The formation of ice in clouds is a prerequisite for the formation of snow and most rainfall, with dust and soot particles able to initiate its formation by acting as ice nuclei. Certain species of bacteria, however, are able to catalyse ice formation at temperatures as warm as -2 degrees C and thus at a temperatures far higher than most non-biological , organic or inorganic substances are able to. Consequently, atmospheric bacteria are likely to play a vital role in initiating rain and snowfall. It has also been suggested that bacteria present in clouds may have evolved to use rainfall as a means of dispersing themselves, in that rain or snow forms their return journey to earth, so that these organisms form part of a constant feedback system between terrestrial ecosystems and clouds.
Here I caught snowflakes and then carefully nurtured their ice-nucleating bacterial cores in the lab. The purfied cores were then introduced into supercooled water to induce ice nucleation and so to trap the bacteria within ice once more.