When a volcano erupts, it ejects ash several kilometers (a few miles) into the atmosphere. This is because of the force of the eruption and because air mixes in with the ash and makes the ash cloud lighter. When ash particles bump into each other, they move in opposite directions, which makes the cloud start to spread out. The wind is stronger higher in the atmosphere, and the ash is very high, so the wind can blow the ash a long way before it falls. Bigger particles fall first and land near the volcano, but smaller particles stay in the air longer and keep moving for a long time.