Detonating nuclear weapons aboveground sends radioactive materials as high as 50 miles into the atmosphere. Large particles fall to the ground near the explosion site, but lighter particles and gases travel into the upper atmosphere. The particles that are swept up into the atmosphere and fall back down to Earth are called fallout. The highest particles can circulate around the world for years until they gradually fall to Earth or are brought back to the surface by precipitation. The path of the locations of the fallout depend on wind and weather patterns.