Wind is caused by a difference in pressure from one area to another area on the surface of the Earth. Air naturally moves from high to low pressure, and when it does so, it is called wind.
Generally, we can say that the cause of the wind is the uneven heating of the Earth’s surface by the Sun. The Earth’s surface is made of different land and water areas, and these varying surfaces absorb and reflect the Sun’s rays unevenly. Warm air rising yields a lower pressure on the Earth, because the air is not pressing down on the Earth’s surface, while descending cooler air produces a higher pressure.
But there are many other factors affecting wind direction. For example, the Earth is spinning, so air in the Northern Hemisphere is deflected to the right by what is known as the Coriolis force. This causes the air, or wind, to flow clockwise around a high-pressure system and counter-clockwise around a low-pressure system.
The closer these low- and high-pressure systems are together, the stronger the “pressure gradient,” and the stronger the winds. Vegetation also plays a role in how much sunlight is reflected or absorbed by the surface of the Earth. Furthermore, snow cover reflects a large amount of radiation back into space. As the air cools, it sinks and causes a pressure increase.
And wind can get even more complex. Some parts of the Earth, near the equator, receive direct sunlight all year long and have a consistently warmer climate. Other parts of the Earth, near the polar regions, receive indirect rays, so the climate is colder.
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As the warm air from the tropics rises, colder air moves in to take the place of the rising warmer air. This movement of air also causes the wind to blow. It’s a dynamic, complex mechanism, which is why weather forecasting is not quite a precise science.
Today we see windmills, used to make electricity, in operation in all parts of the United States, but especially along our coasts. Coastal regions tend to have fairly strong winds blowing in from ocean to land during the day and out from land to ocean during the night. The cause of this phenomenon is that land heats up and cools down faster than water, again creating a pressure gradient.
From the book, "Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works"; Copyright © Larry Scheckel, 2013. Publishes December 17; available wherever books are sold.