Artificial Rain or Cloud Seeding
When moist clouds come in contact with silver iodide or other such chemicals, they dissipate into rain. The chemicals could be fired into the clouds from the ground or, more effectively, dropped over the clouds from aircrafts. Called cloud seeding, this method of creating artificial rain has been around for decades.
This can work only if there are moist clouds in the atmosphere in the first place. Beijing has found the method to be so effective it uses it whenever there are international conferences or national days to be celebrated in Beijing. It was used, most famously, in the 2008 Beijing Olympics to bring about clear blue skies, not only to wash smog away but also to make sure it doesn't rain during the open-air ceremony.
Cloud seeding has been found to be environmentally safe. China has a full department of weather modification that has been cloud seeding extensively. There are those who think the Chinese government exaggerates claims of cloud seeding's success, but there is growing scientific evidence that it works. more