SciTech

How Things Work: Cloud Seeding

Here in Pittsburgh, we?re very familiar with clouds. During the winter months, Pittsburgh averages over 70 percent daily cloud cover. We aren?t strangers of rain, either: The Three Rivers have crested over 30 feet more than 25 times since charting began in the 1760s. With this abundance of rain, it?s hard to imagine not having enough. But what if you were a farmer, your livelihood dependent on the rainfall of your region? What if you were plagued by droughts? Once upon a time, you might have been tempted to perform a rain dance. Today, we need no rain dance: We can make our own rain. Are the X-Men real? Does Storm rent out her rain-making services to the highest bidder? No, we?ve got technology. Here?s how things work:
This technology, known as cloud seeding, has been around since 1946. It was then that Vincent Schaefer of General Electric Labs performed the first experiments in man-made weather atop Mt. Washington in New Hampshire. Schaefer observed cloud formation atop the mountain and tried many times to reproduce the effect within a freezer box in his laboratory. Previous theorists had proposed that precipitation occurred when super-cooled water high in the clouds accumulated around tiny particles of dust. While this is true, Schaefer was unable to reproduce the effect in his laboratory, even after the addition of all sorts of chemicals. One day, however, he tried adding a sample of dry ice. This resulted in the instant formation of a miniature snow storm in the freezer box! It was discovered that the very low temperature of the dry ice caused ambient moisture in the air to instantly crystallize. What potential did this hold?
The basic theory of cloud seeding revolves around increasing the number of ice nuclei in the upper levels of clouds. When a sufficient number of these nuclei are present in a cloud, they become ice crystals, which then, grow larger and fall. The ice crystals melt into rain as they reach warmer air at lower altitudes. The majority of clouds don?t contain enough of these ice nuclei to form precipitation. By artificially increasing their numbers ? ?seeding? them ? rain can sometimes be induced.
Today, several methods exist to artificially produce precipitation from existing clouds. As with Schaefer?s early experiments, dry ice can be seeded into clouds. This is often done from airplanes, so the upper levels of the clouds are cooled to near minus 40 degrees Fahrenheit ? the temperature at which ice nuclei form spontaneously.
Cloud seeding can also be completed with hygroscopic chemicals ? a fancy term for substances that attract water. The most common hygroscopic materials used are salts: potassium chloride and sodium chloride, common table salt. Dispersed into a cloud from air or ground, these salts will attract water vapor to their surfaces, forming the base for ice crystal formation. Finally, a chemical known as silver iodide mimics the molecular structure of ice, tricking water molecules in the cloud to grow on it as if it were an ice nucleus itself. Silver iodide has been shown to be the most effective cloud seeding chemical.
While the potential for cloud seeding may seem boundless, there are many limitations. Unfortunately, only specific types of clouds are seedable. Cloud seeding is often limited to puffy cumulus clouds or layered stratus clouds. On top of this, very specific conditions must exist within these clouds for seeding to actually produce precipitation: Specific temperature, low wind speeds, tall enough clouds, and the presence of enough water vapor are all necessary for seeding to be successful. This complex set of ever-changing variables has long frustrated scientists trying to make cloud seeding economically viable. These factors are so variable, in fact, that the effectiveness of cloud seeding is hard to determine because experiments can not be repeated!
Nevertheless, large?scale experiments in cloud seeding have been attempted all around the world. The U.S. ran an interesting project from 1962 until 1982 known as Project Stormfury. For the project, planes would seed the outer walls of hurricanes in the hopes of weakening the interior of the storm. Although the seeding of four hurricanes produced observable results, they were later discounted when knowledge of hurricane phenomena increased.
Harnessing a system as complex as our atmosphere will always be difficult. The limitations of cloud seeding are reflective of this complexity. But with further development of cloud seeding, we may one day be able to control rain for the benefit of people around the world.