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Wind

Wind blowing dandelion spores away
Wind blowing dandelion spores away

Wind is the movement of air.

Air is a mixture of gases that the eye cannot see. Air is about 78 percent nitrogen and 21 percent oxygen and 1 percent other gases. The gases that make air are made up of tiny particles called molecules. Some molecules are heavy and some are light.

Wind begins with air pressure. Air pressure goes up and down in response to the Earth’s atmosphere. The atmosphere is the blanket of gases that surround the Earth. The atmosphere is held in place by the rotation of the Earth and gravity.

Air near the Earth’s surface is weighed down by all of the air in the atmosphere above it–this is called air pressure. Air closer to the Earth experiences heavier pressure than air far above the Earth. When air molecules are pressed or squeezed heavy air pressure is the result. The pressure on air high in the atmosphere is very light or low; this is called low air pressure.

Wind blowing snow off the top of a mountain

Air is pushed and pulled by the rotation of the Earth and gravity. It is also heated by the sun. Not only does the sun heat up the air, it heats up the surface of the Earth; the heated surface, in turn, reflects warmth into the air. The air around the Earth and the surface of the Earth are warming and cooling all of the time. That means air pressure around the Earth is never even; it is warm in some places and cool in others. The air pressure is high in some places and low in others.

When the Earth and air are warmed by the sun, air molecules move apart and rise; as a result, cooler air sinks to takes its place. When the Earth and air cool down, air molecules move more slowly; they move closer together and become heavy and drop or sink. Around the earth, some air is rising and some air is falling all of the time. The movement of air can be fast or slow depending on how hot or cold air molecules are. The movement of air from areas of high pressure to areas of low pressure is what we call wind.

The push and pull of wind by the rotation of the Earth and gravity and by the warming of the Earth and air by the sun create what we call wind currents.

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