“I have, as everyone has, often made my own clouds. I’ve made clouds hovering at the top of my bathroom ceiling taking a hot shower. Sometimes the cloud would grow so large that it would roll out the open doorway and into the next room. I also made short-lived clouds by exhaling deep breaths into frosty air. As kids, we used to pretend we were smoking cigarettes, taking long draws through our fingers and letting it go through pursed lips as we had seen adults do on TV. The mechanics of these little clouds seemed simple enough and I did not think about them. So I fooled myself. I thought I had a basic understanding about what I was seeing, but as I sat down to write about clouds I realized I didn’t have a clue as to what was going on. I started reading. I pulled a stack of papers about cloud formation from the internet and I couldn’t understand anything they were saying. Pulled some more and read those. In fact, the more I poked around, the more confused I got about this cloud business.
Clouds are filled with bewildering complexity and I was stuck—there wasn’t a word I could write about their nature until the confusion lifted. Those papers fell into two basic categories. The first must have been written by people who didn’t actually understand the nature they were describing. These articles gave the reader some basic ideas but didn’t offer a deeper understanding and, in fact, were often mixed up on the details, so I couldn’t trust what they said. I picked one of these simplified articles at random from the floor of my office and read the first two sentences. Answers.com claimed, “Cloud condensation nuclei or CCNs are small particles (typically 0.00002 mm, or 1/100th the size of a cloud droplet) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapor to a liquid.” In the first sentence we get the idea of some connection between particles and condensation of water, but all the other information given was mixed up in its essential detail. They used the word “droplets” when they should have used “water vapor,” for example. It is more important to know that aerosols are picking up water vapor molecules and not droplets. This sentence also gives the wrong impression that water attaches best to some kind of solid particle, which is entirely wrong. The particles that dissolve when they get wet are the most important. So the first sentence is pretty bad.
But if the first sentence sent you down the wrong road, the second sentence was a bold-faced lie. It was a blasphemy of nature—it couldn’t possibly be true. Individual water molecules are attracted to each other and it is idiotic to suggest that water vapor molecules are incapable of sticking together in the atmosphere. They wanted me to know the important idea that it is rare for droplets to form without some type of aerosol being present, but they carried it too far. So the simple articles were all mixed up in this way, because the authors were in the same boat I was in—they didn’t understand it either.
The other group of articles assumed you knew what they were talking about. To give an idea, one paper I read was called “Cloud Droplet Activation and Surface Tension of Mixtures of Slightly Soluble Organics and Inorganic Salt.” I wasn’t sure I understood what “cloud activation” was, and I had no idea about any kind of connection between clouds and “surface tension.” This type of article tended to focus on just one or two narrow aspects of the overall nature of cloud formation. They were full of intimidating jargon and calculus and, since they were detailing in one small piece and ignoring the rest of the wider picture, these articles were just as confusing. If both styles shared anything at all, it was in how well they succeeded in keeping me from knowing what was going on, or so it seemed.
In situations like these, I just had to go back to the basics and keep digging. Slowly, between the articles and flipping through some chemistry and physics textbooks, things started to clear up. It was in Feynman’s three-volume lecture series that things began to make more sense—in a chapter on evaporation, of all things. Feynman gave me the idea that clouds could be understood in terms of evaporation and condensation. Now I could see “cloud activation” and “surface tension” as code words for evaporation and condensation!
I decided, first, to mess around with the clouds I could make on my own and see if everything checked out before sticking my head into a real atmospheric cloud. What about the pretend-to-smoke-a-cigarette clouds? What happens, when I take a deep breath of cold air, between the water vapor in the inhaled air and the liquid water coating my lungs? Will more water be jumping out of my lungs into inhaled air or the other way around? Inside, water molecules jiggle and jump from the lungs into inhaled air, and water vapor molecules bombard the lungs. So the answer to my question is going to lie in the respective rates that water leaves or arrives at the surface of the lung water. What can we know about the two rates?
Of course, we have a temperature difference. The water in the inhaled air is from outside and far cooler. We know from the behavior of gases that the mean velocity of a gas molecule is directly proportional only to temperature and the relationship looks like this.
mVave2 = 3kT
Where Vave is the average velocity, m is the mass of the gas, and k is the gas constant. The velocity of a gas is therefore closely approximated by temperature alone. Voila`! I can be sure, then, that the rate of the bombarding molecules has to be lower than the rate of the leaving molecules, just by knowing the temperature difference! So more molecules are leaving than arriving. Equilibrium is reached as the inhaled air warms and picks up water molecules from the lungs and the rate of bombarding molecules starts to equal the rate of leaving molecules.
Here was an important mechanic about clouds. When I exhale, warm moist air is injected into cold outside air and a short-lived silvery cloud plume forms. The exhaled air is immediately cooled and water vapor, at a sudden lower temperature, has become supersaturated—it likes to condense at the new temperature. The vapor pressure in the exhaled air has collapsed and now, when water vapor molecules accidentally collide and stick together, there are more arriving molecules than leaving molecules, so these blobs of liquid water grow and grow and begin to reflect light and our own personal cloud materializes. Inside the cloud, the amount of available water vapor decreases and the amount of liquid in droplets increases until a new equilibrium is reached. Our little cloud doesn’t last long, of course. Water vapor is free to move in all directions; as it randomly disperses, things reverse: More water molecules leave the droplets than arrive and they completely evaporate.
Suppose that instead of outside air, I inhaled a warm cloud, like you can make in a hot shower or something? Further, let’s assume our inhaled cloud is the same temperature as the water in our lungs. Okay, fine. What happens to the inhaled cloud droplets under these circumstances? Will they grow, evaporate, or stay the same? I could make a good argument that they would stay the same size by reasoning that the droplets would be bombarded with arriving water molecules at the same rate as the water on the lung surface and, since everything is at the same temperature, no changes. But when I go inside to measure for droplets, I find they have all disappeared. Since this was unexpected, this tells me I have missed something important!
This missing idea is one that is essential to understanding cloud formation. My mistake was not made with arriving water molecules; it was in failing to recognize that water would evaporate from droplets and lung water at different rates, even when they are the same temperature. Interestingly, evaporation rates increase for the same liquid as curvature of its surface increases. In other words, the greater curvature of the water droplets meant they would have a higher evaporation rate. Curvature of a water surface lessens the amount of effort it takes to kick a water molecule loose from water. Each water molecule at the surface of a droplet is a little more exposed, as opposed, to a flatter surface, and it takes, on the average, a little less energy to pry one of these molecules out of the droplet. So what I learned is that the evaporation rate of the droplets is higher than the lung water, so they lose more water and disappear!
One cold day I made a puzzling observation. I happened to be outside near an idling car when I noticed I could make a very thick cloud when I exhaled. I moved away from the car and made another cloud and it was markedly thinner. Interesting. What was it about car exhaust that caused clouds to thicken? Breathing outside air mixed with exhaust fumes must be changing something about evaporation and/or condensation rates. In the inhaled air, dry automobile aerosol is bombarded by arriving water molecules. Some of these aerosols have electrical forces that repel water, but others like to dissolve into arriving water. In these tiny droplets with dissolved aerosol I find something curious: The water vapor pressure of these droplets has dropped.
If I go back and repeat my inhaled cloud experiment, but this time with some car exhaust piped into my bathroom, again careful to have all the water at the same temperature, how will it go? When I looked at the cloud droplets, what did I see? Aha, this time the droplets haven’t evaporated and, in fact, might even be growing in size! Very curious, indeed. What is missing from our understanding this time around? It’s a well-known observation in nature that non-evaporative molecules—molecules reluctant to form into gas—lower the vapor pressure of the liquid into which they are dissolved. Dissolved automobile exhaust aerosol has lowered the evaporation rate of the water in the droplets enough to allow them to grow instead of completely evaporate.
The dissolved aerosol molecules have come to occupy a certain percentage of the surface area of the droplet. Electrically, they are geared for particle and not gas formation, and do not evaporate, so, in this way, they present an additional hurdle for any water molecules trying to escape. Dissolved aerosol also changes the balance of attractive forces within the droplet. Dissolving molecules into water with a stronger attraction to water than water has to itself strengthens intermolecular forces within the droplet, and this lowers evaporation. In the same way, dissolving molecules with a weaker attraction to water, than water has to itself, increases evaporation. It is not an accident that water mixed with densely dissolved sugars in jam or honey remain fluid for so long on your kitchen table; all that sugar causes the water inside to evaporate very slowly! Car aerosol was acting to lower droplet evaporation rates and now, when I exhaled, my breath held more water and larger droplets and a thicker cloud formed. So, with just a few observations from ordinary life, I gained a good deal of insight into real cloud formation.
Naturally I have to make some adjustments from these everyday examples to be more faithful to actual cloud formation high in the atmosphere, but they are relatively minor.
In real clouds, parcels of air cool, mainly, by rising to higher altitudes. The rate at which they rise keeps the water vapor within a supersaturated state ahead of the natural dispersal of its vapor to drier areas of the atmosphere. Higher rates of rise increase the degree of supersaturation in water vapor and favor more intense cloud formation.
Also, a parcel of air, once it moves away from its source of evaporated water, will not likely be recharged with more water vapor. Shifts in condensation or evaporation rates mean that favoring one comes at the expense of the other; so water droplet growth comes at the expense of water vapor within the cloud, and vice versa. As water vapor declines, droplet growth and formation will be halted unless the cloud parcel continues to rise and cool.
Probably the biggest difference between our examples and real clouds is that real clouds are much drier. Real clouds have a relatively low degree of supersaturation. In fact, our everyday examples could have been supersaturated by as much as 10 or 20 percent, whereas real clouds are closer to 1 or 2 percent. This fact about the nature of clouds lies at the heart of why aerosols are such an influential factor in cloud formation high in the atmosphere. In typical situations, the marginal supersaturation of cloud water vapor, and naturally high evaporation rate inherent to embryonic droplets, make it difficult for water to coalesce on its own accord in the atmosphere. This extra reluctance of water vapor to condense into droplets can be overcome, in one way, by increasing the supersaturation of water vapor.
It turns out our examples are extreme compared to real clouds; they were supercharged with far more moisture from hot water, holding my breath, or undergoing quick drops in temperature. Such degrees of supersaturation in real clouds would be exceptional. My everyday clouds readily formed without the aid of dissolved aerosol; but in drier real clouds, aerosols become critically influential. Water molecules always have a certain probability to come together in the open atmosphere and form clumps of water 10, 20, 50, or whatever, molecules in size. But the chances of their continued growth to critical mass are not favored because the spherical shape they take on immediately bumps up the evaporation rate, dooming the vast majority of these spontaneous clusters to dissipation. It is the water-soluble aerosol in the atmosphere that lowers evaporation rates, tipping the balance in favor of droplet formation in clouds. It is easy to see now how water soluble particulate emissions, released as a by-product by a vast hydrocarbon society, would be able to create additional cloud cover in the Earth system—clouds that reflect light, dim light at the Earth’s surface, and favor surface cooling. The most water-soluble aerosol that we know of in the atmosphere is sulfate aerosol.”
Excerpt from “Climate Trek” by Mike Tidwell