Read more: “Strange skies: Seven wonders of the atmosphere“
The ice and water droplets of a cloud need seeds to start growing. While these may be mineral grains or even bacteria (see “Strange skies: Invisible beings that live far above”), some researchers suggest cosmic rays might be involved. They claim that when more of these high-energy particles from space hit the atmosphere, more clouds form, with consequences for our climate. An experiment at CERN, near Geneva, Switzerland, reported last year that The result has encouraged proponents of the theory, though the dust particles seem too small to seed ice crystals.
Some cloud mysteries have been solved recently, though, including how aircraft manage to punch 50-kilometre-wide holes in cloud layers. A plane’s passage can trigger the formation of ice crystals, releasing latent heat and launching a doughnut of warm air that evaporates the cloud as it rolls outwards.
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Another oddity is the “open cell” cloud formation, a huge often seen near coastlines. This is now thought to be created when rain drives jets of air down onto the sea’s surface. Each jet then spreads out until it collides with other jets. Where this happens, the jets rise up again in a honeycomb pattern to spawn the characteristic cloud.
A more stubborn puzzle hovers in the mesosphere around 80 kilometres above ground, far above ordinary clouds. Noctilucent clouds are diaphanous layers of fine ice crystals, most easily seen at dawn or dusk. They are becoming more common, and a NASA satellite called AIM has found that they are also appearing nearer the equator.
These changes might be because climate change has led to a cooling of the mesosphere, encouraging ice formation here. Another possibility is that methane emissions from livestock and industry might react to form water in the mesosphere. Or are changes in solar radiation to blame? No one is sure.