How and why moist air reduces aerodynamic lift

[James McGinn]

How and why moist air reduces aerodynamic lift

James McGinn:
There is a general misconception that the reduction in lift that aircraft experience in moist air is a result of moist air being lower in density than dry air. Supposedly this confirms the notion that the moisture in moist air is monomolecular H2O, the molecular equivalent of steam (gaseous H2O). Actually, this is impossible. As confirmed by hundreds of years of experimental evidence, steam (gaseous H2O) can only exist above the boiling temperature/pressure of H2O. Obviously earth's atmosphere never gets hot enough (and the pressure never gets low enough) to support the existence of steam which has a boiling point temperature greater than any temperature found in the atmosphere. So, contrary to what people generally assume, all of the moisture in earth's atmosphere consists of microdroplets/clusters of H2O. And as long as the diameter of these microdroplets stay smaller than the length of a photon they are invisible--just as invisible as steam.

Nevertheless, aircraft do experience reduced lift in moist air. And (due to the fact that moist air contains droplets of H2O that are larger than the N2 and O2 molecules that they replace) moist air is denser (heavier) than dry air. And so, given that it is generally true that denser air provides more lift than less dense air there appears to be a contradiction here. If all of this is true, why does heavier, denser, moist air not increase lift? Why does it reduce lift?
Before we can answer that question there are two other truths about aerodynamics that we need to clarify:
1) The energy that causes lift in an airplane doesn't come from the engine of the airplane. The energy that comes from the engine of the airplane is used to overcome drag. The energy that causes lift comes from air pressure--it comes from the atmosphere itself; And,
2) There is a finite amount of energy per volume of air. And, therefore, there is a finite amount of energy that can be efficiently extracted from a volume of air. Flying faster allows you to use more air and, thereby, extract more energy from it. But flying faster doesn't increase the amount of energy per volume that an airplane gets out of the air.
The reason lift is reduced in moist air is because liquid H2O has a very high heat capacity, which is just a fancy way of saying that it absorbs energy. Specifically, it is harder to extract the finite amount of energy from a volume of air because so much of it is soaked up by the liquid water that is in the air. So, stating that you know moist air is lighter because it reduces lift is, well, nonsense. It is an ignorance based assertion.

Michael Moroney
Standard aerodynamics is that humid air is less dense than dry air. Complete with temperature/humidity tables and the decrease in density, which any pilot knows.

James McGinn
Yes, you are right in regard to it being a standard BELIEF that humid air is less dense than dry air, as I indicated. And yes you are correct that this standard BELIEF is mirrored in standard temperature/humidity tables taught to all pilots. This is very unfortunate because it makes flying less safe than if pilots had the proper understanding. Who knows how many lives might have been saved if not for this superstition. (Kobe Bryant, JFK jr.) Bad science kills.
This falsehood has been promoted by the meteorological lobby. The truth is that moist air is heavier, not lighter, than dry air. And claiming that reduced drag associated with moist air is evidence that substantiates the notion that moist air is lighter is an invalid argument. Because it fails to account for other factors that may actually be causing the reduction in lift. The fact is, as indicated, water's ability to absorb energy is greater than that of any other liquid. This is the reason why moist air reduces lift. And it is one of the reasons pilots should avoid clouds (another good reason is because clouds tend to hide high energy vortices and wind shear). And if it was true that denser air increases lift that wouldn't be the case, pilots would be advised to go skipping across the atmosphere on clouds, taking advantage of the increased lift provided by their increased density.

Jim Pennino:
Hundreds of thousands of pilots can attest to the facts of humid versus dry air. Kobe Bryant died because they were in a hurry and didn't just wait until the fog and clouds lifted. JFK Jr died because he was a rich kid that bought an airplane that required more experience than he had.

James McGinn:
Hundreds of thousands of pilots? LOL. You need only find one that can explain the experimental basis of these alleged "facts."
The best way to know the weight/volume of moist air is to measure it. Nobody ever has. Meteorologists absolutely refuse to measure it. Being ensconced in a paradigm dominated by group-think stupidity, meteorologists measure the relative humidity and think that that also indicates the weight/volume. And that simply ain't so. (Meteorologists look at things very differently than real scientists. They have their narrative that they hide from the public. They are more concerned about looking scientific than they are being factual.) If you measure, you know. If you guess based on indirect evidence you are just guessing. Reality is too complex. People are gullible. People are easily fooled. People miss details and the devil is in the details. People tend to be easily convinced by anecdotes and experts. And they don't realize that very often the experts are using the same flawed methods to come to the same flawed conclusion. When meteorologists are talking about "density," you never know when they are talking about 1) the ratio of water to air or 2) the weight of air per volume. These are two very different concepts that they employ interchangeably. Sometimes they use them interchangeably in the same paragraph or the same sentence. And if you try to get them to clarify they refuse.
This is very unfortunate because it makes flying less safe than if pilots had the proper understanding. Who knows how many lives might have been saved if not for this superstition.

Paul Alsing:
Superstition? Here is what the FAA has to say about it…
James McGinn:
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/13_phak_ch11.pdf
"Water vapor is lighter than air; consequently, moist air is lighter than dry air. Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance. It is lightest or least dense when, in a given set of conditions, it contains the maximum amount of water vapor... Expect a decrease in overall performance in high humidity conditions."
James McGinn:
LOL, gee golly, the FAA is a government agency. And we all know the government is never wrong.
Paul Alsing:
It is a well-known *fact* that if the air is really humid that it takes a lot more runway for a plane to get into the air. Explain that.

James McGinn:
So, you need to have it explained to you that reduced lift requires more runway?

Paul Alsing:
I make no claims that vary from mainstream science. That is what *you* do. My claims have tons of evidence, just look in any textbook for that evidence. You are the guy making the extraordinary claims so it is up to you to support those claims, as dictated by the scientific method. Are you familiar with that concept?

James McGinn:
Don’t you think it is an extraordinary claim to assert that water becomes gaseous at temperatures far below the boiling temperature/pressure that is indicated in the H2O phase diagram?

Paul Alsing:
Let's pretend that I don't know the answer and I want you to explain it to me, Jim. Why is the lift of an airplane reduced when the air is humid?

James McGinn:
The energy that keeps an airplane from falling out of the sky does not come from the thrust of the airplane's engines, it comes from air pressure. The thrust of the airplane's engines provides the energy that allows the airplane to overcome drag and achieve a velocity that enables laminar flow over the top of the plane's air-foil wings.
And so, if we were to model the pathway of the energy that causes the lift on an aircraft it originates in the atmosphere. It does not originate with the airplane's engine. (This is a common misconception.) It is extracted from the atmosphere by the air foil. Nevertheless, the atmosphere does not experience a net loss in energy as a result of the aircraft passing through it but actually experiences a net gain in that the amount of energy associated with thrust/drag more than compensates for the loss of energy associated with lift.

Understand that air molecules are moving 700 to 1100 miles per hour in earth's atmosphere. The net effect of this is called air pressure. The bump on the leading edge of the air foil causes the air molecules going over the top of the wing to ricochet upwards. They eventually begin to fall back toward the top of the wing but they mostly miss or just kind of skip across the wing, making little impact, falling behind the wing. This skipping is called laminar flow. Since the impact of these molecules is very slight it results in low pressure. On the bottom of the wing there is no bump and there is a slight angle of attack. This causes air molecules to impact the bottom of the wing with full force, 700 to 1100 miles per hour. The imparted force of impact causes lift.
And there is another factor that seems to indicate even more of a nonlinear relationship with respect to these questions surrounding moist air and lift. As I indicated above, moist air reduces lift because moisture in moist air absorbs energy. But even this isn't quite so straight forward. It might also be the case that extremely small H2O nanodroplets effectuate a net increase in lift. And this has to do with the fact that H2O has high surface tension and very small nanodroplets are mostly surface. In other words, when H2O nanodroplets are very small–which occurs under warmer temperatures–the H2O therein acts a lot like relatively heavy gas molecules, effectually making the atmosphere more dense and therefore more capably of providing lift than it would if it had zero moisture.

Paul Alsing:
I make no claims that vary from mainstream science.

James McGinn:
Right. You believe what everybody believes. Scientific methods were created for the sole purpose of breaking free from the stultifying effects of this bias. But scientific methods only work if you apply them.

Paul Alsing:
Why don't you use the scientific method and produce your evidence of your claims, and break meteorology free from these "stultifying effects" of everyone believing what everyone else does? My claims have tons of evidence, just look in any textbook for that evidence. You are the guy making the extraordinary claims so it is up to you to support those claims:

James McGinn:
LOL, so you expect me to comply with rules you refuse to follow?

James McGinn / CEO of Solving Tornadoes

[James McGinn]