Most common weather modification projects augment normal snow or rainfall, to refill reservoirs, generate snow for ski resorts or relieve droughty crops. Fog over busy airports may be modified by dropping dry ice. Future developments may be draining energy from budding hurricanes and hailstorms, or creating rain from a clear blue sky.
At least 29 states in the USA had licensed weather modification programmes in 1999.
Military researchers are also attempting to influence the weather by adding small amounts of energy at just the right time and space. An experimental U.S. Navy and Air Force ionospheric heater known as the High-Frequency Active Auroral Research Program (HAARP) has been projecting tightly-focused beams of intense radio-frequency energy into the atmosphere for the past several years. Artificial "mirrors", formed of polymers sprayed behind high-flying aircraft, could could be held aloft by the pressure exerted by such radiation beams. By absorbing microwave energy, these structures could heat the air and induce changes in the weather.