The key to making rain where it wouldn't ordinarily fall could be an extremely large black tarp.
Researchers from the University of Brussels and Ben Gurion University of the Negev want to cover several kilometers of the desert in a special solar-absorbing material that they say could cause clouds to form and rain to fall. Unknown ecological disasters aside for the moment, the technique could be used to transform barren regions into farming centers.
The material, from a company called Acktar, is a thin, relatively light black-colored polymer that can stretched onto frames. The frames would then be placed in the desert. Once in place, the material would absorb heat. The air thermals rising off the tarp would carry water vapor into the upper atmosphere and cause clouds to form. Rain would follow. You could do something similar with asphalt but asphalt can't be moved.
The researchers estimate they would need to cover five to nine square kilometers to make it rain over an area of 40 to 100 square kilometers. Covering such a large area, however, would require about 80 million Euros.
The researchers right now are tinkering with computer models and will try to put together a 3000 square meter pilot next year in the Negev desert.