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Cirrus are certainly beautiful. Floating high in the sky, well above other clouds, cirrus sometimes have a translucent silky sheen to them. Often they are drawn out into long bands or delicate wispy filaments, as if painted with a fine brush. But cirrus don’t produce rain, snow or storms, which is probably why scientists took little notice of them until recently. When they did, it was usually because of the odd optical phenomena that cirrus can produce, including haloes and bright spots called sun dogs.

Now that has all changed. A large NASA-led study of tropical cirrus called Crystal-Face, employing ships, planes and satellites, has just ended in Florida. And two new cloud satellites are now being built.

Why all the excitement? Unlike other clouds, cirrus are made entirely of ice. They are a loose collection of billions of ice crystals floating in the cold air more than 6000 metres above the ground. Those crystals reflect and refract light, creating the haloes and sun dogs. More significantly they give cirrus a key role in regulating the planet’s temperature.

High in the sky, the clouds of ice crystals can reflect sunlight back into space, cooling the Earth. But they can also intercept heat radiation from below, and blanket the Earth, keeping it warmer. Depending on the thickness of the cloud and its optical properties, one or other process might dominate.

If cirrus were rare, it might not matter. But observations from space show there is far more of it that scientists had thought. In some tropical regions it shows up in 75 per cent of the views taken from space. And the amount of cirrus can quickly change.

How do we know? In part due to a huge unplanned experiment. Condensation trails from high-flying airliners trigger the spread of cirrus, creating lanes of cloud along busy routes (Â鶹´«Ã½, 19 October, p 6). Strikingly, when jet traffic halted after 11 September, daily temperature variation across the US grew by up to 3 °C over the next two days as the cirrus blanket shrank, allowing more sunlight in by day and more heat out by night.

Global warming might have a more dramatic long-term effect on cirrus. Warmer temperatures might loft more moisture into the high atmosphere, particularly in tropical thunderstorms, creating more cirrus in a set of currently unpredictable feedbacks. But it could all go the other way. As temperature rises, more rain might fall, the amount of cirrus reduce and heat vent back into space.

Cirrus has the power to exert a disproportionate influence on the changing energy balance of the planet. But in which direction? Project Crystal-Face will begin to provide answers, but for the global picture we will have to wait until the new satellites – Picasso and CloudSat – get aloft in 2004. Only then will we know if those wispy clouds might stabilise or accelerate climate change.