真正的以人为本的科技：瑞士科学家用激光造云

http://www.newscientist.com/data/images/ns/cms/dn18848/dn18848-1_300.jpg

A laser pulse (red) ionises air and triggers the condensation of water droplets to create a cloud, which is illuminated by a second, green laser (Image: Jean-Pierre Wolf/University of Geneva)

A laser has been used to generate small clouds on demand in lab, and real-world experiments suggest this could be a way to call down rain when it's needed.

People have experimented with cloud seeding for decades in the hope of boosting rainfall, usually by sprinkling silver iodide crystals into clouds high in the atmosphere.

These crystals encourage large water droplets to form around them, and the droplets then fall as rain – in theory, at least. "The efficiency of this technique is controversial," says Jérôme Kasparian at the University of Geneva, Switzerland, one member of a research team that think lasers may be a better way to trigger rain on demand.

Kasparian and colleagues have just reported the first successful use of this technique to summon clouds from air both in the lab and in the skies over Berlin, Germany.

激光的用途很多，大至为核融合点火，小至拿来显示时间，都有用得上它的地方。而现在激光的用途又多了一个：造云！瑞士日内瓦大学的科学家发现，将 5TW（5兆瓩）等级的红外线激光以极短的脉冲（100 飞秒）打向水气饱和，温度摄氏零下 24 度的水气团时，能将周围水分子的电子扯掉，形成氢氧基。这些氢氧基和空气中的硫、二氧化氮等作用，就能产生出做为云种的化合物。

和所有目前的造云技术一样，激光造云只是提供降雨的条件，并且增加降雨的机率，并不能保证降雨。但相较于碘化银之类的化学物造雨法，用激光不仅环保得多，也方便得多（不用飞飞机上去），更重要的，激光的频率、强度等都有可以调整的空间，除了未来还有有可能继续改进外，或许还可以为不同的天气条件最大化降雨机率呢！