Trillions of bacteria, fungi, viruses and single-celled organisms travel the globe high in the atmosphere. Scientists are discovering they play a vital role in the weather and even our health. Clouds are our lifelong companions. Sometimes they drift overhead as wispy filigrees. On other days, they darken the sky and dump rain on us. But for all our familiarity with these veils of water vapour, they have been keeping a secret from us. Clouds are actually floating islands of life, home to trillions of organisms from thousands of species. (BBC News)