Making Rain with Laser Beams

Scientists have found a way of controlling the formation of water droplets using a high-powered laser. This technique could potentially be used to cause or prevent rainfall.

Human society has a strong dependence on the weather. Without rain, crops fail and wells run dry. Too much rain causes problems with flooding, which destroys homes in low-lying areas, spreads disease, and forces people to evacuate or be swept away.

For millennia, humans have dreamed of being able to control the weather. "Rain dances" are an integral part of Native American tribal culture and were also commonly used in Ancient Egypt. Even staunch secularists living in the modern urbanized world have been known to say a quick prayer for rain to hold off during an important event, such as a wedding, sporting event, or camping trip.

Could it be that science has finally delivered the answer to the pleas that the rain gods have so often ignored? A group of researchers working at the University of Geneva have been carrying out field tests of their "rain maker" on the banks of the River Rhone, and have reported encouraging results.

The researchers have developed a technique that fires a laser beam into the clouds. Along the path of the laser beam, particles of nitric acid form. These particles bind with water vapour in the air, causing it to condense into liquid water. They also keep the water droplets stable, preventing them from re-evaporating.

The technology needs to be developed and improved before it can be used to generate a downpour. So far, the researchers have only managed to create tiny droplets of water, less than 0.01 millimeters in diameter. For the particles to be heavy enough fall as rain, they would have to be at least one hundred times bigger.

One way around this problem could be to take advantage of natural features, such as mountains. As wet air is forced to rise over the mountain range, it cools, meaning that the small seeded droplets will grow bigger until they are large enough to fall.

Another fascinating application of this technology is that it could actually be used to prevent rainfall. This works by blasting the clouds with a specially tuned laser in order to scatter the water into tiny droplets, so that they all remain too small to fall as rain.

Previous attempts to force clouds to produce rain have involved shooting tiny particles of silver iodide, or dry ice, into the clouds. The idea is that the particles act as nucleation sites onto which water begins to condense in the same way that steam in your bathroom condenses onto the cold mirror. However, throwing vast amounts of silver iodide into the sky is not a very precise way of controlling the weather. It is difficult to tell whether attempts to use silver iodide to induce rainfall have been successful, or whether it would have rained anyway.

There may be environmental problems associated with leaving large amounts of silver iodide in the atmosphere, especially because it is likely to drift away from the site where it was dispersed. The laser-beam technique is potentially an improved system, because the laser can be precisely focussed and turned on and off as required. Lasers also have a very large range, of several kilometers or more, which means that they can be beamed directly into the regions of the atmosphere where rain formation takes place.