Dust consists of dry, solid particles less than 0.0625 millimeter (1/400 inch) in diameter--smaller than the tiniest grains of sand. Most dust is mineral material that comes from bare soil, plowed fields, river flood plains, and floors of desert basins. Dust also comes from ocean spray, meteoroid fragments, smoke and ash produced by fires, volcanic eruptions, and decaying organic materials. The wind easily lifts dust particles and often carries them for long distances. Winds in the stratosphere can carry fine volcanic dust around the world several times.