Scientists in Europe have recently used lasers in labs to condense water molecules to form clouds. They have also reproduced this feat in the skies over Berlin, Germany. The lasers work by getting rid of the electrons in the atoms in the air, forming hydroxyl radicals. These form particles which are the base of water droplets. As long as there is enough humidity in the air, it works. The concept of encouraging rain is not new. In the past, scientists have used silver iodine crystals. They shoot them up into the clouds or drop them out of planes. The lasers are more reliable than the old silver iodine method, although both are controversial, because it is hard to measure if either actually has a significant effect on creating rain. The rain making process is still in its infancy, as they are still only creating condensation where the laser hits, not over large areas. It is also possible that they will never be able to actually make it rain in real world conditions, only in a lab, although they might be able to encourage precipitation. Read more about it here or here. Watch a video of a laser creating clouds here.
If we were to get rain on demand, imagine the implications. If we order rain over one location, what about the land where that water would have fallen? Whenever humankind has manipulated the earth in the past by the building of dams, deforestation or other types of marring, there were almost always unforeseen negative consequences. Calling down rain where it suits us is unfair to those without the same technology. The more practical use of the laser is measuring the amount of condensation produced when it is shone through the air and then predicting the chance of rain from that measurement.