' gas you have intermixed into the air, the less dense the air. So, yes, humid air is less dense than dry air.
That's exactly it - diatomic molecules N2 and O2 have approximate molecular masses of 28 and 32 (and air is 78+% N and 21+% O with traces of other gases), which in humid air are displaced by H2O, which has a molecular mass of 18.

## 1. How does humidity affect density?

Humidity affects density by influencing the amount of water vapor in the air. As humidity increases, the amount of water vapor in the air also increases. This added water vapor takes up space, causing the air to become less dense.

## 2. Does humidity affect the density of all substances?

No, humidity only affects the density of gases such as air. Solids and liquids are not affected by humidity.

## 3. Why does humidity decrease density?

Humidity decreases density because the water vapor in the air is less dense than dry air. As more water vapor is added, it takes up more space, causing the overall density of the air to decrease.

## 4. How does humidity affect air pressure?

Humidity affects air pressure by changing the density of the air. As humidity increases, the air becomes less dense, which can lead to a decrease in air pressure. This can also affect weather patterns.

## 5. Can humidity affect the density of a gas other than air?

Yes, humidity can affect the density of other gases besides air. For example, the density of water vapor is affected by humidity, as the amount of water vapor in the air directly affects the density of the gas.

• Mechanical Engineering
Replies
8
Views
3K
• Mechanical Engineering
Replies
2
Views
2K
• Other Physics Topics
Replies
42
Views
5K
• Classical Physics
Replies
1
Views
978
• Mechanical Engineering
Replies
18
Views
3K
• Classical Physics
Replies
8
Views
2K
• Chemistry
Replies
5
Views
1K
• Other Physics Topics
Replies
2
Views
1K
• DIY Projects
Replies
5
Views
2K
• Mechanical Engineering
Replies
1
Views
2K