Clouds and what holds them together
When the IPCC announced its latest climate report with a big aplomb at the beginning of February, apparently a surprise for many skeptics and non-scientists, I happened to be sitting in a lecture on cloud formation and climate. The environmental physicist Thomas Leisner, who has just been appointed to Heidelberg, reported on water droplets in the physics colloquium - and what they have to do with weather, clouds and the climate. (More on this in our next Spectrum special, Energy, out April 20, 2007.)
Clouds – isn't that something from your childhood, when you lie in a meadow in summer and watch the cotton balls hustle and bustle in the sky? And Leisner promptly introduced his lecture with "Children's Questions". Why are clouds sometimes seemingly fixed in the sky even though a strong foehn is blowing? Why do clouds have sharp edges? What actually holds clouds together?
The secret of the cloud lies, that's an old hat, in its droplets and the condensation nuclei that allow them to form in the first place. As they move into wetter zones, they grow, freeze, fall, melt, and leave the cloud as rain. But the details of the inner-cloud dynamics are still complex and in part elude theoretical access. Lord Rayleigh conducted experiments with electrically charged droplets of the kind that occur in clouds in 1882 – and Leisner is still discovering new aspects today: for example, highly charged droplets that emit two symmetrical, hairpin-thin water jets when they evaporate and thus dissolve.
What does climate have to do with it? The earth is also just, I must have forgotten that again after my studies, a huge spherical capacitor (with 105 Coulomb), which spans a field of 100 volts/meter between the earth and the atmosphere. A badly insulated capacitor! The leakage current through the galactic cosmic radiation, which constantly perforates the earth with ions, would discharge the atmosphere within a quarter of an hour if the capacitor were not constantly recharged by tropical thunderstorms at the equator. These constantly inject positive charges into the ionosphere, strongly modulated by solar magnetic fields and the solar wind. These, in turn, vary with the 11-year sunspot cycle. And these cycles also have their ups and downs, as most recently in the Spöhrer and Maunder minimums …
Oh yeah, why are clouds sometimes stuck in the sky? They are not structures, but areas, says Thomas Leisner happily, that nothing holds together. With higher humidity and falling temperature, the droplets condense just and not elsewhere. And her sharp edge? The growth of the droplets at the edge of the cloud is almost abrupt, almost like a phase transition. From still invisible germs, the droplets grow so jerkily in size that they suddenly become visible - like a kind of white cotton candy that already delights children's eyes.