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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 A less-than-reliable (i.e. not peer-reviewed) source raises some interesting questions about what causes clouds (see [1]).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."