March 11, 2009 In years gone by it was thought we could alter the weather and induce rain through dances and other rituals, while traveling showmen made their attempts throughout during the drought of the American West and Midwest in the 1930’s. Rainmaking has advanced since those days with the advent of cloud seeding, but while cloud seeding has been shown to change the structure and size of clouds, it’s still debatable whether the practice actually has any effect on rainfall. After all, even if precipitation does occur after cloud seeding there’s no way of knowing whether it would have rained anyway. This uncertainty hasn’t stopped widespread use of cloud seeding in countries around the world including the US, Russia, Australia and China, which boasts the largest cloud seeding system in the world. Now a breakthrough by an international team of scientists could help in the development of new materials which could be used to enhance the process.

Presently the most common materials for cloud seeding are dry ice and silver iodide, which has a hexagonal crystalline structure similar to that of ice. Introducing silver iodide into a cloud that contains supercooled water is supposed to induce freezing nucleation, whereas the introduction of dry ice cools the air so much that ice crystal can form spontaneously from the vapor phase. The structure of regular ice is well known at the macroscale with ice structures usually built out of simple hexagonal arrangements of water molecules - this hexagonal building block motif is easily observed in the structures of snowflakes. Ice structures however, are much more mysterious and less well understood at the nanoscale - particularly when ice forms at an interface with matter as is the case in the higher atmosphere with particles of dust. Silver iodide is used because of its hexagonal crystalline structure, but as reported in Nature Materials (March 2009) scientists led by researchers at the London Centre for Nanotechnology (LCN) at UCL have discovered a novel one dimensional ice chain structure built from pentagons showing that ice can build an extended one dimensional chain structure entirely from pentagons and not hexagons.

"This discovery leads to fundamental new understanding about the nature of hydrogen bonding at interfaces (there is no a priori rule that hexagons should form) and suggests that when people are searching for new ice nucleating agents which can be used to seed clouds and cause rain, they do not necessarily need to focus on materials that have hexagonal surfaces - other types of surfaces may be good too," says Dr Michaelides. "It is important to understand the structure of ice on the nanoscale, and in particular up against solid surfaces because this is how ice crystals form," explains the paper's first author Dr Javier Carrasco. "We need to understand the structure of ice crystals in the upper atmosphere because they play an important role in the formation of clouds and precipitation." And cloud seeding is not the only area to be affected by the discovery. The formation of nanoscale ice crystals (i.e. nucleation) plays a key role in fields as diverse as atmospheric chemistry and biology.

In order to address the challenge of characterizing ice on the nanoscale, the team from the LCN joined up with a team of experimentalists from the University of Liverpool (lead by Professor Andrew Hodgson) to examine ice formation on a very well defined, atomically flat copper surface. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular-scale on a well-defined, plane interface. A common feature of structural models for such films of ice is that they are built from hexagonal arrangements of molecules. The Liverpool group performed scanning tunneling microscopy experiments and the LCN and Berlin teams carried out ab initio calculations to predict what the microscopy results would be. Only through the combination of these two state-of-the-art approaches were they able to definitively show that the ice structures that form are made from pentagons.

The team’s discovery that ice chain structures can be made from pentagons offers the possibility of replacing silver iodide as a seeding agent, which has also raised some (small) concerns from some quarters regarding the uptake of silver in the environment. Now as to whether cloud seeding is effective in the first place I’ll leave for you to decide, but until strong evidence proving its effectiveness or otherwise is found, it seems certain that cloud seeding will continue.

Darren Quick

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