Artificial Rain a Cure for Droughts?

In response to Tony's question about the Chinese creating rain:

BEIJING (AP) -- Chinese weather specialists used chemicals to engineer Beijing's heaviest rainfall of the year, helping to relieve drought and rinse dust from China's capital, the official Xinhua News Agency reported Friday.

Technicians with the Beijing Weather Modification Office fired seven rocket shells containing 163 cigarette-size sticks of silver iodide over the city's skies on Thursday, Xinhua said.

The reaction that occurred brought as much as four-tenths of an inch of rain, the heaviest rainfall this year, helping to "alleviate drought, add soil moisture and remove dust from the air for better air quality," Xinhua said.

Pretty cool, eh? Here's how it works.

The Chinese weather specialists shot the rocket shells high into the air. They then exploded, releasing tiny fragments of silver iodide. You might know something about silver iodide if you've ever developed your own photographs in a dark room.

To understand this you need to know how clouds form. Condensation into raindrops requires some sort of surface. Water molecules in the form of gas (water vapor) cannot just spontaneously come together to form a drop (unless the molecules are cooled below freezing). They need something to start the process. This usually comes in the form of a tiny particulate in the air, called a condensation nuclei. These are microscopic in size and are usually just dust, dirt, or pollution found in the air.

This is where the silver iodide comes in. The tiny particles provide condensation nuclei for water droplets to form around. Thus, the main purpose is to get the water vapor out of the air, and condensed into droplets. As these droplets get heavier, they fall as rain. This is the idea anyways.

It is not known if so called "seeding" a cloud actually does increase rain because no one knows how much rain that seeded cloud would have produced.

In the US, we have been using cloud seeding to increase precipitation in areas experiencing drought, reduce the size of hailstones that form in thunderstorms, and reduce the amount of fog in and around airports. Cloud seeding is even used sometimes over ski resorts to induce snow fall.

Another interesting application, based on this same idea, is to diminish the strength of hurricanes by dropping particulates into the air. However, these particulates work the opposite way. They soak up the water molecules, thus lowering the intensity of the storm. Tests have proven successful, but only lowering the severity of the storm by about 10 mph. I forget what the stuff if called, or what it's composed of, but if I find it, I will edit this post.

As for the side effects, or environmental concern, I really don't know. The only thing I can think of would be if the particulate shot into the air in some way was bad for the environment. Probably the biggest concern would be if it found its way into the water supply. As for the hurricane dampening stuff, I've heard that it's environmentally friendly and biodegradable. It falls into the ocean and dissociates, causing no problems.