The Quest to Control Clouds

In order to keep world leaders dry during the festivities marking the 60th anniversary of the end of World War II in Moscow on May 9, Russian planes attacked dark rain clouds above Moscow with chemicals. The clouds actually disappeared, but it's unclear whether the planes had anything to do with that.

Theoretically speaking, it's possible to dissolve rain clouds, according to weather experts. Clouds could be manipulated to drop their load in places where it doesn't bother anyone.

"Rain drops that float in the air are only about 100 to 200 micrometers in size," said Stephan Borrmann, a cloud expert and professor at the Institute for Atmospheric Physics in Mainz.

"In order for them to fall, they have to reach a size of about one millimeter," he added. "That's what we call rain."

Dry ice and other secrets

Using dry ice would be one possible way to exploit this.

"The cloud turns to ice and ice particles absorb steam quicker," said Jost Heintzenberg, the director of the Institute for Tropospheric Research in Leipzig.

This leads the particles grow in size, making them heavier and causing them to fall. If the air below the cloud is warm, the particles would come down as rain. If it's cold, it would hail. Heintzenberg added that about 80 percent of rain on earth develops this way. By attacking clouds with dry ice, they could be forced to drop their loads in specific places.

A second possible strategy would be to spray clouds with a special kind of salt called silver iodine. It's used because its structure is very similar to that of ice, Heintzenberg said.

"That makes it easier for water molecules to attach," he added.

This again creates large ice cubes that melt and rain down. It's not a cheap option, however: Silver iodine is expensive and would probably harm the environment if used in large quantities, Borrmann said.

A third option would be to keep the clouds to releasing rain by spiking them with additional particles -- so-called aerosol nuclei. They cause steam to divide up in a greater number of smaller drops, which melt and evaporate.

Ships as weather-makers?

It's an effect that's also caused by large ships, Heintzenberg said.

"The droplets become smaller because of soot particles released by the ships," he said, adding that this also prevents clouds from releasing rain.

But the opposite could also happen, he added, saying that warm soot particles create turbulences that mix up cold and warm layers of air that were separated before.

Searching for decades

All of this is highly theoretical and can hardly be used in real life, however. Researchers have been trying to come up with solutions since the 1950s -- especially in dry parts of the earth.

"Clouds form around Cape Town's Table Mountain and their water content is dragged to earth via wires," Borrmann said. "But that's just a few liters each time."

Agriculture could actually profit from some of these tests.

"The one thing that can be done with aerosol nuclei is to turn hail clouds into rain clouds," Borrmann said.

Few hopes for success

But he and Heintzenberg both said they doubted that scientists could really manipulate the weather.

"You cannot prove statistically that a cloud would have behaved differently if it hadn't been treated," Heintzenberg said, adding that he couldn't imagine an effective way to treat thick layers of clouds in any case.

"And besides, legal issues also have to be taken into consideration," he said. "What are you going to do if it rains in the wrong place?"

In the end, a cloud will remain a majestic miracle.

"If a could doesn't feel like raining, it just won't do it," Borrmann said.