This part of the story might be titled “On the cause of atmospheric heterogeneous ice nucleation.” It is biological story. 80% the bacteria known to have ice nucleation activities inhabit the phyllosphere.
Atmospheric ice nuclei from decomposing vegetation. Nature 236:163-165. By R. C. Schnell and G. Vali.
How many biogenic ice nuclei make it from litter to the atmosphere? Schnell (1974), Bul. of Am. Metero. Soc. 55(6)670 reports fluxes from 10 to 1000 ice nuclei per square cm per day. When snow covers the ground, very few ice nuclei are loaded into the atmosphere. As the snow melts more and more ice nuclei are detected in the air. Ice nuclei in the air above a grass surface on a calm day was 1 nucleus per liter of air and would cause freezing at temperatures as warm as -6.5 C. Following mechanical disturbance of the grass surface 10 times as many ice nuclei were recorded in the air over the surface. Following a three day rain with the grass surface wet there were three orders of magnitude fewer ice nuclei in the air above the canopy than on dry days. Schnell also indicated that the nuclei may be volatile or liquid in nature. Just what are ice nuclei made of? Chemical tests of ice nuclei derived from leaf material indicate that they are insoluble in water, stable in all common organic solvents and that nucleating activity is lost upon heating above 60 C. They are about 0.1 micrometers in size (so they would scatter in the blue wave lengths). Terpines have been shown to be ice nuclei. Schnell reports that ice nuclei abundance in rain water matches biogenic availability in litter from the same geographic region. He thinks that this correlation indicates that a large proportion of the earth's atmospheric ice nuclei are biogenic. Schnell has also found ocean derived ice nuclei that are active at temperatures as high as -5 C. He notes that seawater rich in phytoplankton produce a large amount of ice nuclei (10s to 100s of millions of ice nuclei per gram of phytoplankton). Oceanic areas of low photoplankton productivity have low ice nuclei production. Do the clouds with ice crystals over the oceans pop up where ocean productivity is high? Sort of interesting! The penultimate papers in the Schnell-Vali team up were: Biogenic Ice Nuclei: Part I. Terrestrial and Marine Sources. J. Atm. Sci. 33:1554-1564. Biogenic Ice Nuclei: Part II. Bacterial Sources. J. Atm. Sci. 33:1565-1570. Those in the know who are interested in ECOSYSTEM CONTROLS ON THE WEATHER AND CLIMATE need these papers in their collection. To quote Schnell and Vali: "The changes in atmospheric ice nuclei concentrations can be related to large scale meteorological effects, thereby suggesting a cause and effect relationship controlled by meteorological characteristics / life history / source region of a particular air mass."
Photo: Jahansell, Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0)