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Naveen's two cents

Actually, there must be some interesting physics involved to produce the wide range of shapes of clouds. For instance, why do some appear to have boundaries, instead of simply diffusing? A less-than-reliable (i.e. not peer-reviewed) source raises some interesting questions about what causes clouds (see [1]). Water droplets are much denser than water, so even updrafts of wind or atmospheric drag forces would not be enough to suspend them. Instead, the author claims that the air between water droplets is warmed by the heat of condensation when the water vapor forms a suspension in the atmosphere. However, even this additional information does not explain the morphology of clouds.

Another non-journal article about the physics of clouds [2] explains how some aspects of clouds are fractal-like due to atmospheric turbulence, but daily temperature cycles, spatial variations in the earth's surface (e.g. land-sea boundary), and Benard cells [3] can introduce additional structures.

On the electrical structure of clouds

In general, clouds have a net positive charge towards the top, and a net negative charge towards its center. There is also a net positive charge towards the bottom of the could present in some clouds. This charge net charge distribution generates an electric field inside the cloud. Several groups have studied these electric fields both from the charge and the from the air. Of course, when lightening occurs, these fields change dramatically.

Question for thought: If we have polar water molecules in an electric field, wouldn't the interaction between the water droplets and the electric field within the cloud be the dominant interaction within the cloud rather than the dipole dipole interactions between the water droplets?

Good reads

I found several good and thorough books about clouds..

I was browsing through and found this book in Cabot:Atmospheric chemistry and physics : from air pollution to climate change / John H. Seinfeld, Spyros N. Pandis. It has quite a few chapters about clouds, but I can't seem to find a copy of it online.

Another good and really thorough discussions about cloud is found here: Microphysics of Clouds and Precipitation By Hans R. Pruppacher, James D. Klett[4]

I wanted to summarize some of these chapters, but realize that it is probably better to refer the books to you guys! Unfortunately, some of the more interesting chapters (ie. Chapter 15). Mckay has the book, but it is checked out.

Also: Cloud dynamics by L T Matveev [5] Available online and in Mckay.

These hopefully should answer all the questions that anyone have about clouds!!

Biology enters!

Last week Brent Christner, assistant professor of biological sciences at LSU, published in PNAS findings that elucidate the role and source of bacteria in nucleating ice crystals in clouds. The following is a newsy article on the subject, and the paper recently published online at PNAS can be found here.

Here's some more information on it!

I imagine that makes them quite soft and, dare I say, a complex fluid.

The sky is not an ethereal, sterile realm. It's teeming with bacteria, and scientists say that the microbes play a powerful role in producing rain and snow.

While the idea that bacteria could prompt precipitation was previously known, a paper published this week in Science shows that they're more important than anyone expected.

Researchers led by Louisiana State University microbiologist Brent Christner analyzed snow samples from around the world, categorizing the content of their "nucleators" -- tiny particles that help water vapor coalesce and freeze.

All snow and most rain begins as ice. Though water is widely thought to have a freezing point of zero degrees Celsius, it's not so simple in the clouds, where pristine vapors only bind to form ice crystals at exceedingly cold temperatures. Nucleators let crystallization happen in the less extreme conditions that prevail in much of Earth's troposphere.

Christner found bacteria, technically known as "biological ice nucleators," in an atmospheric context. High levels of bacteria were present in nearly every sample.

"Atmospheric scientists haven't previously recognized that these particles are so widely distributed," he said.

The findings raise the question of how climate change and human activities will affect bacterial balances in the sky. More immediately, they're a starting point for research on bacterial contributions to cloud formation and precipitation.

In its latest report, the International Panel on Climate Change said that the impact of feedback loops involving clouds on global weather patterns are the "largest source of uncertainty" in current predictions of climate change.

Christner's findings won't overturn the IPCC's fundamental conclusions -- a high probability of dramatically rising global temperatures -- but they should spur research that will help scientists predict the changes in greater detail, said Princeton University climate scientist Leo Donner, who was not involved in the study.

Donner agreed that climate scientists had not appreciated the ubiquity of precipitation-causing bacteria in the atmosphere.

"One of the real uncertainties in the climate system is how cloud particles are nucleated," he said. "Climate models need information on nucleators. This is especially relevant for understanding how clouds change as atmospheric composition changes."

The fact that bacteria could cause snow and rain was discovered almost by accident in the 1970s by study co-author David Sands, a Montana State University plant pathologist, during his research on Pseudomonas syringae, a microbe that causes ice to form on leaves.

Unable to discover the source of repeatedly infected fields, Sands exasperatedly took to the skies. He did the scientific equivalent of dragging a cup through the clouds -- and lo and behold, there was P. syringae.

P. syringae is not the only biological ice nucleator, but it is the most common, and all varieties share a protein structure that provides a scaffold for free-floating water molecules. Once bound to the bacteria and to each other, the water vapors are able to freeze, and eventually fall back to Earth.

In a pure state, water vapors freeze at temperatures below -35 degrees Celsius. Nucleators allow this to happen in warmer conditions, and Christner's study found that bacteria are the most common warm-temperature nucleators of all.

Researchers never realized bacteria could be so widespread in the clouds, said Christner, because the technologies used to measure fine dust -- traditionally seen as the most important nucleator -- ignore microbe-sized particles.

"It's not that these atmospheric scientists are idiots -- they're not," he said. "But biological nucleators were not previously recognized as being that abundant or important. They're going to have to revise that."

And more:


[edit] Formation

The amount of water that can exist as vapor in a given volume is proportional to the temperature. When the amount of water vapor is in equilibrium above a flat surface of water the level of vapor pressure is called saturation and the relative humidity is 100%. At this equilibrium there are equal numbers of molecules evaporating from the water as there are condensing back into the water. If the relative humidity becomes greater than 100%, it is called supersaturated. Supersaturation occurs in the absence of condensation nuclei, for example the flat surface of water.

Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air. The difference between these values is the basis for the formation of clouds. When saturated air cools, it can no longer contain the same amount of water vapor. If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached. Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in supersaturation.

[edit] Supersaturation

Supersaturation of more than 1-2% relative to water is rarely seen in the atmosphere.[1] For high levels of supersaturation there must be no condensation nuclei for the water vapor to condense on.

Supersaturation can also occur relative to ice. This is much more common in the atmosphere than supersaturation relative to water. Water droplets are able to maintain supersaturation relative to ice (remain as ice water droplets and not freeze) because of the high surface tension of each microdroplet, which prevents them from expanding to form larger ice crystals. Without ice nuclei supercooled liquid water droplets can exist down to about -40 C/F, at which point they will spontaneously freeze.

[edit] Collision-Coalescence

One theory explaining how the behavior of individual droplets leads to the formation of clouds is the collision-coalescence process. Droplets suspended in the air will interact with each other, either by colliding and bouncing off each other or by coalescing -- combining -- to form a larger droplet. Eventually, the droplets become large enough that they fall to the earth as precipitation. The collision-coalescence process does not make up a significant part of cloud formation for the same reason that water droplets have a relatively high surface tension, which prevents them from coalescing on a large scale before they eventually fall to the earth.

[edit] Bergeron Process

The primary mechanism for the formation of ice clouds was discovered by Tor Bergeron. The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can hold, depends on what the vapor is interacting with. Specifically, the saturation vapor pressure of air with respect to ice is lower than the saturation vapor pressure with respect to water. Air interacting with a water droplet may be saturated (at 100% RH) when interacting with a water droplet, but the same air would be supersaturated when interacting with an ice particle.[2] The air will attempt to return to equilibrium, so the extra water vapor will condense into ice on the surface of the particle. These ice particles end up as the nuclei of larger ice crystals. This process only happens at temperatures around -40 °C. The surface tension of the water allows the droplet to stay liquid well below its normal freezing point. When this happens, it is now supercooled liquid water. The Bergeron process relies on supercooled liquid water interacting with ice nuclei to form larger particles. If there are few ice nuclei compared to the amount of SLW, droplets will be unable to form. A process whereby scientists seed a cloud with artificial ice nuclei to encourage precipitation is known as cloud seeding. This can help cause precipitation in clouds that otherwise may not rain. Adding excess artificial ice nuclei -- overseeding a cloud -- shifts the balance so that there are many nuclei compared to the amount of supercooled liquid water. An overseeded cloud will form many particles, but each will be very small. This can be done as a preventative measure for areas that are at risk for hail storms.

[edit] Dynamic Phase Hypothesis

The second critical point in the formation of clouds is their dependence on updrafts. As particles group together to form water droplets, they will quickly be pulled down to earth by the force of gravity. The droplets would quickly dissipate and the cloud will never form. However, if warm air interacts with cold air, an updraft can form. Warm air is less dense than colder air, so the warm air rises. The air travelling upward buffers the falling droplets, and can keep them in the air much longer than they would otherwise stay. In addition, the air cools as it rises, so any moisture in the updraft will then condense into liquid form, adding to the amount of water available for precipitation. Violent updrafts can reach speeds of up to 180 mph (300 km/h).[3] A frozen ice nucleus can pick up 1/2" in size traveling through one of these updrafts and can cycle through several updrafts before finally becoming so heavy that it falls to the ground. Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of super-cooled water. Hailstones have been found with diameters of up to 7" (17.8 cm).[4]