Humid Air Weights Less Than Dry Air

[JESMKS]

This is an interesting fact that I probably never knew. I've always believed that humid air, being muggier, was heavier than dry air. Water, when vaporized into dry air, causes the combined mixture to become lighter. There is a law in the physics of gases that any volume of gas, under the same pressure, always contains the same number of gas molecules, no matter what the gas, or mixture of gases might be. As a water vapor molecule weights less than either a nitrogen or an O2 molecule, when water is vaporized into dry air, the water molecule displaces either a nitrogen or an O2 molecule and the new mixture per unit volume and pressure becomes lighter. I wonder if this is in anyway related to the creation of oceanic lows that form hurricanes?

[Larry Jacks]

I don't know about oceanic flows that form hurricanes, but pilots have long known about humid air being less dense than dry air. Humid air increases the density altitude, meaning wings produce less lift for a given airspeed and engines produce less power. This can increase takeoff distances and reduce rate of climb. For lightplanes, humidity is a relatively small factor - altitude above sea level and temperature are more significant. For large planes, humidity might be factored into performance calculations.

Here's a link to a density altitude calculator. It doesn't ask for humidity directly but humidity is related to the dew point. Plug in some figures for elevation, temperature, and altimeter setting, say a summer day at my home airfield (Meadowlake OOV, 6874 feet elevation, 85 degrees F, 29.92 altimeter setting) and play with the dew point to see how the calculated density altitude varies.

More info is available here.

[Peter Wilson]

I thought you were going to use this to explain why fly balls don't carry well in Dodger stadium after the sun goes down...

[swansont]

I thought you were going to use this to explain why fly balls don't carry well in Dodger stadium after the sun goes down...

I've heard many broadcasters claim that a ball isn't carrying well because of the heavy, wet air. Ugh.

I'd speculate that the less-dense air means the Magnus effect gives less lift, but I've don't recall a broadcaster ever uttering the phrase "Magnus effect." I think also that people have studied the effect of baseballs stored in dry vs humid conditions, and the balls stored dry carried further under otherwise identical conditions.

[01101001]

Wikipedia: Water vapor

Thus the density of water vapor is 0.804g/l, which is significantly less than that of dry air, 1.27g/l at STP.

[trinitree88]

Wikipedia: Water vapor

01101001. Which is why clouds form at altitude. Water vapor is second only to methane 18/16...ignoring helium and hydrogen which are trivially present in the air. The science books that emphasize the temperature effect should pay more attention to the fact that water vapor floats upward naturally.

[Romanus]

This is also why "dry lines" can trigger thunderstorms--because the lower, denser, dry air can literally act as kind of cold front, a lifting mechanism.

[01101001]

01101001. Which is why clouds form at altitude.

Heh. Spend more time in the San Francisco Bay Area. Clouds form on the ground, too. We call them fog.

(Which is not to argue against your altitude statement. Any reason that seemed directed at me? I was just putting numbers on the earlier fascinating facts.)

[trinitree88]

Heh. Spend more time in the San Francisco Bay Area. Clouds form on the ground, too. We call them fog.

(Which is not to argue against your altitude statement. Any reason that seemed directed at me? I was just putting numbers on the earlier fascinating facts.)

01101001. No particular direction intended....It's just that as a chemistry teacher I find almost every kid knows helium is lighter than air, and almost none of them are aware that water is.....they have that sun-driving-it-up with heat thing going on in their heads. So I try to post it in a teaching forum to slow that rumor down a bit. Pete.

[Romanus]

It took me a while to wrap my mind around this factoid as well. It seemed counterintuitive at the time: salt water and muddy water are denser than fresh water, so why not humid air? Such are the different properties of gas and liquid solutions.

[snarkophilus]

I think also that people have studied the effect of baseballs stored in dry vs humid conditions, and the balls stored dry carried further under otherwise identical conditions.

This is just because water from the vapour diffuses into the ball, making it heavier. Not so much a density consideration.

[penguinman007]

When I was 2 years old and saw my first clouds, I realized that either Humid air was lighter than dry air, or little fairy s carried the water up into the sky with buckets.

[George]

As a water vapor molecule weights less than either a nitrogen or an O2 molecule, when water is vaporized into dry air, the water molecule displaces either a nitrogen or an O2 molecule and the new mixture per unit volume and pressure becomes lighter.

How does a water molecule weigh less than an O2 molecule?

Perhaps we anthropomorphize too much; it is always much more difficult to run fast in humid air vs. dry.

[Lord Jubjub]

This is an interesting fact that I probably never knew. I've always believed that humid air, being muggier, was heavier than dry air. Water, when vaporized into dry air, causes the combined mixture to become lighter. There is a law in the physics of gases that any volume of gas, under the same pressure, always contains the same number of gas molecules, no matter what the gas, or mixture of gases might be. As a water vapor molecule weights less than either a nitrogen or an O2 molecule, when water is vaporized into dry air, the water molecule displaces either a nitrogen or an O2 molecule and the new mixture per unit volume and pressure becomes lighter. I wonder if this is in anyway related to the creation of oceanic lows that form hurricanes?

Yes.

It isn't merely related to the creation of hurricanes, it is the primary factor in the creation of hurricanes.

[Noclevername]

How does a water molecule weigh less than an O2 molecule?

H2O vs. O2 Two hydrogen atoms are lighter than one oxygen atom. So one oxygen atom plus two tiny little hydrogens for a water molecule, against two big, heavy oxygen atoms per gaseous oxygen molecule.

Put two bowling balls in a bag. Now put one bowling ball and a couple of tennis balls in the other bag. Which is heavier?

[George]

H2O vs. O2 Two hydrogen atoms are lighter than one oxygen atom. So one oxygen atom plus two tiny little hydrogens for a water molecule, against two big, heavy oxygen atoms per gaseous oxygen molecule.

Oops, I was thinking HO2 for water for some strange reason Perhaps if I drink this formula I will have more oxygen to reason with.

[Romanus]

Is it related to hurricanes?

Well, yes and no. Yes, in that there cannot be a hurricane without warm, humid air. No, in that humid air alone is insufficient; there must be broader synoptic support for one, such as a preexisting disturbance with favorable upper-level conditions. However, with certain systems--especially in the Eastern Hemisphere--one can get into chicken-or-the-egg problems...

[Noclevername]

--one can get into chicken-or-the-egg problems...

(Completely off topic, I've never understood that metaphor. Dinosaur eggs, fish eggs, arthropod eggs.... Chickens are johnny-come-latelies to the egg game.

Rant over, back on topic.)

[AndreH]

This is an interesting fact that I probably never knew. I've always believed that humid air, being muggier, was heavier than dry air. Water, when vaporized into dry air, causes the combined mixture to become lighter. There is a law in the physics of gases that any volume of gas, under the same pressure, always contains the same number of gas molecules, no matter what the gas, or mixture of gases might be. As a water vapor molecule weights less than either a nitrogen or an O2 molecule, when water is vaporized into dry air, the water molecule displaces either a nitrogen or an O2 molecule and the new mixture per unit volume and pressure becomes lighter. I wonder if this is in anyway related to the creation of oceanic lows that form hurricanes?

Never thought about this. I think if asked, I would have said hunid air is heavier. (If you put down the molecules and the weights it is clear it must be lighter). I guess it is a misconception, because one thinks of fog or condensed water vapor when talking about humid air.

[Noclevername]

Humid air feels heavier. It's like breathing Jello. So it actually being lighter is slightly counterintuitive, at first glance.

[Swordfish]

It does seem counter intuitive, that humid air is less dense than dry air, because humid air certainly feels thicker, but it is, in fact, less dense. That being said, there are a few more confusions or misconceptions that I noticed, mainly about clouds.

First off, and most basic, clouds form because a parcel of air is saturated, causing water vapor to condense onto small particles in the air. This can be done in two basic ways: cooling the air to its dewpoint, or adding water vapor into the air.

The reason that most of the clouds you see are above ground level isn't really the density difference between dry and moist air. For the most part, clouds form in an area that is experiencing upward motion (which causes air to cool) for any number of reasons. Now, one of those possible reasons for upward motion is the density difference between dry and moist air, but you also have convective, frontal, and orthographic lifting, as well as convergence. In fact, humid air rising over dry air is actually an example of frontal lifting, and this normally happens, as Romanus said, along a Dry Line.

What it comes down to is that humidity is only going to be an important factor concerning vertical motion if there is little variation in temperature.

Now, as far as hurricanes are concerned, well, that'd be opening a whole new bag of things here. Hurricanes actually require a preexisting disturbance in order to form. The disturbances that you find over oceans which will eventually become hurricanes are mostly formed via convergence. The rest form due to convection with a small percentage resulting from something that spun off of a front (or "sheer line" as they're called in the tropics). The density difference of humid and dry air doesn't really factor into making hurricanes because humidity over the ocean usually doesn't vary enough to create a strong enough lifting force.

It's late, so I hope I wrote that clearly enough to understand.