By comparing rates of seed formation during the different phases of the experiments, the researchers have been able to put a figure on cosmic rays' contribution to the process.

ECONOMIST: Climate science (II)

The results, reported in this week's Nature, suggest naturally occurring rays enhance seed-formation rates by a factor of ten.

ECONOMIST: Climate science (II): Clouds in a jar | The

And thus the debate stood, until a Danish scientist named Henrik Svensmark suggested something outrageous that cosmic rays might seed cloud formation.

FORBES: Did CLOUD Just Rain on the Global Warming Parade?

One way to increase the probability of rain is to seed the formation not of water droplets, but of ice crystals, an important step in precipitation.

ECONOMIST: Aerobiology

To their surprise, they discovered that the seed-formation rates for sulphuric acid and ammonia are between a tenth and a thousandth of those needed to account for the cloud seeding actually seen in the atmosphere.

ECONOMIST: Climate science (II)

But one of--a major constituent of comets, we believe, are stardust, interstellar grains, particles that formed 'round other stars and were the original seed of the star formation of the solar system.

NPR: Stardust Set to Deliver Comet Samples