Bad Clouds FAQ

Preamble

This FAQ (Frequently Asked Questions) is written by Alistair B. Fraser. It is in response to questions posed over the years by readers of the Bad Meteorology pages. If you have arived on this page without having read those pages or the other Bad Science pages, then what follows, will probably make little sense.

Although the questions presented here are often ones asked by a specific person, each is chosen to characterize a group of similar questions which have been asked about the topic.

Issues discussed below (arising out of the Bad Clouds page)

Air is a sponge
But, air does have a holding capacity for water vapor
A correct prediction implies a correct reasoning
The air-holding water explanation is just a simplification
But, what about the relative humidity?
What about boiling? It clearly depends upon the air pressure.

Questions arising out of Bad Clouds:

Air is a sponge

Question:
When I was taught about the formation of clouds, I was given a physical reason for why the cold air cannot hold as much water vapor. I was taught that with decreasing temperature, there is not as much room between the air molecules and so the water vapor gets squeezed out (like water in a sponge). This makes sense to me. How can you say that the air is irrelevant when we actually know how the air squeezes the water vapor out?

Answer:
This sponge analogy is an attempt to provide a physical explanation for something which does not actually occur. The distance between the molecules in the air is very large. There is far more than adequate room for lots more water vapor or anything else for that matter. As Dalton pointed out in the nineteenth century, the gases behave independently of one another: one does not squeeze out the other.

Yet, there is a simple way to convince yourself of this without even making recourse to books: just watch the formation of cumulus clouds. These are the puffy white clouds which form on a summer’s day over the Sun-warmed ground. The clouds form at the top of rising columns of air. As the air rises to a region of lower pressure, its density drops (the molecules get farther apart) and yet that is where the cloud forms. If you were to believe the silly explanation of the water being squeezed out because the molecules were getting closer together, then you should also expect that the clouds would have formed, not in the rising air, but in the sinking air, because it is there that the air density is increasing.

So, casual observation of the formation of many clouds shows that the issue is not one of air density (the closeness of the air molecules) but one of temperature, and not the temperature of the air, but of the water, itself.

But, air does have a holding capacity for water vapor

Question:
You criticise the phrase, "The reason clouds form when air cools is because cold air cannot hold as much water vapor as warm air." Yet, contrary to what you say, this phrase states the issue correctly. The fact that, as you argue, the temperature dependence of condensaton results from an intrinsic property of water, and not of air, does not prevent the phrase to be logically true.

Answer:
Yikes! Well here we really part company. The phrase is categorical nonsense. Heck, even Dalton knew better when he pointed out that the gases are independent. The idea that air has a holding capacity is an eighteenth century speculation that likened water vapor in air to salt in water. Air is not holding the water vapor in any sense. Further, if all the air is removed, the relationship between the equilibrium vapor pressure and temperature remains the same. Empirically, the air is irrelevant. So, how can you justify telling folks that something which has no bearing on the issue (because in its absence the system behaves the same way) is the causative agent.

A correct prediction implies a correct reasoning

Question:
The fact that one gets the correct physical behavior (a cloud forms when air is cooled) from an application of the idea of air having a temperature-dependent holding capacity for water vapor is sufficient proof that the explanation is, in fact, also correct.

Answer:
Your suggestion is specious on two counts: the fact that you sometimes will get the correct answer from applying the reasoning is not a vindication of the logic; the reasoning often produces the wrong answer (that is, a result not in accord with experiment).

A simple illustration of the first problem was given on the Bad Clouds page itself: just because you get the correct answer by trying to reduce the fraction 16/64 by canceling the 6s does not vindicate the technique or assure one that it will work under other circumstances. In short, it is so easy to get the right answer for the wrong reason, that one should always be skeptical of any assertion that a correct result implies that the process by which it was attained was also correct.

Examples of the second problem abound. The observed behavior of all manner of natural phenomena refute it: the transformation of a water cloud into an ice cloud, the formation of haze, the metamorphism of a snow pack,the formation of steam fog, etc. Indeed, so many are the examples that we sometimes pose a problem for our students: describe the behavior of the weather in a world in which the physical processes actually behaved as that described by the air-holding-water myth. Most meteorology students have no difficulty in covering many sheets of paper with descriptions of how different would be the weather in this fantasy world.

The air-holding water explanation is just a simplification

Question:
But, describing the process in terms of the air is merely a simplification to make it easier for the student to understand. How can you object to simplifications?

Answer:
I don’t object to simplifications. However, you must make a distinction between something which has been made simple (stripped to its essence), and something which has been made simplistic (stripped of its essence). The explanation which attributes the formation of clouds to the inability of the air to hold as much water vapor at lower temperatures is not a simplification, it is categorically wrong! It bears no more correspondence to the behavior of nature than if I were to explain the process as one in which the water molecules were held in the arms of angels who, upon being chilled, begin to shiver and drop them. Just because the former explanation has the patina of science (rather than religion) does not make it correct.

I am not in favor of telling lies to students, nor will I accept the justification that lies are acceptable if they seem easier to grasp than the truth. One is not obliged to provide an explanation (you could merely state what happens rather than why it happens), but if you do provide an explanation, you are obliged to get it right.

The amazing thing is that this is an issue which was settled in the nineteenth century and is handled correctly in virtually every thermodynamics textbook in the world and yet nearly two centuries later, a disproved eighteenth century speculation continues to be presented as fact in school text books and by teachers.

But what about the relative humidity?

Question:
Based on your explanation, how is relative humidity explained? Everything I have read describes it as being the amount of water vapor in the air compared to the amount of water vapor the air at that temperature could hold.

Answer:
The relative humidity is a useful measure of some aspects of water vapor. The flaw is not in the concept, but in the way some incompetent authors present it to their readers as a percentage of the air’s holding capacity. The relative humidity is the vapor pressure divided by the equilibrium vapor pressure (times 100%). The equilibrium vapor pressure occurs when there is an equal (thus the word equilibrium) flow of water molecules arriving and leaving the condensed phase (the liquid or ice). Thus there is no net condensation or evaporation. If the vapor pressure is greater than the equilibrium value, there is a net condensation. And that is not because the air cannot hold the water, but merely because there is a greater flow into the condensed phase than out of it.

What about boiling? It clearly depends upon the air pressure.

Question:
You list four things that affect the rate at which water molecules leave the surface. I understand Dalton's law of partial pressures (I think) but I have also seen water boil as the container around it is evacuated. How is this reconciled?

Answer:
When I was in grade school, I was taught that water freezes if its temperature is below 32 F and evaporates if it is above 212 F. Between those temperatures, it remains a liquid. This was all patent nonsense.

The evaporation rate of water is a continuously varying function of the temperature of the water. It does not have an abrupt transition at the boiling point, say, going from zero to some very high value. This might seem like an odd statement given that we all apparently have seen the evaporation increase abruptly at the boiling point, but let me explain.

By evaporation rate, I mean (what is normally meant), the number of molecules leaving a unit area of the water surface in a unit time. And this evaporation rate changes slowly and smoothly through the boiling point (as it does for every other temperature value). However, when boiling begins, the surface area of the liquid increases discontinuously. This leads to a vastly greater loss of molecules from the liquid, but not because the loss per unit area has increased. Boiling just means bubbling: vapor bubbles can now survive inside the liquid. This, of course, happens when the vapor pressure (from the evaporating molecules) becomes greater than the pressure which would collapse the bubble. At lower temperatures and vapor pressures, any incipient bubbles which form in the liquid are immediately crushed by the surrounding pressure (which will be slightly higher than the pressure on the liquid itself --- often caused by the atmosphere). At higher temperatures and vapor pressures, a bubble survives, grows, rises to the top, breaks, and releases its vapor.

The supposed upper limit of the temperature of the liquid sometimes attributed to the boiling point is merely an approximate consequence of the increased loss of vapor and so latent heat cooling. When the cooling from this increased evaporation matches the heating from, say, the stove, then the temperature does not rise any further. But, there is no actual temperature bound; it merely takes greater heating to keep up with the latent heat loss.

If one places water in a container and lowers the pressure, then the temperature at which boiling takes place is also lowered. However, the evaporation rate (that is molecules leaving per unit time per unit area) is just as it would have been outside the chamber. But now bubbles survive.

But, none of this has anything in particular to do with the formation of clouds other than the fact that the relationship between temperature and vapor pressure is of interest in both processes.