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Physicists Solve Cloud Formation Puzzle

Clouds sometimes form more quickly than the laws of physics seem to allow. Now atmospheric physicists think they know how

One of the biggest challenges in atmospheric physics is to explain how clouds form.

Physicists know the basics, of course: that at a certain temperature and pressure, water vapour condenses into droplets which combine to form rain drops heavy enough to fall to Earth.

The devil is in the detail. This process of droplet aggregation sometimes happens so quickly that it defies explanation. Most people will have seen clouds form in a matter of minutes and rain appear almost from nowhere.

Researchers have even measured this process. They commonly see droplets with a diameter of 15 micrometres–too small for rain–grow to 50 micrometres or more in less than half an hour. That’s big enough to trigger a downpour.

The question is how this growth occurs. No standard models of droplet formation can explain it ( (at least, in the absence of ice formation). But today we have a solution thanks to the work of Vassilios Dallas and Christos Vassilicos at Imperial College, London.

At the heart of this problem is a quantity called the Stokes number, after the Irish mathematician, George Stokes, who invented it in the 19th century. The Stokes number is a dimensionless quantity related to inertia that describes how water droplets bang together in a flow of gas. It is hugely sensitive to the scale at which these collisions occur.

When the Stokes number is small, a droplet follows the flow of the gas as it moves around another droplet and so they rarely collide. When the number is large, the droplets have greater inertia and so cannot avoid banging into each other.

Here’s the problem. Before clouds form, the droplets are small and the Stokes number is tiny. Therefore the droplets rarely collide. After clouds form, the droplets are large and the Stokes number is huge, meaning that the particles easily combine, creating rain. But how does this transition occur?

There’s a chicken and egg problem here. The droplets cannot grow quickly unless the Stokes number is large but the Stokes number cannot be large unless the droplets are big.

The breakthrough that Dallas and Vassilicos have made is to show how turbulence changes this relationship. They say that turbulence occurs over a huge range of scales, including the micrometre scales at which droplets form. The effect of this turbulence is to create big variations in the Stokes number on the micrometer scale. This, they say, is what makes the tiny droplets to collide more often.

Essentially, Dallas and Vassilicos are saying that micrometer scale turbulence accelerates cloud formation and triggers rain showers.

That’s an interesting, although not entirely unexpected result that should lead to better weather forecasts. Perhaps more significantly, it could also have a big impact on climate models. Clouds have a big effect on the amount of light Earth reflects back into space. Being able to better calculate when they form is important.

And it plugs an embarrassing hole in our understanding of one of the most basic atmospheric phenomena.

Ref: arxiv.org/abs/1012.0578: Rain Initiation In Warm Clouds

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