(c) Anthony K. Grafton 2003

What Makes the Wind Blow?

Asking simple questions can teach us a great deal about the world around us. Getting to the answer can take us through interesting twists and turns and teach us things we might never have known otherwise. For instance, why does the wind blow?

Wind is just the movement of air from one place to another. Wind speeds can range from practically nothing all the way beyond 500 kilometers per hour. Wind can help kites, birds, and airplanes fly, and it can damage or destroy crops, buildings and trees depending on how hard it blows. But wind, like all weather on Earth, has a common source: the sun.

When the sun shines on Earth, it warms the ground, the water, and the air. When air or any other gas is heated without being sealed in a container, it expands. When some mass of air is warmed, its expansion causes it to be less dense than the surrounding, cool air. This makes the warmer air rise just like the way a hot-air balloon rises into the sky.

When a big volume of air rises up away from the ground, nature doesn’t just leave an empty vacuum in its place. Instead, surrounding air moves in to fill the space. When this surrounding air moves, we feel the wind blow.

In the same way, air that is cooled will sink toward the ground. The sinking air pushes outward to make room, and we also feel the wind blow. Rising and sinking air are the basic causes behind the low-pressure and high-pressure systems shown on weather maps. Differences in the way different parts of the Earth heat up and cool down result in big volumes of air constantly expanding and rising and cooling and sinking all over the planet.

Consider this: during the day, water is warmed by the sun more slowly than land, and during the night, water holds the heat longer than the land. Given the effect of heating and cooling on the air just above the land and water, which way should the wind near ocean shores known as “sea breezes” blow during the day and which way should it blow at night? Send your answer to Science Corner.