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.
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